1 | =head1 NAME |
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2 | |
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3 | perlsyn - Perl syntax |
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4 | |
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5 | =head1 DESCRIPTION |
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6 | |
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7 | A Perl script consists of a sequence of declarations and statements. |
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8 | The sequence of statements is executed just once, unlike in B<sed> |
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9 | and B<awk> scripts, where the sequence of statements is executed |
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10 | for each input line. While this means that you must explicitly |
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11 | loop over the lines of your input file (or files), it also means |
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12 | you have much more control over which files and which lines you look at. |
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13 | (Actually, I'm lying--it is possible to do an implicit loop with |
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14 | either the B<-n> or B<-p> switch. It's just not the mandatory |
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15 | default like it is in B<sed> and B<awk>.) |
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16 | |
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17 | Perl is, for the most part, a free-form language. (The only exception |
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18 | to this is format declarations, for obvious reasons.) Text from a |
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19 | C<"#"> character until the end of the line is a comment, and is |
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20 | ignored. If you attempt to use C</* */> C-style comments, it will be |
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21 | interpreted either as division or pattern matching, depending on the |
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22 | context, and C++ C<//> comments just look like a null regular |
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23 | expression, so don't do that. |
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24 | |
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25 | =head2 Declarations |
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26 | |
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27 | The only things you need to declare in Perl are report formats |
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28 | and subroutines--and even undefined subroutines can be handled |
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29 | through AUTOLOAD. A variable holds the undefined value (C<undef>) |
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30 | until it has been assigned a defined value, which is anything |
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31 | other than C<undef>. When used as a number, C<undef> is treated |
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32 | as C<0>; when used as a string, it is treated the empty string, |
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33 | C<"">; and when used as a reference that isn't being assigned |
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34 | to, it is treated as an error. If you enable warnings, you'll |
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35 | be notified of an uninitialized value whenever you treat C<undef> |
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36 | as a string or a number. Well, usually. Boolean ("don't-care") |
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37 | contexts and operators such as C<++>, C<-->, C<+=>, C<-=>, and |
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38 | C<.=> are always exempt from such warnings. |
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39 | |
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40 | A declaration can be put anywhere a statement can, but has no effect on |
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41 | the execution of the primary sequence of statements--declarations all |
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42 | take effect at compile time. Typically all the declarations are put at |
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43 | the beginning or the end of the script. However, if you're using |
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44 | lexically-scoped private variables created with C<my()>, you'll |
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45 | have to make sure |
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46 | your format or subroutine definition is within the same block scope |
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47 | as the my if you expect to be able to access those private variables. |
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48 | |
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49 | Declaring a subroutine allows a subroutine name to be used as if it were a |
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50 | list operator from that point forward in the program. You can declare a |
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51 | subroutine without defining it by saying C<sub name>, thus: |
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52 | |
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53 | sub myname; |
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54 | $me = myname $0 or die "can't get myname"; |
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55 | |
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56 | Note that my() functions as a list operator, not as a unary operator; so |
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57 | be careful to use C<or> instead of C<||> in this case. However, if |
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58 | you were to declare the subroutine as C<sub myname ($)>, then |
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59 | C<myname> would function as a unary operator, so either C<or> or |
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60 | C<||> would work. |
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61 | |
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62 | Subroutines declarations can also be loaded up with the C<require> statement |
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63 | or both loaded and imported into your namespace with a C<use> statement. |
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64 | See L<perlmod> for details on this. |
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65 | |
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66 | A statement sequence may contain declarations of lexically-scoped |
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67 | variables, but apart from declaring a variable name, the declaration acts |
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68 | like an ordinary statement, and is elaborated within the sequence of |
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69 | statements as if it were an ordinary statement. That means it actually |
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70 | has both compile-time and run-time effects. |
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71 | |
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72 | =head2 Simple statements |
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73 | |
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74 | The only kind of simple statement is an expression evaluated for its |
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75 | side effects. Every simple statement must be terminated with a |
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76 | semicolon, unless it is the final statement in a block, in which case |
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77 | the semicolon is optional. (A semicolon is still encouraged there if the |
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78 | block takes up more than one line, because you may eventually add another line.) |
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79 | Note that there are some operators like C<eval {}> and C<do {}> that look |
