1 | =head1 NAME |
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2 | |
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3 | perlform - Perl formats |
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4 | |
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5 | =head1 DESCRIPTION |
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6 | |
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7 | Perl has a mechanism to help you generate simple reports and charts. To |
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8 | facilitate this, Perl helps you code up your output page close to how it |
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9 | will look when it's printed. It can keep track of things like how many |
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10 | lines are on a page, what page you're on, when to print page headers, |
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11 | etc. Keywords are borrowed from FORTRAN: format() to declare and write() |
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12 | to execute; see their entries in L<perlfunc>. Fortunately, the layout is |
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13 | much more legible, more like BASIC's PRINT USING statement. Think of it |
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14 | as a poor man's nroff(1). |
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15 | |
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16 | Formats, like packages and subroutines, are declared rather than |
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17 | executed, so they may occur at any point in your program. (Usually it's |
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18 | best to keep them all together though.) They have their own namespace |
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19 | apart from all the other "types" in Perl. This means that if you have a |
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20 | function named "Foo", it is not the same thing as having a format named |
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21 | "Foo". However, the default name for the format associated with a given |
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22 | filehandle is the same as the name of the filehandle. Thus, the default |
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23 | format for STDOUT is named "STDOUT", and the default format for filehandle |
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24 | TEMP is named "TEMP". They just look the same. They aren't. |
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25 | |
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26 | Output record formats are declared as follows: |
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27 | |
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28 | format NAME = |
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29 | FORMLIST |
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30 | . |
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31 | |
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32 | If name is omitted, format "STDOUT" is defined. FORMLIST consists of |
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33 | a sequence of lines, each of which may be one of three types: |
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34 | |
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35 | =over 4 |
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36 | |
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37 | =item 1. |
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38 | |
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39 | A comment, indicated by putting a '#' in the first column. |
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40 | |
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41 | =item 2. |
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42 | |
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43 | A "picture" line giving the format for one output line. |
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44 | |
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45 | =item 3. |
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46 | |
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47 | An argument line supplying values to plug into the previous picture line. |
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48 | |
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49 | =back |
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50 | |
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51 | Picture lines are printed exactly as they look, except for certain fields |
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52 | that substitute values into the line. Each field in a picture line starts |
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53 | with either "@" (at) or "^" (caret). These lines do not undergo any kind |
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54 | of variable interpolation. The at field (not to be confused with the array |
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55 | marker @) is the normal kind of field; the other kind, caret fields, are used |
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56 | to do rudimentary multi-line text block filling. The length of the field |
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57 | is supplied by padding out the field with multiple "E<lt>", "E<gt>", or "|" |
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58 | characters to specify, respectively, left justification, right |
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59 | justification, or centering. If the variable would exceed the width |
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60 | specified, it is truncated. |
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61 | |
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62 | As an alternate form of right justification, you may also use "#" |
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63 | characters (with an optional ".") to specify a numeric field. This way |
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64 | you can line up the decimal points. If any value supplied for these |
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65 | fields contains a newline, only the text up to the newline is printed. |
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66 | Finally, the special field "@*" can be used for printing multi-line, |
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67 | nontruncated values; it should appear by itself on a line. |
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68 | |
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69 | The values are specified on the following line in the same order as |
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70 | the picture fields. The expressions providing the values should be |
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71 | separated by commas. The expressions are all evaluated in a list context |
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72 | before the line is processed, so a single list expression could produce |
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73 | multiple list elements. The expressions may be spread out to more than |
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74 | one line if enclosed in braces. If so, the opening brace must be the first |
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75 | token on the first line. If an expression evaluates to a number with a |
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76 | decimal part, and if the corresponding picture specifies that the decimal |
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77 | part should appear in the output (that is, any picture except multiple "#" |
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78 | characters B<without> an embedded "."), the character used for the decimal |
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79 | point is B<always> determined by the current LC_NUMERIC locale. This |
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80 | means that, if, for example, the run-time environment happens to specify a |
