1 | =head1 NAME |
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2 | |
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3 | perldata - Perl data types |
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4 | |
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5 | =head1 DESCRIPTION |
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6 | |
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7 | =head2 Variable names |
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8 | |
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9 | Perl has three built-in data types: scalars, arrays of scalars, and |
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10 | associative arrays of scalars, known as "hashes". Normal arrays |
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11 | are ordered lists of scalars indexed by number, starting with 0 and with |
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12 | negative subscripts counting from the end. Hashes are unordered |
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13 | collections of scalar values indexed by their associated string key. |
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14 | |
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15 | Values are usually referred to by name, or through a named reference. |
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16 | The first character of the name tells you to what sort of data |
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17 | structure it refers. The rest of the name tells you the particular |
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18 | value to which it refers. Usually this name is a single I<identifier>, |
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19 | that is, a string beginning with a letter or underscore, and |
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20 | containing letters, underscores, and digits. In some cases, it may |
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21 | be a chain of identifiers, separated by C<::> (or by the slightly |
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22 | archaic C<'>); all but the last are interpreted as names of packages, |
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23 | to locate the namespace in which to look up the final identifier |
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24 | (see L<perlmod/Packages> for details). It's possible to substitute |
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25 | for a simple identifier, an expression that produces a reference |
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26 | to the value at runtime. This is described in more detail below |
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27 | and in L<perlref>. |
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28 | |
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29 | Perl also has its own built-in variables whose names don't follow |
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30 | these rules. They have strange names so they don't accidentally |
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31 | collide with one of your normal variables. Strings that match |
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32 | parenthesized parts of a regular expression are saved under names |
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33 | containing only digits after the C<$> (see L<perlop> and L<perlre>). |
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34 | In addition, several special variables that provide windows into |
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35 | the inner working of Perl have names containing punctuation characters |
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36 | and control characters. These are documented in L<perlvar>. |
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37 | |
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38 | Scalar values are always named with '$', even when referring to a |
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39 | scalar that is part of an array or a hash. The '$' symbol works |
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40 | semantically like the English word "the" in that it indicates a |
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41 | single value is expected. |
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42 | |
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43 | $days # the simple scalar value "days" |
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44 | $days[28] # the 29th element of array @days |
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45 | $days{'Feb'} # the 'Feb' value from hash %days |
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46 | $#days # the last index of array @days |
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47 | |
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48 | Entire arrays (and slices of arrays and hashes) are denoted by '@', |
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49 | which works much like the word "these" or "those" does in English, |
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50 | in that it indicates multiple values are expected. |
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51 | |
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52 | @days # ($days[0], $days[1],... $days[n]) |
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53 | @days[3,4,5] # same as ($days[3],$days[4],$days[5]) |
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54 | @days{'a','c'} # same as ($days{'a'},$days{'c'}) |
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55 | |
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56 | Entire hashes are denoted by '%': |
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57 | |
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58 | %days # (key1, val1, key2, val2 ...) |
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59 | |
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60 | In addition, subroutines are named with an initial '&', though this |
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61 | is optional when unambiguous, just as the word "do" is often redundant |
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62 | in English. Symbol table entries can be named with an initial '*', |
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63 | but you don't really care about that yet (if ever :-). |
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64 | |
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65 | Every variable type has its own namespace, as do several |
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66 | non-variable identifiers. This means that you can, without fear |
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67 | of conflict, use the same name for a scalar variable, an array, or |
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68 | a hash--or, for that matter, for a filehandle, a directory handle, a |
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69 | subroutine name, a format name, or a label. This means that $foo |
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70 | and @foo are two different variables. It also means that C<$foo[1]> |
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71 | is a part of @foo, not a part of $foo. This may seem a bit weird, |
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72 | but that's okay, because it is weird. |
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73 | |
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74 | Because variable references always start with '$', '@', or '%', the |