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80 | like compound statements, but aren't (they're just TERMs in an expression), |
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81 | and thus need an explicit termination if used as the last item in a statement. |
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82 | |
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83 | Any simple statement may optionally be followed by a I<SINGLE> modifier, |
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84 | just before the terminating semicolon (or block ending). The possible |
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85 | modifiers are: |
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86 | |
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87 | if EXPR |
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88 | unless EXPR |
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89 | while EXPR |
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90 | until EXPR |
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91 | foreach EXPR |
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92 | |
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93 | The C<if> and C<unless> modifiers have the expected semantics, |
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94 | presuming you're a speaker of English. The C<foreach> modifier is an |
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95 | iterator: For each value in EXPR, it aliases C<$_> to the value and |
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96 | executes the statement. The C<while> and C<until> modifiers have the |
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97 | usual "C<while> loop" semantics (conditional evaluated first), except |
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98 | when applied to a C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE |
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99 | statement), in which case the block executes once before the |
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100 | conditional is evaluated. This is so that you can write loops like: |
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101 | |
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102 | do { |
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103 | $line = <STDIN>; |
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104 | ... |
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105 | } until $line eq ".\n"; |
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106 | |
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107 | See L<perlfunc/do>. Note also that the loop control statements described |
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108 | later will I<NOT> work in this construct, because modifiers don't take |
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109 | loop labels. Sorry. You can always put another block inside of it |
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110 | (for C<next>) or around it (for C<last>) to do that sort of thing. |
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111 | For C<next>, just double the braces: |
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112 | |
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113 | do {{ |
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114 | next if $x == $y; |
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115 | # do something here |
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116 | }} until $x++ > $z; |
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117 | |
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118 | For C<last>, you have to be more elaborate: |
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119 | |
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120 | LOOP: { |
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121 | do { |
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122 | last if $x = $y**2; |
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123 | # do something here |
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124 | } while $x++ <= $z; |
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125 | } |
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126 | |
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127 | =head2 Compound statements |
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128 | |
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129 | In Perl, a sequence of statements that defines a scope is called a block. |
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130 | Sometimes a block is delimited by the file containing it (in the case |
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131 | of a required file, or the program as a whole), and sometimes a block |
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132 | is delimited by the extent of a string (in the case of an eval). |
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133 | |
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134 | But generally, a block is delimited by curly brackets, also known as braces. |
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135 | We will call this syntactic construct a BLOCK. |
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136 | |
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137 | The following compound statements may be used to control flow: |
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138 | |
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139 | if (EXPR) BLOCK |
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140 | if (EXPR) BLOCK else BLOCK |
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141 | if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK |
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142 | LABEL while (EXPR) BLOCK |
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143 | LABEL while (EXPR) BLOCK continue BLOCK |
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144 | LABEL for (EXPR; EXPR; EXPR) BLOCK |
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145 | LABEL foreach VAR (LIST) BLOCK |
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146 | LABEL foreach VAR (LIST) BLOCK continue BLOCK |
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147 | LABEL BLOCK continue BLOCK |
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148 | |
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149 | Note that, unlike C and Pascal, these are defined in terms of BLOCKs, |
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150 | not statements. This means that the curly brackets are I<required>--no |
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151 | dangling statements allowed. If you want to write conditionals without |
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152 | curly brackets there are several other ways to do it. The following |
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153 | all do the same thing: |
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154 | |
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155 | if (!open(FOO)) { die "Can't open $FOO: $!"; } |
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156 | die "Can't open $FOO: $!" unless open(FOO); |
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157 | open(FOO) or die "Can't open $FOO: $!"; # FOO or bust! |
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158 | open(FOO) ? 'hi mom' : die "Can't open $FOO: $!"; |
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159 | # a bit exotic, that last one |
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160 | |
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161 | The C<if> statement is straightforward. Because BLOCKs are always |
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162 | bounded by curly brackets, there is never any ambiguity about which |
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163 | C<if> an C<else> goes with. If you use C<unless> in place of C<if>, |
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164 | the sense of the test is reversed. |
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165 | |
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166 | The C<while> statement executes the block as long as the expression is |
