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Re^2: (Mis)Understanding <c>grep...eq $_<c>

by ikegami (Patriarch)
on Feb 24, 2009 at 07:09 UTC ( #745920=note: print w/replies, xml ) Need Help??


in reply to Re: (Mis)Understanding <c>grep...eq $_<c>
in thread (Mis)Understanding grep...eq $_

[grep] searches arrays or lists.

I don't think so. Do you have any reason to believe grep knows anything about arrays?

>perl -le"@a = qw( a b c d ); @b = grep { push(@a,'!') if !$i++; 1 } @ +a; print @b" abcd

Contrast with for which does:

>perl -le"@a = qw( a b c d ); for (@a) { push(@a,'!') if !$i++; push @ +b, $_ } print @b" abcd!

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Re^3: (Mis)Understanding <c>grep...eq $_<c>
by Marshall (Canon) on Feb 25, 2009 at 08:22 UTC
    Grep could have been called "filter" in array context.
    Most common form is @output = grep{anon_sub_returning_True_False}@input;
    This code:
    #!/usr/bin/perl -w use strict; my @a = qw( a b c d ); my $i=0; my @b = grep { push(@a,'!') if !$i++; 1 } @a; print "@b\n"; print "@a\n"; __END__ output: a b c d a b c d !
    Means take each item in @a and put it into the anon grep subroutine. If that subroutine returns true, then move that item to @b. Absent an explicit return statement, the return value of a Perl sub is the last statement. In this case it is "1", or always true. So this is the same as: @a=@b; as far as grep{} is concerned. This push statement gets executed but has no bearing upon the action of grep{}.

    So, YES grep{} is meant to process/filter lists! I would argue that putting this push statement inside the grep is a weird thing to do.

    grep does have a scalar context also! This is useful in some cases, here this is just a "dumb" example.

    my $num = grep{1}@a; print $num; ##prints 5 (num of elements in @a) ie, abcd!
    But this can be used to say count number of matches in @a.
    $num = grep {/some_regex/}@a;
    But of course most common use would be @a=grep{/regex/}@b; which moves any item from @b which matches regex to @a.

    Update:Perl will not allow the input list to be modified within the grep{} subroutine. This could create a circular endless loop and that just won't happen.

      So, YES grep{} is meant to process/filter lists! I would argue that putting this push statement inside the grep is a weird thing to do.

      Obviously processes/filters lists. Obviously a weird thing to do. Jethro said grep also works on arrays, not just lists, and I was demonstrating that it only works on lists.

        grep works just fine on arrays (and other lists). You live in a bizarre world if you interpret "grep works on arrays" to mean "grep is optimized to do something special if passed a single array similar to how foreach (in some versions of Perl) is optimized" (something that you can only detect by doing "weird" things). *shrug*

        - tye        

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