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75 | "reserved" words aren't in fact reserved with respect to variable |
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76 | names. They I<are> reserved with respect to labels and filehandles, |
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77 | however, which don't have an initial special character. You can't |
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78 | have a filehandle named "log", for instance. Hint: you could say |
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79 | C<open(LOG,'logfile')> rather than C<open(log,'logfile')>. Using |
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80 | uppercase filehandles also improves readability and protects you |
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81 | from conflict with future reserved words. Case I<is> significant--"FOO", |
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82 | "Foo", and "foo" are all different names. Names that start with a |
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83 | letter or underscore may also contain digits and underscores. |
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84 | |
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85 | It is possible to replace such an alphanumeric name with an expression |
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86 | that returns a reference to the appropriate type. For a description |
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87 | of this, see L<perlref>. |
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88 | |
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89 | Names that start with a digit may contain only more digits. Names |
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90 | that do not start with a letter, underscore, or digit are limited to |
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91 | one character, e.g., C<$%> or C<$$>. (Most of these one character names |
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92 | have a predefined significance to Perl. For instance, C<$$> is the |
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93 | current process id.) |
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94 | |
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95 | =head2 Context |
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96 | |
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97 | The interpretation of operations and values in Perl sometimes depends |
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98 | on the requirements of the context around the operation or value. |
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99 | There are two major contexts: list and scalar. Certain operations |
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100 | return list values in contexts wanting a list, and scalar values |
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101 | otherwise. If this is true of an operation it will be mentioned in |
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102 | the documentation for that operation. In other words, Perl overloads |
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103 | certain operations based on whether the expected return value is |
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104 | singular or plural. Some words in English work this way, like "fish" |
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105 | and "sheep". |
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106 | |
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107 | In a reciprocal fashion, an operation provides either a scalar or a |
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108 | list context to each of its arguments. For example, if you say |
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109 | |
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110 | int( <STDIN> ) |
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111 | |
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112 | the integer operation provides scalar context for the <> |
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113 | operator, which responds by reading one line from STDIN and passing it |
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114 | back to the integer operation, which will then find the integer value |
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115 | of that line and return that. If, on the other hand, you say |
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116 | |
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117 | sort( <STDIN> ) |
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118 | |
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119 | then the sort operation provides list context for <>, which |
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120 | will proceed to read every line available up to the end of file, and |
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121 | pass that list of lines back to the sort routine, which will then |
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122 | sort those lines and return them as a list to whatever the context |
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123 | of the sort was. |
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124 | |
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125 | Assignment is a little bit special in that it uses its left argument |
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126 | to determine the context for the right argument. Assignment to a |
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127 | scalar evaluates the right-hand side in scalar context, while |
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128 | assignment to an array or hash evaluates the righthand side in list |
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129 | context. Assignment to a list (or slice, which is just a list |
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130 | anyway) also evaluates the righthand side in list context. |
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131 | |
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132 | When you use the C<use warnings> pragma or Perl's B<-w> command-line |
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133 | option, you may see warnings |
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134 | about useless uses of constants or functions in "void context". |
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135 | Void context just means the value has been discarded, such as a |
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136 | statement containing only C<"fred";> or C<getpwuid(0);>. It still |
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137 | counts as scalar context for functions that care whether or not |
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138 | they're being called in list context. |
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139 | |
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140 | User-defined subroutines may choose to care whether they are being |
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141 | called in a void, scalar, or list context. Most subroutines do not |
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142 | need to bother, though. That's because both scalars and lists are |
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143 | automatically interpolated into lists. See L<perlfunc/wantarray> |
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144 | for how you would dynamically discern your function's calling |
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145 | context. |