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167 | true (does not evaluate to the null string C<""> or C<0> or C<"0">). |
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168 | The LABEL is optional, and if present, consists of an identifier followed |
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169 | by a colon. The LABEL identifies the loop for the loop control |
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170 | statements C<next>, C<last>, and C<redo>. |
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171 | If the LABEL is omitted, the loop control statement |
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172 | refers to the innermost enclosing loop. This may include dynamically |
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173 | looking back your call-stack at run time to find the LABEL. Such |
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174 | desperate behavior triggers a warning if you use the C<use warnings> |
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175 | praga or the B<-w> flag. |
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176 | Unlike a C<foreach> statement, a C<while> statement never implicitly |
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177 | localises any variables. |
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178 | |
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179 | If there is a C<continue> BLOCK, it is always executed just before the |
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180 | conditional is about to be evaluated again, just like the third part of a |
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181 | C<for> loop in C. Thus it can be used to increment a loop variable, even |
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182 | when the loop has been continued via the C<next> statement (which is |
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183 | similar to the C C<continue> statement). |
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184 | |
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185 | =head2 Loop Control |
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186 | |
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187 | The C<next> command is like the C<continue> statement in C; it starts |
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188 | the next iteration of the loop: |
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189 | |
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190 | LINE: while (<STDIN>) { |
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191 | next LINE if /^#/; # discard comments |
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192 | ... |
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193 | } |
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194 | |
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195 | The C<last> command is like the C<break> statement in C (as used in |
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196 | loops); it immediately exits the loop in question. The |
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197 | C<continue> block, if any, is not executed: |
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198 | |
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199 | LINE: while (<STDIN>) { |
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200 | last LINE if /^$/; # exit when done with header |
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201 | ... |
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202 | } |
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203 | |
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204 | The C<redo> command restarts the loop block without evaluating the |
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205 | conditional again. The C<continue> block, if any, is I<not> executed. |
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206 | This command is normally used by programs that want to lie to themselves |
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207 | about what was just input. |
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208 | |
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209 | For example, when processing a file like F</etc/termcap>. |
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210 | If your input lines might end in backslashes to indicate continuation, you |
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211 | want to skip ahead and get the next record. |
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212 | |
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213 | while (<>) { |
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214 | chomp; |
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215 | if (s/\\$//) { |
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216 | $_ .= <>; |
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217 | redo unless eof(); |
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218 | } |
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219 | # now process $_ |
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220 | } |
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221 | |
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222 | which is Perl short-hand for the more explicitly written version: |
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223 | |
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224 | LINE: while (defined($line = <ARGV>)) { |
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225 | chomp($line); |
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226 | if ($line =~ s/\\$//) { |
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227 | $line .= <ARGV>; |
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228 | redo LINE unless eof(); # not eof(ARGV)! |
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229 | } |
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230 | # now process $line |
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231 | } |
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232 | |
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233 | Note that if there were a C<continue> block on the above code, it would get |
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234 | executed even on discarded lines. This is often used to reset line counters |
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235 | or C<?pat?> one-time matches. |
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236 | |
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237 | # inspired by :1,$g/fred/s//WILMA/ |
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238 | while (<>) { |
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239 | ?(fred)? && s//WILMA $1 WILMA/; |
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240 | ?(barney)? && s//BETTY $1 BETTY/; |
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241 | ?(homer)? && s//MARGE $1 MARGE/; |
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242 | } continue { |
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243 | print "$ARGV $.: $_"; |
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244 | close ARGV if eof(); # reset $. |
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245 | reset if eof(); # reset ?pat? |
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246 | } |
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247 | |
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248 | If the word C<while> is replaced by the word C<until>, the sense of the |
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249 | test is reversed, but the conditional is still tested before the first |
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250 | iteration. |
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251 | |
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252 | The loop control statements don't work in an C<if> or C<unless>, since |
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253 | they aren't loops. You can double the braces to make them such, though. |
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254 | |
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255 | if (/pattern/) {{ |
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256 | next if /fred/; |
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257 | next if /barney/; |