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81 | German locale, "," will be used instead of the default ".". See |
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82 | L<perllocale> and L<"WARNINGS"> for more information. |
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83 | |
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84 | Picture fields that begin with ^ rather than @ are treated specially. |
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85 | With a # field, the field is blanked out if the value is undefined. For |
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86 | other field types, the caret enables a kind of fill mode. Instead of an |
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87 | arbitrary expression, the value supplied must be a scalar variable name |
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88 | that contains a text string. Perl puts as much text as it can into the |
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89 | field, and then chops off the front of the string so that the next time |
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90 | the variable is referenced, more of the text can be printed. (Yes, this |
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91 | means that the variable itself is altered during execution of the write() |
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92 | call, and is not returned.) Normally you would use a sequence of fields |
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93 | in a vertical stack to print out a block of text. You might wish to end |
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94 | the final field with the text "...", which will appear in the output if |
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95 | the text was too long to appear in its entirety. You can change which |
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96 | characters are legal to break on by changing the variable C<$:> (that's |
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97 | $FORMAT_LINE_BREAK_CHARACTERS if you're using the English module) to a |
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98 | list of the desired characters. |
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99 | |
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100 | Using caret fields can produce variable length records. If the text |
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101 | to be formatted is short, you can suppress blank lines by putting a |
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102 | "~" (tilde) character anywhere in the line. The tilde will be translated |
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103 | to a space upon output. If you put a second tilde contiguous to the |
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104 | first, the line will be repeated until all the fields on the line are |
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105 | exhausted. (If you use a field of the at variety, the expression you |
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106 | supply had better not give the same value every time forever!) |
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107 | |
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108 | Top-of-form processing is by default handled by a format with the |
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109 | same name as the current filehandle with "_TOP" concatenated to it. |
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110 | It's triggered at the top of each page. See L<perlfunc/write>. |
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111 | |
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112 | Examples: |
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113 | |
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114 | # a report on the /etc/passwd file |
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115 | format STDOUT_TOP = |
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116 | Passwd File |
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117 | Name Login Office Uid Gid Home |
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118 | ------------------------------------------------------------------ |
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119 | . |
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120 | format STDOUT = |
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121 | @<<<<<<<<<<<<<<<<<< @||||||| @<<<<<<@>>>> @>>>> @<<<<<<<<<<<<<<<<< |
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122 | $name, $login, $office,$uid,$gid, $home |
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123 | . |
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124 | |
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125 | |
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126 | # a report from a bug report form |
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127 | format STDOUT_TOP = |
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128 | Bug Reports |
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129 | @<<<<<<<<<<<<<<<<<<<<<<< @||| @>>>>>>>>>>>>>>>>>>>>>>> |
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130 | $system, $%, $date |
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131 | ------------------------------------------------------------------ |
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132 | . |
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133 | format STDOUT = |
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134 | Subject: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< |
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135 | $subject |
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136 | Index: @<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< |
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137 | $index, $description |
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138 | Priority: @<<<<<<<<<< Date: @<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< |
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139 | $priority, $date, $description |
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140 | From: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< |
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141 | $from, $description |
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142 | Assigned to: @<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< |
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143 | $programmer, $description |
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144 | ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< |
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145 | $description |
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146 | ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< |
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147 | $description |
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148 | ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< |
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149 | $description |
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150 | ~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<< |
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151 | $description |
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152 | ~ ^<<<<<<<<<<<<<<<<<<<<<<<... |
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153 | $description |
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154 | . |
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155 | |
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156 | It is possible to intermix print()s with write()s on the same output |
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157 | channel, but you'll have to handle C<$-> (C<$FORMAT_LINES_LEFT>) |
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158 | yourself. |
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159 | |
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160 | =head2 Format Variables |
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161 | |
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162 | The current format name is stored in the variable C<$~> (C<$FORMAT_NAME>), |
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163 | and the current top of form format name is in C<$^> (C<$FORMAT_TOP_NAME>). |