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146 | |
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147 | =head2 Scalar values |
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148 | |
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149 | All data in Perl is a scalar, an array of scalars, or a hash of |
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150 | scalars. A scalar may contain one single value in any of three |
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151 | different flavors: a number, a string, or a reference. In general, |
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152 | conversion from one form to another is transparent. Although a |
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153 | scalar may not directly hold multiple values, it may contain a |
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154 | reference to an array or hash which in turn contains multiple values. |
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155 | |
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156 | Scalars aren't necessarily one thing or another. There's no place |
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157 | to declare a scalar variable to be of type "string", type "number", |
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158 | type "reference", or anything else. Because of the automatic |
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159 | conversion of scalars, operations that return scalars don't need |
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160 | to care (and in fact, cannot care) whether their caller is looking |
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161 | for a string, a number, or a reference. Perl is a contextually |
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162 | polymorphic language whose scalars can be strings, numbers, or |
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163 | references (which includes objects). Although strings and numbers |
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164 | are considered pretty much the same thing for nearly all purposes, |
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165 | references are strongly-typed, uncastable pointers with builtin |
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166 | reference-counting and destructor invocation. |
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167 | |
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168 | A scalar value is interpreted as TRUE in the Boolean sense if it is not |
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169 | the null string or the number 0 (or its string equivalent, "0"). The |
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170 | Boolean context is just a special kind of scalar context where no |
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171 | conversion to a string or a number is ever performed. |
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172 | |
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173 | There are actually two varieties of null strings (sometimes referred |
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174 | to as "empty" strings), a defined one and an undefined one. The |
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175 | defined version is just a string of length zero, such as C<"">. |
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176 | The undefined version is the value that indicates that there is |
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177 | no real value for something, such as when there was an error, or |
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178 | at end of file, or when you refer to an uninitialized variable or |
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179 | element of an array or hash. Although in early versions of Perl, |
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180 | an undefined scalar could become defined when first used in a |
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181 | place expecting a defined value, this no longer happens except for |
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182 | rare cases of autovivification as explained in L<perlref>. You can |
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183 | use the defined() operator to determine whether a scalar value is |
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184 | defined (this has no meaning on arrays or hashes), and the undef() |
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185 | operator to produce an undefined value. |
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186 | |
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187 | To find out whether a given string is a valid non-zero number, it's |
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188 | sometimes enough to test it against both numeric 0 and also lexical |
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189 | "0" (although this will cause B<-w> noises). That's because strings |
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190 | that aren't numbers count as 0, just as they do in B<awk>: |
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191 | |
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192 | if ($str == 0 && $str ne "0") { |
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193 | warn "That doesn't look like a number"; |
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194 | } |
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195 | |
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196 | That method may be best because otherwise you won't treat IEEE |
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197 | notations like C<NaN> or C<Infinity> properly. At other times, you |
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198 | might prefer to determine whether string data can be used numerically |
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199 | by calling the POSIX::strtod() function or by inspecting your string |
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200 | with a regular expression (as documented in L<perlre>). |
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201 | |
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202 | warn "has nondigits" if /\D/; |
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203 | warn "not a natural number" unless /^\d+$/; # rejects -3 |
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204 | warn "not an integer" unless /^-?\d+$/; # rejects +3 |
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205 | warn "not an integer" unless /^[+-]?\d+$/; |
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206 | warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2 |
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207 | warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/; |
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208 | warn "not a C float" |
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209 | unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/; |
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210 | |
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211 | The length of an array is a scalar value. You may find the length |
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212 | of array @days by evaluating C<$#days>, as in B<csh>. Technically |
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213 | speaking, this isn't the length of the array; it's the subscript |
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214 | of the last element, since there is ordinarily a 0th element. |
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215 | Assigning to C<$#days> actually changes the length of the array. |