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258 | # so something here |
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259 | }} |
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260 | |
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261 | The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer |
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262 | available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>. |
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263 | |
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264 | =head2 For Loops |
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265 | |
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266 | Perl's C-style C<for> loop works exactly like the corresponding C<while> loop; |
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267 | that means that this: |
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268 | |
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269 | for ($i = 1; $i < 10; $i++) { |
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270 | ... |
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271 | } |
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272 | |
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273 | is the same as this: |
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274 | |
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275 | $i = 1; |
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276 | while ($i < 10) { |
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277 | ... |
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278 | } continue { |
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279 | $i++; |
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280 | } |
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281 | |
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282 | (There is one minor difference: The first form implies a lexical scope |
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283 | for variables declared with C<my> in the initialization expression.) |
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284 | |
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285 | Besides the normal array index looping, C<for> can lend itself |
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286 | to many other interesting applications. Here's one that avoids the |
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287 | problem you get into if you explicitly test for end-of-file on |
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288 | an interactive file descriptor causing your program to appear to |
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289 | hang. |
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290 | |
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291 | $on_a_tty = -t STDIN && -t STDOUT; |
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292 | sub prompt { print "yes? " if $on_a_tty } |
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293 | for ( prompt(); <STDIN>; prompt() ) { |
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294 | # do something |
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295 | } |
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296 | |
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297 | =head2 Foreach Loops |
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298 | |
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299 | The C<foreach> loop iterates over a normal list value and sets the |
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300 | variable VAR to be each element of the list in turn. If the variable |
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301 | is preceded with the keyword C<my>, then it is lexically scoped, and |
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302 | is therefore visible only within the loop. Otherwise, the variable is |
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303 | implicitly local to the loop and regains its former value upon exiting |
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304 | the loop. If the variable was previously declared with C<my>, it uses |
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305 | that variable instead of the global one, but it's still localized to |
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306 | the loop. |
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307 | |
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308 | The C<foreach> keyword is actually a synonym for the C<for> keyword, so |
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309 | you can use C<foreach> for readability or C<for> for brevity. (Or because |
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310 | the Bourne shell is more familiar to you than I<csh>, so writing C<for> |
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311 | comes more naturally.) If VAR is omitted, C<$_> is set to each value. |
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312 | If any element of LIST is an lvalue, you can modify it by modifying VAR |
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313 | inside the loop. That's because the C<foreach> loop index variable is |
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314 | an implicit alias for each item in the list that you're looping over. |
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315 | |
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316 | If any part of LIST is an array, C<foreach> will get very confused if |
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317 | you add or remove elements within the loop body, for example with |
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318 | C<splice>. So don't do that. |
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319 | |
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320 | C<foreach> probably won't do what you expect if VAR is a tied or other |
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321 | special variable. Don't do that either. |
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322 | |
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323 | Examples: |
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324 | |
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325 | for (@ary) { s/foo/bar/ } |
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326 | |
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327 | for my $elem (@elements) { |
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328 | $elem *= 2; |
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329 | } |
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330 | |
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331 | for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') { |
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332 | print $count, "\n"; sleep(1); |
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333 | } |
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334 | |
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335 | for (1..15) { print "Merry Christmas\n"; } |
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336 | |
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337 | foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) { |
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338 | print "Item: $item\n"; |
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339 | } |
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340 | |
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341 | Here's how a C programmer might code up a particular algorithm in Perl: |
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342 | |
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343 | for (my $i = 0; $i < @ary1; $i++) { |
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344 | for (my $j = 0; $j < @ary2; $j++) { |
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345 | if ($ary1[$i] > $ary2[$j]) { |
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346 | last; # can't go to outer :-( |
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347 | } |
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348 | $ary1[$i] += $ary2[$j]; |
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349 | } |
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350 | # this is where that last takes me |
