Getting a result from "foreach" block

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

Dear colleagues,

I tried to google the answer, but haven't successed, so I'll be grateful for your help.

How can I get some list of results of some "foreach" loop? Just something like that:

my @results = foreach (@data) {
    $_ = $_ * 13;
    # some command to add the element to the resulting list
    # kind of push(), but how to get the reference
    # to the resulting array?
}
[download]

Right now I'm using the following construction that I don't like:

@results = eval {
    my @r;
    foreach (@data) {
        $_ = $_ * 13;
        push(@r);
    }
    return(@r);
}
[download]

Of course, I'm not too happy with that. How does the best way look like?

Lots of thanks to all for hints, tips and ideas!

V.Melnik

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Re: Getting a result from "foreach" block by AppleFritter (Vicar) on Jun 25, 2014 at 11:47 UTC
May the more enlightened members of the monastery correct me if I'm wrong, but `foreach` is a control construct. It does not return a value; it controls the execution of your code. What you want is map, which takes a list, performs a operation on each element and returns the resulting list of processed elements: `#!/usr/bin/perl use strict; my @array = (1, 2, 3, 4, 5); my @results = map { $_ * 13 } @array; # @results is (13, 26, 39, 52, 65)` [download]	[reply] [d/l]
Re^2: Getting a result from "foreach" block by v_melnik (Scribe) on Jun 25, 2014 at 11:50 UTC
Wow, great, thank you! Such a shame I have never used map() before! V.Melnik	[reply]
Re^3: Getting a result from "foreach" block by AppleFritter (Vicar) on Jun 25, 2014 at 11:53 UTC
You're welcome! Keep map and grep in mind, they're among the most useful functions you'll ever see for list processing. For more useful list-related functions, see e.g. List::Util. (In particular, this contains a `reduce` function that sadly does not exist as a builtin.)	[reply]
Re: Getting a result from "foreach" block by CountZero (Bishop) on Jun 25, 2014 at 14:28 UTC
What stops you from doing the following: `my @results; foreach (@data) { $_ = $_ * 13; push @results, $_; }` [download] or `my @results; push @results, $_13 for @data;` [download] You will find the "return" of the loop in `@results`. The most "perlish" way of doing such a thing of course uses `map` (as AppleFritter already said]: `my @results = map $_13, @data;` [download] CountZero A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James My blog: Imperial Deltronics	[reply] [d/l] [select]
Re^2: Getting a result from "foreach" block by AnomalousMonk (Archbishop) on Jun 25, 2014 at 18:51 UTC
What stops you ... Nothing stops the beginner from using the first approach or from pitching headlong into the yawning chasm it opens up, for the `foreach` loop is mutating the `@data` "source" array. Of course, map also aliases `$_` to the items of the list over which it iterates and so presents the same pitfall, but I agree it is the idiomatic and preferable approach — but let beginners beware! `c:\@Work\Perl>perl -wMstrict -MData::Dump -le "my @data = 1 .. 4; dd \@data; ;; my @results; foreach (@data) { $_ = $_ * 13; push @results, $_; } ;; dd \@data; dd \@results; " [1 .. 4] [13, 26, 39, 52] [13, 26, 39, 52]` [download]	[reply] [d/l] [select]
Re^3: Getting a result from "foreach" block by RonW (Parson) on Jun 26, 2014 at 17:18 UTC
Then why not: `my @results; foreach (@data) { push @results, $_ * 13; }` [download] To avoid the mutation of @data ?	[reply] [d/l]
Re^4: Getting a result from "foreach" block by AnomalousMonk (Archbishop) on Jun 26, 2014 at 18:31 UTC
Re: Getting a result from "foreach" block by toolic (Bishop) on Jun 25, 2014 at 12:28 UTC
push(@r); Tip #1 from the Basic debugging checklist: warnings `Useless use of push with no values at ...` [download]	[reply] [d/l]
Re: Getting a result from "foreach" block by Anonymous Monk on Jun 25, 2014 at 12:02 UTC
Or, for more general cases, simply use `foreach`, and/or the `each()` function, to control an ordinary loop, and do whatever needs to be done within that loop. The aforementioned functions are of course very efficient and are great for avoiding the necessity of explicit loops. Feel free to do it either way as suits the occasion.	[reply]