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216 | Shortening an array this way destroys intervening values. Lengthening |
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217 | an array that was previously shortened does not recover values |
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218 | that were in those elements. (It used to do so in Perl 4, but we |
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219 | had to break this to make sure destructors were called when expected.) |
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220 | |
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221 | You can also gain some miniscule measure of efficiency by pre-extending |
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222 | an array that is going to get big. You can also extend an array |
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223 | by assigning to an element that is off the end of the array. You |
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224 | can truncate an array down to nothing by assigning the null list |
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225 | () to it. The following are equivalent: |
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226 | |
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227 | @whatever = (); |
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228 | $#whatever = -1; |
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229 | |
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230 | If you evaluate an array in scalar context, it returns the length |
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231 | of the array. (Note that this is not true of lists, which return |
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232 | the last value, like the C comma operator, nor of built-in functions, |
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233 | which return whatever they feel like returning.) The following is |
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234 | always true: |
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235 | |
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236 | scalar(@whatever) == $#whatever - $[ + 1; |
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237 | |
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238 | Version 5 of Perl changed the semantics of C<$[>: files that don't set |
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239 | the value of C<$[> no longer need to worry about whether another |
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240 | file changed its value. (In other words, use of C<$[> is deprecated.) |
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241 | So in general you can assume that |
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242 | |
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243 | scalar(@whatever) == $#whatever + 1; |
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244 | |
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245 | Some programmers choose to use an explicit conversion so as to |
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246 | leave nothing to doubt: |
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247 | |
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248 | $element_count = scalar(@whatever); |
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249 | |
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250 | If you evaluate a hash in scalar context, it returns false if the |
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251 | hash is empty. If there are any key/value pairs, it returns true; |
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252 | more precisely, the value returned is a string consisting of the |
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253 | number of used buckets and the number of allocated buckets, separated |
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254 | by a slash. This is pretty much useful only to find out whether |
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255 | Perl's internal hashing algorithm is performing poorly on your data |
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256 | set. For example, you stick 10,000 things in a hash, but evaluating |
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257 | %HASH in scalar context reveals C<"1/16">, which means only one out |
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258 | of sixteen buckets has been touched, and presumably contains all |
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259 | 10,000 of your items. This isn't supposed to happen. |
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260 | |
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261 | You can preallocate space for a hash by assigning to the keys() function. |
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262 | This rounds up the allocated bucked to the next power of two: |
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263 | |
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264 | keys(%users) = 1000; # allocate 1024 buckets |
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265 | |
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266 | =head2 Scalar value constructors |
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267 | |
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268 | Numeric literals are specified in any of the following floating point or |
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269 | integer formats: |
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270 | |
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271 | 12345 |
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272 | 12345.67 |
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273 | .23E-10 # a very small number |
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274 | 4_294_967_296 # underline for legibility |
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275 | 0xff # hex |
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276 | 0377 # octal |
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277 | 0b011011 # binary |
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278 | |
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279 | String literals are usually delimited by either single or double |
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280 | quotes. They work much like quotes in the standard Unix shells: |
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281 | double-quoted string literals are subject to backslash and variable |
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282 | substitution; single-quoted strings are not (except for C<\'> and |
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283 | C<\\>). The usual C-style backslash rules apply for making |
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284 | characters such as newline, tab, etc., as well as some more exotic |
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285 | forms. See L<perlop/"Quote and Quote-like Operators"> for a list. |
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286 | |
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287 | Hexadecimal, octal, or binary, representations in string literals |
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288 | (e.g. '0xff') are not automatically converted to their integer |
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289 | representation. The hex() and oct() functions make these conversions |
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290 | for you. See L<perlfunc/hex> and L<perlfunc/oct> for more details. |