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351 | } |
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352 | |
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353 | Whereas here's how a Perl programmer more comfortable with the idiom might |
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354 | do it: |
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355 | |
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356 | OUTER: for my $wid (@ary1) { |
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357 | INNER: for my $jet (@ary2) { |
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358 | next OUTER if $wid > $jet; |
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359 | $wid += $jet; |
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360 | } |
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361 | } |
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362 | |
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363 | See how much easier this is? It's cleaner, safer, and faster. It's |
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364 | cleaner because it's less noisy. It's safer because if code gets added |
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365 | between the inner and outer loops later on, the new code won't be |
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366 | accidentally executed. The C<next> explicitly iterates the other loop |
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367 | rather than merely terminating the inner one. And it's faster because |
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368 | Perl executes a C<foreach> statement more rapidly than it would the |
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369 | equivalent C<for> loop. |
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370 | |
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371 | =head2 Basic BLOCKs and Switch Statements |
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372 | |
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373 | A BLOCK by itself (labeled or not) is semantically equivalent to a |
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374 | loop that executes once. Thus you can use any of the loop control |
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375 | statements in it to leave or restart the block. (Note that this is |
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376 | I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief |
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377 | C<do{}> blocks, which do I<NOT> count as loops.) The C<continue> |
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378 | block is optional. |
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379 | |
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380 | The BLOCK construct is particularly nice for doing case |
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381 | structures. |
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382 | |
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383 | SWITCH: { |
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384 | if (/^abc/) { $abc = 1; last SWITCH; } |
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385 | if (/^def/) { $def = 1; last SWITCH; } |
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386 | if (/^xyz/) { $xyz = 1; last SWITCH; } |
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387 | $nothing = 1; |
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388 | } |
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389 | |
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390 | There is no official C<switch> statement in Perl, because there are |
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391 | already several ways to write the equivalent. In addition to the |
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392 | above, you could write |
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393 | |
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394 | SWITCH: { |
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395 | $abc = 1, last SWITCH if /^abc/; |
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396 | $def = 1, last SWITCH if /^def/; |
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397 | $xyz = 1, last SWITCH if /^xyz/; |
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398 | $nothing = 1; |
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399 | } |
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400 | |
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401 | (That's actually not as strange as it looks once you realize that you can |
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402 | use loop control "operators" within an expression, That's just the normal |
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403 | C comma operator.) |
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404 | |
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405 | or |
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406 | |
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407 | SWITCH: { |
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408 | /^abc/ && do { $abc = 1; last SWITCH; }; |
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409 | /^def/ && do { $def = 1; last SWITCH; }; |
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410 | /^xyz/ && do { $xyz = 1; last SWITCH; }; |
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411 | $nothing = 1; |
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412 | } |
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413 | |
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414 | or formatted so it stands out more as a "proper" C<switch> statement: |
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415 | |
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416 | SWITCH: { |
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417 | /^abc/ && do { |
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418 | $abc = 1; |
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419 | last SWITCH; |
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420 | }; |
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421 | |
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422 | /^def/ && do { |
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423 | $def = 1; |
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424 | last SWITCH; |
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425 | }; |
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426 | |
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427 | /^xyz/ && do { |
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428 | $xyz = 1; |
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429 | last SWITCH; |
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430 | }; |
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431 | $nothing = 1; |
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432 | } |
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433 | |
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434 | or |
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435 | |
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436 | SWITCH: { |
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437 | /^abc/ and $abc = 1, last SWITCH; |
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438 | /^def/ and $def = 1, last SWITCH; |
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439 | /^xyz/ and $xyz = 1, last SWITCH; |
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440 | $nothing = 1; |
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441 | } |
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442 | |
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443 | or even, horrors, |
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444 | |
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445 | if (/^abc/) |
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446 | { $abc = 1 } |
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447 | elsif (/^def/) |
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448 | { $def = 1 } |
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449 | elsif (/^xyz/) |
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450 | { $xyz = 1 } |
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451 | else |
