On Mon, Jan 25, 2016 at 12:24:04AM +0100, lee wrote:
> 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?
Beyond what I have written above, I have had little use for it in C and
less in Perl. But I have no doubt that were you to ask in the
appropriate forum then someone could provide a compelling argument.
> 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.
You could add to your subroutine:
die "Only call in list context" unless wantarray;
--
Paul Johnson - p...@pjcj.net
http://www.pjcj.net
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