in reply to Map Vs Foreach

map{} will not be faster than foreach!!! map is a slow but very powerful critter. map{} takes an input array and makes a new output array. foreach my $x (@input){} iterates over an existing array. @output = map{...}@input, makes a new array from the input - it transforms one array into a new array. Don't use map{} if you don't use the output of map{}, ie there should always be some use of the map array output.

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Re^2: Map Vs Foreach
by chromatic (Archbishop) on Nov 26, 2009 at 07:34 UTC
    map is a slow

    Why do you say that?

    map{} takes an input array and makes a new output array

    map transforms lists, not only arrays. Likewise, for iterates over lists.

    As well, map doesn't have to produce any output. A void context map is poor style in my book, but there's no performance penalty in modern Perls.

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      map{} is a wrong idea if the output isn't being used. That is why it can be "slow". But I am hearing that there is no performance penalty for a "void map{}"... meaning that Perl won't create and allocate memory for the output of map{} if is not being used? I didn't know about that. When did this performance enhancement happen?

      I think we both agree that a "void map{}" is bad style. I looked at the post from Ratazong and saw the following code being bench marked: 

      my @sqrt_results = map { sqrt ($_ ) } @input;

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      This is a perfect thing for map{}: translate one thing to another. map{} can also make 1 to many and many to one translations. In general, I use map{} when the transformation can be expressed as a "one liner+" and foreach() when the code is longer. As a matter of style, this allows me to put foreach(@input) at the top instead of at the end of the program text, compare with: @output = map{lots of lines}@input.
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        map{} is a wrong idea if the output isn't being used.
        Why? We use functions and operators without using their output all the time (print for instance). What makes map special that's not using its return value is a sin?
        I didn't know about that. When did this performance enhancement happen?
        6 years ago (5.8.1). After many years of people using the performance penalty of map-in-void-context as an argument not to use map in void context, someone actually produced a one line patch to eliminate that. Didn't stop people from ranting against using map-in-void-context though. Didn't even stop people using the performance argument either.

        Besides, wasn't it Larry himself who said that using things in other ways than originally intended isn't bad in itself? That one can enjoy sex, even if you aren't using it to reproduce yourself?

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        You asked about the difference between lists and arrays. Here's an example of map operating on lists, not arrays:

        say for map { "$_ squared is " . $_ ** 2 } 1 .. 10;

        If an operator operates on lists, Perl's list context ensures that it can handle arrays. If an operator operates on arrays (push, splice), it won't necessarily operate on lists.

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