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452 | { $nothing = 1 } |
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453 | |
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454 | A common idiom for a C<switch> statement is to use C<foreach>'s aliasing to make |
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455 | a temporary assignment to C<$_> for convenient matching: |
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456 | |
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457 | SWITCH: for ($where) { |
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458 | /In Card Names/ && do { push @flags, '-e'; last; }; |
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459 | /Anywhere/ && do { push @flags, '-h'; last; }; |
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460 | /In Rulings/ && do { last; }; |
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461 | die "unknown value for form variable where: `$where'"; |
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462 | } |
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463 | |
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464 | Another interesting approach to a switch statement is arrange |
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465 | for a C<do> block to return the proper value: |
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466 | |
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467 | $amode = do { |
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468 | if ($flag & O_RDONLY) { "r" } # XXX: isn't this 0? |
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469 | elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" } |
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470 | elsif ($flag & O_RDWR) { |
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471 | if ($flag & O_CREAT) { "w+" } |
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472 | else { ($flag & O_APPEND) ? "a+" : "r+" } |
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473 | } |
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474 | }; |
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475 | |
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476 | Or |
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477 | |
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478 | print do { |
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479 | ($flags & O_WRONLY) ? "write-only" : |
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480 | ($flags & O_RDWR) ? "read-write" : |
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481 | "read-only"; |
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482 | }; |
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483 | |
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484 | Or if you are certainly that all the C<&&> clauses are true, you can use |
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485 | something like this, which "switches" on the value of the |
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486 | C<HTTP_USER_AGENT> envariable. |
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487 | |
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488 | #!/usr/bin/perl |
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489 | # pick out jargon file page based on browser |
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490 | $dir = 'http://www.wins.uva.nl/~mes/jargon'; |
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491 | for ($ENV{HTTP_USER_AGENT}) { |
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492 | $page = /Mac/ && 'm/Macintrash.html' |
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493 | || /Win(dows )?NT/ && 'e/evilandrude.html' |
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494 | || /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html' |
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495 | || /Linux/ && 'l/Linux.html' |
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496 | || /HP-UX/ && 'h/HP-SUX.html' |
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497 | || /SunOS/ && 's/ScumOS.html' |
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498 | || 'a/AppendixB.html'; |
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499 | } |
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500 | print "Location: $dir/$page\015\012\015\012"; |
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501 | |
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502 | That kind of switch statement only works when you know the C<&&> clauses |
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503 | will be true. If you don't, the previous C<?:> example should be used. |
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504 | |
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505 | You might also consider writing a hash of subroutine references |
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506 | instead of synthesizing a C<switch> statement. |
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507 | |
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508 | =head2 Goto |
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509 | |
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510 | Although not for the faint of heart, Perl does support a C<goto> |
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511 | statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and |
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512 | C<goto>-&NAME. A loop's LABEL is not actually a valid target for |
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513 | a C<goto>; it's just the name of the loop. |
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514 | |
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515 | The C<goto>-LABEL form finds the statement labeled with LABEL and resumes |
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516 | execution there. It may not be used to go into any construct that |
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517 | requires initialization, such as a subroutine or a C<foreach> loop. It |
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518 | also can't be used to go into a construct that is optimized away. It |
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519 | can be used to go almost anywhere else within the dynamic scope, |
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520 | including out of subroutines, but it's usually better to use some other |
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521 | construct such as C<last> or C<die>. The author of Perl has never felt the |
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522 | need to use this form of C<goto> (in Perl, that is--C is another matter). |
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523 | |
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524 | The C<goto>-EXPR form expects a label name, whose scope will be resolved |
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525 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't |
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526 | necessarily recommended if you're optimizing for maintainability: |
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527 | |
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528 | goto(("FOO", "BAR", "GLARCH")[$i]); |
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529 | |
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530 | The C<goto>-&NAME form is highly magical, and substitutes a call to the |
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531 | named subroutine for the currently running subroutine. This is used by |
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532 | C<AUTOLOAD()> subroutines that wish to load another subroutine and then |
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533 | pretend that the other subroutine had been called in the first place |
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534 | (except that any modifications to C<@_> in the current subroutine are |
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535 | propagated to the other subroutine.) After the C<goto>, not even C<caller()> |
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536 | will be able to tell that this routine was called first. |
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537 | |
