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This article will explain in detail how perl determines whether there is an element in the array. The editor thinks it is very practical, so I share it with you for reference. I hope you can get something after reading this article.

The grep function can solve this problem. Here's how to use grep:

The grep function takes two parameters. A code block and a list.

For each element in the list, its value is assigned to $_ (the scalar default of Perl), and the code block is executed. If the return value of the code block is false, the corresponding value is discarded. If the return value of the code block is true, the corresponding value is used as one of the return values.

Note: there is no comma between the code block and the second parameter!

Take a look at a few grep columns:

Filter out the small number my @ numbers = qw (8 2 5 3 1 7); my @ big_numbers = grep {$_ > 4} @ numbers;print "@ big_numbers\ n"; # (8, 5, 7)

Grep returns values greater than 4 and filters out values not greater than 4.

Filter out the new file my @ files = glob "* .log"; my @ old_files = grep {- M $_ > 365} @ files;print join "\ n", @ old_files

Glob "* .log" returns all files with .log extension for the current file.

-M $path_to_file returns the number of days since the file was last modified.

This example filters out files that have been modified within 365 days and gets files that have existed for at least a year.

Does the array contain an element?

Another interesting application of grep is to check whether an element is included in an array. For example, you have a list and want to know if a given name is among them.

Use strict;use warnings;my @ names = qw (Foo Bar Baz); my $visitor =; chomp $visitor;if (grep {$visitor eq $_} @ names) {print "Visitor $visitor is in the guest list\ n";} else {print "Visitor $visitor is NOT in the guest list\ n";}

In this example, the grep function is in a scalar context. In a scalar context, grep returns the number of elements that have been filtered. We check if $visitor is equal to the current element, and grep returns the same number of times.

If the return value is 0, the expression is false, if it is any positive number, the expression is true.

This method can solve the problem, but because it involves context, it may not be very clear to some friends. Let's take a look at another scenario: the any function of the List::MoreUtils module.

Are there any matching elements?

The any function has the same syntax as, accepting a block and a list of values, but it only returns true or false. True, if the block gives true for any of the values. False if none of them match. It also short circuits so on large lists this can be a lot faster.

The syntax of the any function is the same as grep, passing in a code fast and a list, but only returning true or false. If any value causes the code block to return true, the function returns true;. If there is no match, it returns false. There is a short circuit operation during processing, so it is faster for larger lists.

Use List::MoreUtils qw (any); if (any {$visitor eq $_} @ names) {print "Visitor $visitor is in the guest list\ n";} else {print "Visitor $visitor is NOT in the guest list\ n";} grep of UNIX and Linux?

A brief explanation:

As I mentioned earlier, the built-in grep function is a generic implementation of the UNIX grep command.

UNIX grep filters every line of a file based on regular expressions.

Perl grep can filter any list based on any criteria.

The following Perl code is a simple implementation version of UNIX grep:

My $regex = shift;print grep {$_ = ~ / $regex/}

The first line reads a regular expression from the command line, and the other parameters on the command line should be the file name.

The diamond operator extracts each line from all files (command line arguments). Grep filters according to the regular formula. Lines that have been filtered are printed.

Grep on Windows

There is no grep program in Window, but you can install one yourself or use the Perl script above.

This is the end of the article on "how perl determines whether there is an element in an array". I hope the above content can be of some help to you, so that you can learn more knowledge. if you think the article is good, please share it for more people to see.

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