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164 | The current output page number is stored in C<$%> (C<$FORMAT_PAGE_NUMBER>), |
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165 | and the number of lines on the page is in C<$=> (C<$FORMAT_LINES_PER_PAGE>). |
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166 | Whether to autoflush output on this handle is stored in C<$|> |
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167 | (C<$OUTPUT_AUTOFLUSH>). The string output before each top of page (except |
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168 | the first) is stored in C<$^L> (C<$FORMAT_FORMFEED>). These variables are |
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169 | set on a per-filehandle basis, so you'll need to select() into a different |
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170 | one to affect them: |
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171 | |
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172 | select((select(OUTF), |
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173 | $~ = "My_Other_Format", |
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174 | $^ = "My_Top_Format" |
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175 | )[0]); |
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176 | |
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177 | Pretty ugly, eh? It's a common idiom though, so don't be too surprised |
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178 | when you see it. You can at least use a temporary variable to hold |
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179 | the previous filehandle: (this is a much better approach in general, |
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180 | because not only does legibility improve, you now have intermediary |
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181 | stage in the expression to single-step the debugger through): |
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182 | |
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183 | $ofh = select(OUTF); |
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184 | $~ = "My_Other_Format"; |
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185 | $^ = "My_Top_Format"; |
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186 | select($ofh); |
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187 | |
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188 | If you use the English module, you can even read the variable names: |
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189 | |
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190 | use English; |
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191 | $ofh = select(OUTF); |
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192 | $FORMAT_NAME = "My_Other_Format"; |
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193 | $FORMAT_TOP_NAME = "My_Top_Format"; |
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194 | select($ofh); |
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195 | |
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196 | But you still have those funny select()s. So just use the FileHandle |
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197 | module. Now, you can access these special variables using lowercase |
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198 | method names instead: |
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199 | |
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200 | use FileHandle; |
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201 | format_name OUTF "My_Other_Format"; |
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202 | format_top_name OUTF "My_Top_Format"; |
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203 | |
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204 | Much better! |
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205 | |
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206 | =head1 NOTES |
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207 | |
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208 | Because the values line may contain arbitrary expressions (for at fields, |
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209 | not caret fields), you can farm out more sophisticated processing |
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210 | to other functions, like sprintf() or one of your own. For example: |
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211 | |
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212 | format Ident = |
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213 | @<<<<<<<<<<<<<<< |
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214 | &commify($n) |
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215 | . |
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216 | |
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217 | To get a real at or caret into the field, do this: |
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218 | |
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219 | format Ident = |
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220 | I have an @ here. |
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221 | "@" |
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222 | . |
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223 | |
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224 | To center a whole line of text, do something like this: |
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225 | |
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226 | format Ident = |
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227 | @||||||||||||||||||||||||||||||||||||||||||||||| |
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228 | "Some text line" |
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229 | . |
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230 | |
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231 | There is no builtin way to say "float this to the right hand side |
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232 | of the page, however wide it is." You have to specify where it goes. |
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233 | The truly desperate can generate their own format on the fly, based |
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234 | on the current number of columns, and then eval() it: |
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235 | |
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236 | $format = "format STDOUT = \n" |
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237 | . '^' . '<' x $cols . "\n" |
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238 | . '$entry' . "\n" |
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239 | . "\t^" . "<" x ($cols-8) . "~~\n" |
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240 | . '$entry' . "\n" |
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241 | . ".\n"; |
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242 | print $format if $Debugging; |
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243 | eval $format; |
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244 | die $@ if $@; |
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245 | |
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246 | Which would generate a format looking something like this: |
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247 | |
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248 | format STDOUT = |
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249 | ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< |
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250 | $entry |
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251 | ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<~~ |
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252 | $entry |
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253 | . |
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254 | |
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255 | Here's a little program that's somewhat like fmt(1): |
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256 | |
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257 | format = |