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538 | In almost all cases like this, it's usually a far, far better idea to use the |
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539 | structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of |
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540 | resorting to a C<goto>. For certain applications, the catch and throw pair of |
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541 | C<eval{}> and die() for exception processing can also be a prudent approach. |
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542 | |
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543 | =head2 PODs: Embedded Documentation |
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544 | |
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545 | Perl has a mechanism for intermixing documentation with source code. |
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546 | While it's expecting the beginning of a new statement, if the compiler |
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547 | encounters a line that begins with an equal sign and a word, like this |
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548 | |
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549 | =head1 Here There Be Pods! |
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550 | |
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551 | Then that text and all remaining text up through and including a line |
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552 | beginning with C<=cut> will be ignored. The format of the intervening |
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553 | text is described in L<perlpod>. |
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554 | |
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555 | This allows you to intermix your source code |
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556 | and your documentation text freely, as in |
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557 | |
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558 | =item snazzle($) |
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559 | |
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560 | The snazzle() function will behave in the most spectacular |
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561 | form that you can possibly imagine, not even excepting |
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562 | cybernetic pyrotechnics. |
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563 | |
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564 | =cut back to the compiler, nuff of this pod stuff! |
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565 | |
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566 | sub snazzle($) { |
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567 | my $thingie = shift; |
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568 | ......... |
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569 | } |
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570 | |
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571 | Note that pod translators should look at only paragraphs beginning |
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572 | with a pod directive (it makes parsing easier), whereas the compiler |
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573 | actually knows to look for pod escapes even in the middle of a |
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574 | paragraph. This means that the following secret stuff will be |
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575 | ignored by both the compiler and the translators. |
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576 | |
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577 | $a=3; |
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578 | =secret stuff |
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579 | warn "Neither POD nor CODE!?" |
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580 | =cut back |
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581 | print "got $a\n"; |
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582 | |
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583 | You probably shouldn't rely upon the C<warn()> being podded out forever. |
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584 | Not all pod translators are well-behaved in this regard, and perhaps |
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585 | the compiler will become pickier. |
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586 | |
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587 | One may also use pod directives to quickly comment out a section |
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588 | of code. |
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589 | |
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590 | =head2 Plain Old Comments (Not!) |
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591 | |
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592 | Much like the C preprocessor, Perl can process line directives. Using |
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593 | this, one can control Perl's idea of filenames and line numbers in |
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594 | error or warning messages (especially for strings that are processed |
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595 | with C<eval()>). The syntax for this mechanism is the same as for most |
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596 | C preprocessors: it matches the regular expression |
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597 | C</^#\s*line\s+(\d+)\s*(?:\s"([^"]+)")?\s*$/> with C<$1> being the line |
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598 | number for the next line, and C<$2> being the optional filename |
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599 | (specified within quotes). |
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600 | |
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601 | Here are some examples that you should be able to type into your command |
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602 | shell: |
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603 | |
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604 | % perl |
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605 | # line 200 "bzzzt" |
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606 | # the `#' on the previous line must be the first char on line |
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607 | die 'foo'; |
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608 | __END__ |
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609 | foo at bzzzt line 201. |
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610 | |
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611 | % perl |
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612 | # line 200 "bzzzt" |
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613 | eval qq[\n#line 2001 ""\ndie 'foo']; print $@; |
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614 | __END__ |
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615 | foo at - line 2001. |
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616 | |
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617 | % perl |
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618 | eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@; |
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619 | __END__ |
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620 | foo at foo bar line 200. |
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621 | |
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622 | % perl |
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623 | # line 345 "goop" |
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624 | eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'"; |
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625 | print $@; |
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626 | __END__ |
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627 | foo at goop line 345. |
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628 | |
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629 | =cut |
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