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Re^2: Problems with map(function, array)

by Redei (Initiate)
on Dec 10, 2012 at 21:26 UTC ( #1008166=note: print w/ replies, xml ) Need Help??


in reply to Re: Problems with map(function, array)
in thread Problems with map(function, array)

My problem is what happens when I don't use $_:

sub twice { 2 * $_[0] } @a = map twice, 1..3; print "@a\n"; ==> 0 0 0


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Re^3: Problems with map(function, array)
by LanX (Canon) on Dec 10, 2012 at 21:30 UTC
    the LIST elements are ALWAYS passed via $_ per iteration.

    you call twice w/o parameters so $_[0] is undef and undef*2 == 0!

    take again a look at my "add"-example to distiguish between parameters and iteration value

    Cheers Rolf

Re^3: Problems with map(function, array)
by eyepopslikeamosquito (Canon) on Dec 10, 2012 at 21:36 UTC

    Your original code with strict and warnings:

    use strict; use warnings; sub twice { 2 * $_[0] } my @a = map twice, 1..3; print "@a\n";
    gives the warning "Use of uninitialized value $_[0] in multiplication (*) at map1.pl line 3".

    This is how it should be written:

    use strict; use warnings; sub twice { 2 * $_[0] } my @a = map twice($_), 1..3; print "@a\n";
    which produces 2 4 6 as expected.

    BTW, Conway, in Perl Best Practices, chapter 8, recommends always using the block form of map, not the expression form that you are using. The expression form might be faster than the block form though (if that matters, you should benchmark it).

      Thank you for the suggestions. I should have been using strict and warnings, of course. Maybe then I would have seen more easily where my problem was.

      I don't have Conway's book, so I can't see in what context he recommends using the block (rather than the expression) form of map. But I do have reasons for wanting the expression form:

      • It's legal syntax, so it should be useable. (And it is, of course, if you don't make the kind of mistake I did.)
      • It's more elegant for a single-parameter function. At least I think so.

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