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291 | |
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292 | You can also embed newlines directly in your strings, i.e., they can end |
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293 | on a different line than they begin. This is nice, but if you forget |
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294 | your trailing quote, the error will not be reported until Perl finds |
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295 | another line containing the quote character, which may be much further |
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296 | on in the script. Variable substitution inside strings is limited to |
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297 | scalar variables, arrays, and array or hash slices. (In other words, |
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298 | names beginning with $ or @, followed by an optional bracketed |
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299 | expression as a subscript.) The following code segment prints out "The |
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300 | price is $Z<>100." |
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301 | |
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302 | $Price = '$100'; # not interpreted |
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303 | print "The price is $Price.\n"; # interpreted |
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304 | |
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305 | As in some shells, you can enclose the variable name in braces to |
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306 | disambiguate it from following alphanumerics. You must also do |
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307 | this when interpolating a variable into a string to separate the |
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308 | variable name from a following double-colon or an apostrophe, since |
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309 | these would be otherwise treated as a package separator: |
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310 | |
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311 | $who = "Larry"; |
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312 | print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n"; |
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313 | print "We use ${who}speak when ${who}'s here.\n"; |
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314 | |
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315 | Without the braces, Perl would have looked for a $whospeak, a |
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316 | C<$who::0>, and a C<$who's> variable. The last two would be the |
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317 | $0 and the $s variables in the (presumably) non-existent package |
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318 | C<who>. |
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319 | |
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320 | In fact, an identifier within such curlies is forced to be a string, |
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321 | as is any simple identifier within a hash subscript. Neither need |
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322 | quoting. Our earlier example, C<$days{'Feb'}> can be written as |
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323 | C<$days{Feb}> and the quotes will be assumed automatically. But |
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324 | anything more complicated in the subscript will be interpreted as |
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325 | an expression. |
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326 | |
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327 | A literal of the form C<v1.20.300.4000> is parsed as a string composed |
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328 | of characters with the specified ordinals. This provides an alternative, |
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329 | more readable way to construct strings, rather than use the somewhat less |
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330 | readable interpolation form C<"\x{1}\x{14}\x{12c}\x{fa0}">. This is useful |
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331 | for representing Unicode strings, and for comparing version "numbers" |
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332 | using the string comparison operators, C<cmp>, C<gt>, C<lt> etc. |
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333 | If there are two or more dots in the literal, the leading C<v> may be |
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334 | omitted. |
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335 | |
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336 | print v9786; # prints UTF-8 encoded SMILEY, "\x{263a}" |
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337 | print v102.111.111; # prints "foo" |
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338 | print 102.111.111; # same |
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339 | |
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340 | Such literals are accepted by both C<require> and C<use> for |
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341 | doing a version check. The C<$^V> special variable also contains the |
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342 | running Perl interpreter's version in this form. See L<perlvar/$^V>. |
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343 | |
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344 | The special literals __FILE__, __LINE__, and __PACKAGE__ |
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345 | represent the current filename, line number, and package name at that |
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346 | point in your program. They may be used only as separate tokens; they |
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347 | will not be interpolated into strings. If there is no current package |
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348 | (due to an empty C<package;> directive), __PACKAGE__ is the undefined |
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349 | value. |
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350 | |
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351 | The two control characters ^D and ^Z, and the tokens __END__ and __DATA__ |
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352 | may be used to indicate the logical end of the script before the actual |
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353 | end of file. Any following text is ignored. |
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354 | |
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355 | Text after __DATA__ but may be read via the filehandle C<PACKNAME::DATA>, |
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356 | where C<PACKNAME> is the package that was current when the __DATA__ |
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357 | token was encountered. The filehandle is left open pointing to the |
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358 | contents after __DATA__. It is the program's responsibility to |
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359 | C<close DATA> when it is done reading from it. For compatibility with |
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360 | older scripts written before __DATA__ was introduced, __END__ behaves |
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361 | like __DATA__ in the toplevel script (but not in files loaded with |
