4.14. Finding All Elements in an Array Matching Certain Criteria4.14.1. ProblemFrom a list, you want only the elements that match certain criteria. This notion of extracting a subset of a larger list is common. It's how you find all engineers in a list of employees, all users in the "staff" group, or all the filenames you're interested in. 4.14.2. SolutionUse grep to apply a condition to all elements in the list and return only those for which the condition was true: @MATCHING = grep { TEST ($_) } @LIST; 4.14.3. DiscussionThis could also be accomplished with a foreach loop: @matching = ( ); foreach (@list) { push(@matching, $_) if TEST ($_); } The Perl grep function is shorthand for all that looping and mucking about. It's not really like the Unix grep command; it doesn't have options to return line numbers or to negate the test, and it isn't limited to regular-expression tests. For example, to filter out just the large numbers from an array or to find out which keys in a hash have very large values: @bigs = grep { $_ > 1_000_000 } @nums; @pigs = grep { $users{$_} > 1e7 } keys %users; Here's something that sets @matching to lines from the who command that start with "gnat ": @matching = grep { /^gnat / } `who`; Here's another example: @engineers = grep { $_->position( ) eq "Engineer" } @employees; It extracts only those objects from the array @employees whose position method returns the string Engineer. You could have even more complex tests in a grep: @secondary_assistance = grep { $_->income >= 26_000 && $_->income < 30_000 } @applicants; But at that point you may decide it would be more legible to write a proper loop instead. 4.14.4. See AlsoThe "For Loops," "Foreach Loops," and "Loop Control" sections of perlsyn(1) and Chapter 4 of Programming Perl; the grep function in perlfunc(1) and Chapter 29 of Programming Perl; your system's who(1) manpage, if it exists; Recipe 4.13 Copyright © 2003 O'Reilly & Associates. All rights reserved. |
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