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P5each cpan:ELIZABETH last updated on 2018-05-19


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P5each - Implement Perl 5's each() built-in


use P5each;


This module tries to mimic the behaviour of the each function of Perl 5 as closely as possible.


each HASH
each ARRAY
each EXPR
        When called on a hash in list context, returns a 2-element list
        consisting of the key and value for the next element of a hash. In
        Perl 5.12 and later only, it will also return the index and value
        for the next element of an array so that you can iterate over it;
        older Perls consider this a syntax error. When called in scalar
        context, returns only the key (not the value) in a hash, or the
        index in an array.

        Hash entries are returned in an apparently random order. The
        actual random order is specific to a given hash; the exact same
        series of operations on two hashes may result in a different order
        for each hash. Any insertion into the hash may change the order,
        as will any deletion, with the exception that the most recent key
        returned by "each" or "keys" may be deleted without changing the
        order. So long as a given hash is unmodified you may rely on
        "keys", "values" and "each" to repeatedly return the same order as
        each other. See "Algorithmic Complexity Attacks" in perlsec for
        details on why hash order is randomized. Aside from the guarantees
        provided here the exact details of Perl's hash algorithm and the
        hash traversal order are subject to change in any release of Perl.

        After "each" has returned all entries from the hash or array, the
        next call to "each" returns the empty list in list context and
        "undef" in scalar context; the next call following that one
        restarts iteration. Each hash or array has its own internal
        iterator, accessed by "each", "keys", and "values". The iterator
        is implicitly reset when "each" has reached the end as just
        described; it can be explicitly reset by calling "keys" or
        "values" on the hash or array. If you add or delete a hash's
        elements while iterating over it, the effect on the iterator is
        unspecified; for example, entries may be skipped or duplicated--so
        don't do that. Exception: It is always safe to delete the item
        most recently returned by "each()", so the following code works

                while (($key, $value) = each %hash) {
                  print $key, "\n";
                  delete $hash{$key};   # This is safe

        This prints out your environment like the printenv(1) program, but
        in a different order:

            while (($key,$value) = each %ENV) {
                print "$key=$value\n";

        Starting with Perl 5.14, "each" can take a scalar EXPR, which must
        hold reference to an unblessed hash or array. The argument will be
        dereferenced automatically. This aspect of "each" is considered
        highly experimental. The exact behaviour may change in a future
        version of Perl.

            while (($key,$value) = each $hashref) { ... }

        As of Perl 5.18 you can use a bare "each" in a "while" loop, which
        will set $_ on every iteration.

            while(each %ENV) {
                print "$_=$ENV{$_}\n";

        To avoid confusing would-be users of your code who are running
        earlier versions of Perl with mysterious syntax errors, put this
        sort of thing at the top of your file to signal that your code
        will work only on Perls of a recent vintage:

            use 5.012;  # so keys/values/each work on arrays
            use 5.014;  # so keys/values/each work on scalars (experimental)
            use 5.018;  # so each assigns to $_ in a lone while test

        See also "keys", "values", and "sort".


Using list assignments in while loops will not work, because the assignment will happen anyway even if an empty list is returned, so that this:

while (($key, $value) = each %hash) { }

will loop forever. There is unfortunately no way to fix this in Perl 6 module space at the moment. But a slightly different syntax, will work as expected:

while each(%hash) -> ($key,$value) { }

Also, this will alias the values in the list, so you don't actually need to define $key and $value outside of the while loop to make this work.


Elizabeth Mattijsen

Source can be located at: . Comments and Pull Requests are welcome.


Copyright 2018 Elizabeth Mattijsen

Re-imagined from Perl 5 as part of the CPAN Butterfly Plan.

This library is free software; you can redistribute it and/or modify it under the Artistic License 2.0.