Paul Johnson <p...@pjcj.net> writes:
> On Sun, Jan 24, 2016 at 05:44:14PM +0200, Shlomi Fish wrote:
>> Hi lee,
>>
>> On Sun, 24 Jan 2016 13:11:37 +0100
>> lee <l...@yagibdah.de> wrote:
>>
>> > Paul Johnson <p...@pjcj.net> writes:
>> > >
>> > > In scalar context the comma operator evaluates its left-hand side,
>> > > throws it away and returns the right-hand side.
>> >
>> > What is the useful use for this operator?
>> >
>>
>> Well, I believe its use was originally inherited from
>> https://en.wikipedia.org/wiki/C_%28programming_language%29 where one can do
>> something like:
>>
>> x = (y++, y+2);
>>
>> In Perl 5 though it is preferable to use do { ... } instead:
>>
>> $x = do { $y++; $y+2; };
>
> In both Perl and C the comma operator is probably most usually (deliberately)
> seen in for statements:
>
> #!/usr/bin/env perl
>
> use strict;
> use warnings;
>
> for (my ($x, $y) = (1, 7); $x < 5; $x++, $y--) {
> print "$x $y\n";
> }
>
> and
>
> #include <stdio.h>
>
> int main() {
> int x, y;
> for (x = 1, y = 7; x < 5; x++, y--)
> printf("%d %d\n", x, y);
> return 0;
> }
>
> both of which produce the output:
>
> 1 7
> 2 6
> 3 5
> 4 4
Ok and how is the comma operator usefully useful? Obviously, I could
use it to create convoluted code, which is usually not my intention.
Consider with these examples that an expression like (1, 3) might
unexpectedly evaluate to 3, and you start to think that you don't like
things like
sub s {
my $a = 1;
my $b = 3;
return ($a, $b);
}
anymore because you could, by mistake (or intentionally), write
my $x = s;
. So let me re-phrase my original question to: How do you /safely/
return arrays?
That means I need an error message for '$x = s;' because I'd write that
only by mistake.
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