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362 | C<require> or C<do>) and leaves the remaining contents of the |
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363 | file accessible via C<main::DATA>. |
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364 | |
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365 | See L<SelfLoader> for more description of __DATA__, and |
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366 | an example of its use. Note that you cannot read from the DATA |
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367 | filehandle in a BEGIN block: the BEGIN block is executed as soon |
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368 | as it is seen (during compilation), at which point the corresponding |
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369 | __DATA__ (or __END__) token has not yet been seen. |
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370 | |
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371 | A word that has no other interpretation in the grammar will |
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372 | be treated as if it were a quoted string. These are known as |
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373 | "barewords". As with filehandles and labels, a bareword that consists |
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374 | entirely of lowercase letters risks conflict with future reserved |
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375 | words, and if you use the C<use warnings> pragma or the B<-w> switch, |
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376 | Perl will warn you about any |
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377 | such words. Some people may wish to outlaw barewords entirely. If you |
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378 | say |
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379 | |
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380 | use strict 'subs'; |
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381 | |
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382 | then any bareword that would NOT be interpreted as a subroutine call |
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383 | produces a compile-time error instead. The restriction lasts to the |
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384 | end of the enclosing block. An inner block may countermand this |
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385 | by saying C<no strict 'subs'>. |
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386 | |
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387 | Arrays and slices are interpolated into double-quoted strings |
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388 | by joining the elements with the delimiter specified in the C<$"> |
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389 | variable (C<$LIST_SEPARATOR> in English), space by default. The |
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390 | following are equivalent: |
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391 | |
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392 | $temp = join($", @ARGV); |
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393 | system "echo $temp"; |
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394 | |
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395 | system "echo @ARGV"; |
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396 | |
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397 | Within search patterns (which also undergo double-quotish substitution) |
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398 | there is an unfortunate ambiguity: Is C</$foo[bar]/> to be interpreted as |
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399 | C</${foo}[bar]/> (where C<[bar]> is a character class for the regular |
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400 | expression) or as C</${foo[bar]}/> (where C<[bar]> is the subscript to array |
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401 | @foo)? If @foo doesn't otherwise exist, then it's obviously a |
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402 | character class. If @foo exists, Perl takes a good guess about C<[bar]>, |
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403 | and is almost always right. If it does guess wrong, or if you're just |
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404 | plain paranoid, you can force the correct interpretation with curly |
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405 | braces as above. |
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406 | |
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407 | A line-oriented form of quoting is based on the shell "here-document" |
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408 | syntax. Following a C<< << >> you specify a string to terminate |
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409 | the quoted material, and all lines following the current line down to |
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410 | the terminating string are the value of the item. The terminating |
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411 | string may be either an identifier (a word), or some quoted text. If |
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412 | quoted, the type of quotes you use determines the treatment of the |
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413 | text, just as in regular quoting. An unquoted identifier works like |
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414 | double quotes. There must be no space between the C<< << >> and |
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415 | the identifier. (If you put a space it will be treated as a null |
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416 | identifier, which is valid, and matches the first empty line.) The |
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417 | terminating string must appear by itself (unquoted and with no |
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418 | surrounding whitespace) on the terminating line. |
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419 | |
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420 | print <<EOF; |
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421 | The price is $Price. |
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422 | EOF |
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423 | |
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424 | print <<"EOF"; # same as above |
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425 | The price is $Price. |
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426 | EOF |
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427 | |
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428 | print <<`EOC`; # execute commands |
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429 | echo hi there |
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430 | echo lo there |
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431 | EOC |
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432 | |
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433 | print <<"foo", <<"bar"; # you can stack them |
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434 | I said foo. |
---|
435 | foo |
---|
436 | I said bar. |
---|
437 | bar |
---|
438 | |
---|
439 | myfunc(<<"THIS", 23, <<'THAT'); |
---|
440 | Here's a line |
---|
441 | or two. |
---|
442 | THIS |
---|
443 | and here's another. |
---|
444 | THAT |
---|
445 | |
---|
446 | Just don't forget that you have to put a semicolon on the end |
