Neither foreach (aka for), while nor do..while change the contents. But what's in the these loops can.

If you modify the variable of foreach (or $_ if a variable isn't specified) as shown in the following code, it will affect that element of the array. This is done very commonly.

@a = qw( food foot );
foreach (@a) { s/foo/bar/g }
# @a is now qw( bard bart )

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The following is a example of a strategy sometimes used with while:

sub visit_breadth_first {
   my ($node, $visitor) = @_;
   # Initial value:
   my @to_visit = ($node);
   # Loop as long as @to_visit isn't empty:
   while (@to_visit) {
      # Remove an element:
      my $current = shift(@to_visit);
      # Maybe add some elements:
      push(@to_visit, @{$current->children()});
      &$visitor($current);
   }
}

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In case you're looking for ways to modify the array, look at map, grep and sort. If you assign the value they return back to the original array, it's going to be modified. For example:

@a = map  { ... transformation ... } @a;
@a = grep { ... filter         ... } @a;
@a = sort { ... equiv test     ... } @a;

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In reply to Re: how do foreach and while affect an array? by ikegami
in thread how do foreach and while affect an array? by Anonymous Monk

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