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258 | ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ~~ |
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259 | $_ |
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260 | |
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261 | . |
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262 | |
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263 | $/ = ''; |
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264 | while (<>) { |
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265 | s/\s*\n\s*/ /g; |
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266 | write; |
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267 | } |
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268 | |
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269 | =head2 Footers |
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270 | |
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271 | While $FORMAT_TOP_NAME contains the name of the current header format, |
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272 | there is no corresponding mechanism to automatically do the same thing |
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273 | for a footer. Not knowing how big a format is going to be until you |
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274 | evaluate it is one of the major problems. It's on the TODO list. |
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275 | |
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276 | Here's one strategy: If you have a fixed-size footer, you can get footers |
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277 | by checking $FORMAT_LINES_LEFT before each write() and print the footer |
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278 | yourself if necessary. |
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279 | |
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280 | Here's another strategy: Open a pipe to yourself, using C<open(MYSELF, "|-")> |
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281 | (see L<perlfunc/open()>) and always write() to MYSELF instead of STDOUT. |
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282 | Have your child process massage its STDIN to rearrange headers and footers |
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283 | however you like. Not very convenient, but doable. |
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284 | |
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285 | =head2 Accessing Formatting Internals |
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286 | |
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287 | For low-level access to the formatting mechanism. you may use formline() |
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288 | and access C<$^A> (the $ACCUMULATOR variable) directly. |
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289 | |
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290 | For example: |
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291 | |
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292 | $str = formline <<'END', 1,2,3; |
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293 | @<<< @||| @>>> |
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294 | END |
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295 | |
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296 | print "Wow, I just stored `$^A' in the accumulator!\n"; |
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297 | |
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298 | Or to make an swrite() subroutine, which is to write() what sprintf() |
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299 | is to printf(), do this: |
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300 | |
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301 | use Carp; |
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302 | sub swrite { |
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303 | croak "usage: swrite PICTURE ARGS" unless @_; |
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304 | my $format = shift; |
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305 | $^A = ""; |
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306 | formline($format,@_); |
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307 | return $^A; |
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308 | } |
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309 | |
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310 | $string = swrite(<<'END', 1, 2, 3); |
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311 | Check me out |
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312 | @<<< @||| @>>> |
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313 | END |
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314 | print $string; |
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315 | |
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316 | =head1 WARNINGS |
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317 | |
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318 | The lone dot that ends a format can also prematurely end a mail |
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319 | message passing through a misconfigured Internet mailer (and based on |
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320 | experience, such misconfiguration is the rule, not the exception). So |
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321 | when sending format code through mail, you should indent it so that |
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322 | the format-ending dot is not on the left margin; this will prevent |
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323 | SMTP cutoff. |
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324 | |
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325 | Lexical variables (declared with "my") are not visible within a |
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326 | format unless the format is declared within the scope of the lexical |
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327 | variable. (They weren't visible at all before version 5.001.) |
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328 | |
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329 | Formats are the only part of Perl that unconditionally use information |
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330 | from a program's locale; if a program's environment specifies an |
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331 | LC_NUMERIC locale, it is always used to specify the decimal point |
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332 | character in formatted output. Perl ignores all other aspects of locale |
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333 | handling unless the C<use locale> pragma is in effect. Formatted output |
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334 | cannot be controlled by C<use locale> because the pragma is tied to the |
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335 | block structure of the program, and, for historical reasons, formats |
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336 | exist outside that block structure. See L<perllocale> for further |
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337 | discussion of locale handling. |
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338 | |
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339 | Inside of an expression, the whitespace characters \n, \t and \f are |
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340 | considered to be equivalent to a single space. Thus, you could think |
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341 | of this filter being applied to each value in the format: |
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342 | |
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343 | $value =~ tr/\n\t\f/ /; |
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344 | |
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345 | The remaining whitespace character, \r, forces the printing of a new |
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346 | line if allowed by the picture line. |
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