---|
447 | to finish the statement, as Perl doesn't know you're not going to |
---|
448 | try to do this: |
---|
449 | |
---|
450 | print <<ABC |
---|
451 | 179231 |
---|
452 | ABC |
---|
453 | + 20; |
---|
454 | |
---|
455 | If you want your here-docs to be indented with the |
---|
456 | rest of the code, you'll need to remove leading whitespace |
---|
457 | from each line manually: |
---|
458 | |
---|
459 | ($quote = <<'FINIS') =~ s/^\s+//gm; |
---|
460 | The Road goes ever on and on, |
---|
461 | down from the door where it began. |
---|
462 | FINIS |
---|
463 | |
---|
464 | =head2 List value constructors |
---|
465 | |
---|
466 | List values are denoted by separating individual values by commas |
---|
467 | (and enclosing the list in parentheses where precedence requires it): |
---|
468 | |
---|
469 | (LIST) |
---|
470 | |
---|
471 | In a context not requiring a list value, the value of what appears |
---|
472 | to be a list literal is simply the value of the final element, as |
---|
473 | with the C comma operator. For example, |
---|
474 | |
---|
475 | @foo = ('cc', '-E', $bar); |
---|
476 | |
---|
477 | assigns the entire list value to array @foo, but |
---|
478 | |
---|
479 | $foo = ('cc', '-E', $bar); |
---|
480 | |
---|
481 | assigns the value of variable $bar to the scalar variable $foo. |
---|
482 | Note that the value of an actual array in scalar context is the |
---|
483 | length of the array; the following assigns the value 3 to $foo: |
---|
484 | |
---|
485 | @foo = ('cc', '-E', $bar); |
---|
486 | $foo = @foo; # $foo gets 3 |
---|
487 | |
---|
488 | You may have an optional comma before the closing parenthesis of a |
---|
489 | list literal, so that you can say: |
---|
490 | |
---|
491 | @foo = ( |
---|
492 | 1, |
---|
493 | 2, |
---|
494 | 3, |
---|
495 | ); |
---|
496 | |
---|
497 | To use a here-document to assign an array, one line per element, |
---|
498 | you might use an approach like this: |
---|
499 | |
---|
500 | @sauces = <<End_Lines =~ m/(\S.*\S)/g; |
---|
501 | normal tomato |
---|
502 | spicy tomato |
---|
503 | green chile |
---|
504 | pesto |
---|
505 | white wine |
---|
506 | End_Lines |
---|
507 | |
---|
508 | LISTs do automatic interpolation of sublists. That is, when a LIST is |
---|
509 | evaluated, each element of the list is evaluated in list context, and |
---|
510 | the resulting list value is interpolated into LIST just as if each |
---|
511 | individual element were a member of LIST. Thus arrays and hashes lose their |
---|
512 | identity in a LIST--the list |
---|
513 | |
---|
514 | (@foo,@bar,&SomeSub,%glarch) |
---|
515 | |
---|
516 | contains all the elements of @foo followed by all the elements of @bar, |
---|
517 | followed by all the elements returned by the subroutine named SomeSub |
---|
518 | called in list context, followed by the key/value pairs of %glarch. |
---|
519 | To make a list reference that does I<NOT> interpolate, see L<perlref>. |
---|
520 | |
---|
521 | The null list is represented by (). Interpolating it in a list |
---|
522 | has no effect. Thus ((),(),()) is equivalent to (). Similarly, |
---|
523 | interpolating an array with no elements is the same as if no |
---|
524 | array had been interpolated at that point. |
---|
525 | |
---|
526 | A list value may also be subscripted like a normal array. You must |
---|
527 | put the list in parentheses to avoid ambiguity. For example: |
---|
528 | |
---|
529 | # Stat returns list value. |
---|
530 | $time = (stat($file))[8]; |
---|
531 | |
---|
532 | # SYNTAX ERROR HERE. |
---|
533 | $time = stat($file)[8]; # OOPS, FORGOT PARENTHESES |
---|
534 | |
---|
535 | # Find a hex digit. |
---|
536 | $hexdigit = ('a','b','c','d','e','f')[$digit-10]; |
---|
537 | |
---|
538 | # A "reverse comma operator". |
---|
539 | return (pop(@foo),pop(@foo))[0]; |
---|
540 | |
---|
541 | Lists may be assigned to only when each element of the list |
---|
542 | is itself legal to assign to: |
---|
543 | |
---|
544 | ($a, $b, $c) = (1, 2, 3); |
---|
545 | |
---|
546 | ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00); |
---|
547 | |
---|
548 | An exception to this is that you may assign to C<undef> in a list. |
---|
549 | This is useful for throwing away some of the return values of a |
---|
550 | function: |
---|
551 | |
---|
552 | ($dev, $ino, undef, undef, $uid, $gid) = stat($file); |
---|
553 | |
---|
554 | List assignment in scalar context returns the number of elements |
---|
555 | produced by the expression on the right side of the assignment: |
---|
556 | |
---|
557 | $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2 |
---|
558 | $x = (($foo,$bar) = f()); # set $x to f()'s return count |
---|
559 | |
---|
560 | This is handy when you want to do a list assignment in a Boolean |
---|
561 | context, because most list functions return a null list when finished, |
---|
562 | which when assigned produces a 0, which is interpreted as FALSE. |
---|
563 | |
---|
564 | The final element may be an array or a hash: |
---|
565 | |
---|
566 | ($a, $b, @rest) = split; |
---|
567 | my($a, $b, %rest) = @_; |
---|
568 | |
---|
569 | You can actually put an array or hash anywhere in the list, but the first one |
---|
570 | in the list will soak up all the values, and anything after it will become |
---|
571 | undefined. This may be useful in a my() or local(). |
---|
572 | |
---|
573 | A hash can be initialized using a literal list holding pairs of |
---|
574 | items to be interpreted as a key and a value: |
---|
575 | |
---|
576 | # same as map assignment above |
---|
577 | %map = ('red',0x00f,'blue',0x0f0,'green',0xf00); |
---|
578 | |
---|
579 | While literal lists and named arrays are often interchangeable, that's |
---|
580 | not the case for hashes. Just because you can subscript a list value like |
---|
581 | a normal array does not mean that you can subscript a list value as a |
---|
582 | hash. Likewise, hashes included as parts of other lists (including |
---|
583 | parameters lists and return lists from functions) always flatten out into |
---|
584 | key/value pairs. That's why it's good to use references sometimes. |
---|
585 | |
---|
586 | It is often more readable to use the C<< => >> operator between key/value |
---|
587 | pairs. The C<< => >> operator is mostly just a more visually distinctive |
---|
588 | synonym for a comma, but it also arranges for its left-hand operand to be |
---|
589 | interpreted as a string--if it's a bareword that would be a legal identifier. |
---|
590 | This makes it nice for initializing hashes: |
---|
591 | |
---|
592 | %map = ( |
---|
593 | red => 0x00f, |
---|
594 | blue => 0x0f0, |
---|
595 | green => 0xf00, |
---|
596 | ); |
---|
597 | |
---|
598 | or for initializing hash references to be used as records: |
---|
599 | |
---|
600 | $rec = { |
---|
601 | witch => 'Mable the Merciless', |
---|
602 | cat => 'Fluffy the Ferocious', |
---|
603 | date => '10/31/1776', |
---|
604 | }; |
---|
605 | |
---|
606 | or for using call-by-named-parameter to complicated functions: |
---|
607 | |
---|
608 | $field = $query->radio_group( |
---|
609 | name => 'group_name', |
---|
610 | values => ['eenie','meenie','minie'], |
---|
611 | default => 'meenie', |
---|
612 | linebreak => 'true', |
---|
613 | labels => \%labels |
---|
614 | ); |
---|
615 | |
---|
616 | Note that just because a hash is initialized in that order doesn't |
---|
617 | mean that it comes out in that order. See L<perlfunc/sort> for examples |
---|
618 | of how to arrange for an output ordering. |
---|
619 | |
---|
620 | =head2 Slices |
---|
621 | |
---|
622 | A common way to access an array or a hash is one scalar element at a |
---|
623 | time. You can also subscript a list to get a single element from it. |
