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map sub to list?

by pldanutz (Acolyte)
on Sep 09, 2013 at 16:43 UTC ( #1053037=perlquestion: print w/replies, xml ) Need Help??

pldanutz has asked for the wisdom of the Perl Monks concerning the following question:

use strict; use warnings; sub f($) { return $_ [0] + 1 }; my @a = (1, 2, 3); map (f, @a)

As some would say, "why not working?" I get Not enough arguments for main::f at -e line 1, near "f," Yet perldoc -f map says

@chars = map(chr, @numbers);

How is one different from the other?

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Re: map sub to list?
by BrowserUk (Patriarch) on Sep 09, 2013 at 16:50 UTC

    chr defaults to using $_, so use: @a = map f( $_ ), @numbers;.

    Or define f() to default to $_:

    sub f(_){ $_[0]+1 };

    Now my @a = map f, @numbers; works.

    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
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      Thanks to everybody who replied! I think I've got it. When writing @a = map f( $_ ), @numbers; this is a case of map BLOCK LIST (as opposed to map EXPR LIST) right?
        No, using the comma makes it a case of EXPR, LIST.
Re: map sub to list?
by toolic (Bishop) on Sep 09, 2013 at 16:48 UTC
    How is one different from the other?
    perldoc -f ord does not require you to explicitly pass an argument to it, but your user-defined sub f requires you to pass an argument to it.
Re: map sub to list?
by Arunbear (Prior) on Sep 09, 2013 at 16:58 UTC
    If you're just adding 1 to each element then defining a separate sub is overkill:
    % perl -de0 Loading DB routines from version 1.32 Editor support available. Enter h or `h h' for help, or `man perldebug' for more help. main::(-e:1): 0 DB<1> @a = (1, 2, 3) DB<2> x @a 0 1 1 2 2 3 DB<3> x map { $_ + 1 } @a 0 2 1 3 2 4 DB<4> q %
Re: map sub to list?
by LanX (Sage) on Sep 09, 2013 at 16:59 UTC
    the canonical way is

    DB<100> sub f { return $_ + 1 }; DB<101> map f, 1..3 => (2, 3, 4) DB<102> map &f, 1..3 => (2, 3, 4)

    Cheers Rolf

    ( addicted to the Perl Programming Language)

Re: map sub to list?
by Laurent_R (Canon) on Sep 09, 2013 at 17:12 UTC

    Don't use function prototypes unless you know how and, more importantly, why. This is not needed here and is the main cause of your problem.

      I've tried removing the prototype, of course; but actually the main cause of the problem was relying on an argument as opposed to $_, right? I prefer functional style as much as possible, coming from a Lisp background and all that :)

        Yes. Assuming you want to keep the overkill of a function, you could either change the relevant line to:

        sub f{ return $_ + 1 };

        or pass the $_ as an argument to the function:

        use strict; use warnings; sub f{ return $_[0] + 1 }; my @a = (1, 2, 3); print join " ", map (f($_), @a);

        I have added the join because you like functional programming. But the more common way to do such things is illustrated in the following Perl one-liner:

        $ perl -e 'print join " ", map {$_ + 1} 1..4' 2 3 4 5

        Talking of functional programming, note that the block of code after the map (the {$_ + 1} part) can be regarded as an anonymous function being applied by map onto each input element.

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