---|
624 | |
---|
625 | $whoami = $ENV{"USER"}; # one element from the hash |
---|
626 | $parent = $ISA[0]; # one element from the array |
---|
627 | $dir = (getpwnam("daemon"))[7]; # likewise, but with list |
---|
628 | |
---|
629 | A slice accesses several elements of a list, an array, or a hash |
---|
630 | simultaneously using a list of subscripts. It's more convenient |
---|
631 | than writing out the individual elements as a list of separate |
---|
632 | scalar values. |
---|
633 | |
---|
634 | ($him, $her) = @folks[0,-1]; # array slice |
---|
635 | @them = @folks[0 .. 3]; # array slice |
---|
636 | ($who, $home) = @ENV{"USER", "HOME"}; # hash slice |
---|
637 | ($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice |
---|
638 | |
---|
639 | Since you can assign to a list of variables, you can also assign to |
---|
640 | an array or hash slice. |
---|
641 | |
---|
642 | @days[3..5] = qw/Wed Thu Fri/; |
---|
643 | @colors{'red','blue','green'} |
---|
644 | = (0xff0000, 0x0000ff, 0x00ff00); |
---|
645 | @folks[0, -1] = @folks[-1, 0]; |
---|
646 | |
---|
647 | The previous assignments are exactly equivalent to |
---|
648 | |
---|
649 | ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/; |
---|
650 | ($colors{'red'}, $colors{'blue'}, $colors{'green'}) |
---|
651 | = (0xff0000, 0x0000ff, 0x00ff00); |
---|
652 | ($folks[0], $folks[-1]) = ($folks[0], $folks[-1]); |
---|
653 | |
---|
654 | Since changing a slice changes the original array or hash that it's |
---|
655 | slicing, a C<foreach> construct will alter some--or even all--of the |
---|
656 | values of the array or hash. |
---|
657 | |
---|
658 | foreach (@array[ 4 .. 10 ]) { s/peter/paul/ } |
---|
659 | |
---|
660 | foreach (@hash{keys %hash}) { |
---|
661 | s/^\s+//; # trim leading whitespace |
---|
662 | s/\s+$//; # trim trailing whitespace |
---|
663 | s/(\w+)/\u\L$1/g; # "titlecase" words |
---|
664 | } |
---|
665 | |
---|
666 | A slice of an empty list is still an empty list. Thus: |
---|
667 | |
---|
668 | @a = ()[1,0]; # @a has no elements |
---|
669 | @b = (@a)[0,1]; # @b has no elements |
---|
670 | @c = (0,1)[2,3]; # @c has no elements |
---|
671 | |
---|
672 | But: |
---|
673 | |
---|
674 | @a = (1)[1,0]; # @a has two elements |
---|
675 | @b = (1,undef)[1,0,2]; # @b has three elements |
---|
676 | |
---|
677 | This makes it easy to write loops that terminate when a null list |
---|
678 | is returned: |
---|
679 | |
---|
680 | while ( ($home, $user) = (getpwent)[7,0]) { |
---|
681 | printf "%-8s %s\n", $user, $home; |
---|
682 | } |
---|
683 | |
---|
684 | As noted earlier in this document, the scalar sense of list assignment |
---|
685 | is the number of elements on the right-hand side of the assignment. |
---|
686 | The null list contains no elements, so when the password file is |
---|
687 | exhausted, the result is 0, not 2. |
---|
688 | |
---|
689 | If you're confused about why you use an '@' there on a hash slice |
---|
690 | instead of a '%', think of it like this. The type of bracket (square |
---|
691 | or curly) governs whether it's an array or a hash being looked at. |
---|
692 | On the other hand, the leading symbol ('$' or '@') on the array or |
---|
693 | hash indicates whether you are getting back a singular value (a |
---|
694 | scalar) or a plural one (a list). |
---|
695 | |
---|
696 | =head2 Typeglobs and Filehandles |
---|
697 | |
---|
698 | Perl uses an internal type called a I<typeglob> to hold an entire |
---|
699 | symbol table entry. The type prefix of a typeglob is a C<*>, because |
---|
700 | it represents all types. This used to be the preferred way to |
---|
701 | pass arrays and hashes by reference into a function, but now that |
---|
702 | we have real references, this is seldom needed. |
---|
703 | |
---|
704 | The main use of typeglobs in modern Perl is create symbol table aliases. |
---|
705 | This assignment: |
---|
706 | |
---|
707 | *this = *that; |
---|
708 | |
---|
709 | makes $this an alias for $that, @this an alias for @that, %this an alias |
---|
710 | for %that, &this an alias for &that, etc. Much safer is to use a reference. |
---|
711 | This: |
---|
712 | |
---|
713 | local *Here::blue = \$There::green; |
---|
714 | |
---|
715 | temporarily makes $Here::blue an alias for $There::green, but doesn't |
---|
716 | make @Here::blue an alias for @There::green, or %Here::blue an alias for |
---|
717 | %There::green, etc. See L<perlmod/"Symbol Tables"> for more examples |
---|
718 | of this. Strange though this may seem, this is the basis for the whole |
---|
719 | module import/export system. |
---|
720 | |
---|
721 | Another use for typeglobs is to pass filehandles into a function or |
---|
722 | to create new filehandles. If you need to use a typeglob to save away |
---|
723 | a filehandle, do it this way: |
---|
724 | |
---|
725 | $fh = *STDOUT; |
---|
726 | |
---|
727 | or perhaps as a real reference, like this: |
---|
728 | |
---|
729 | $fh = \*STDOUT; |
---|
730 | |
---|
731 | See L<perlsub> for examples of using these as indirect filehandles |
---|
732 | in functions. |
---|
733 | |
---|
734 | Typeglobs are also a way to create a local filehandle using the local() |
---|
735 | operator. These last until their block is exited, but may be passed back. |
---|
736 | For example: |
---|
737 | |
---|
738 | sub newopen { |
---|
739 | my $path = shift; |
---|
740 | local *FH; # not my! |
---|
741 | open (FH, $path) or return undef; |
---|
742 | return *FH; |
---|
743 | } |
---|
744 | $fh = newopen('/etc/passwd'); |
---|
745 | |
---|
746 | Now that we have the C<*foo{THING}> notation, typeglobs aren't used as much |
---|
747 | for filehandle manipulations, although they're still needed to pass brand |
---|
748 | new file and directory handles into or out of functions. That's because |
---|
749 | C<*HANDLE{IO}> only works if HANDLE has already been used as a handle. |
---|
750 | In other words, C<*FH> must be used to create new symbol table entries; |
---|
751 | C<*foo{THING}> cannot. When in doubt, use C<*FH>. |
---|
752 | |
---|
753 | All functions that are capable of creating filehandles (open(), |
---|
754 | opendir(), pipe(), socketpair(), sysopen(), socket(), and accept()) |
---|
755 | automatically create an anonymous filehandle if the handle passed to |
---|
756 | them is an uninitialized scalar variable. This allows the constructs |
---|
757 | such as C<open(my $fh, ...)> and C<open(local $fh,...)> to be used to |
---|
758 | create filehandles that will conveniently be closed automatically when |
---|
759 | the scope ends, provided there are no other references to them. This |
---|
760 | largely eliminates the need for typeglobs when opening filehandles |
---|
761 | that must be passed around, as in the following example: |
---|
762 | |
---|
763 | sub myopen { |
---|
764 | open my $fh, "@_" |
---|
765 | or die "Can't open '@_': $!"; |
---|
766 | return $fh; |
---|
767 | } |
---|
768 | |
---|
769 | { |
---|
770 | my $f = myopen("</etc/motd"); |
---|
771 | print <$f>; |
---|
772 | # $f implicitly closed here |
---|
773 | } |
---|
774 | |
---|
775 | Another way to create anonymous filehandles is with the Symbol |
---|
776 | module or with the IO::Handle module and its ilk. These modules |
---|
777 | have the advantage of not hiding different types of the same name |
---|
778 | during the local(). See the bottom of L<perlfunc/open()> for an |
---|
779 | example. |
---|
780 | |
---|
781 | =head1 SEE ALSO |
---|
782 | |
---|
783 | See L<perlvar> for a description of Perl's built-in variables and |
---|
784 | a discussion of legal variable names. See L<perlref>, L<perlsub>, |
---|
785 | and L<perlmod/"Symbol Tables"> for more discussion on typeglobs and |
---|
786 | the C<*foo{THING}> syntax. |
---|