On Fri, Mar 14, 2014 at 6:03 PM, Ralph Grove <grov...@jmu.edu> wrote:
> I'm trying to determine how Perl evaluates operands for expressions. I
> expected the following code to output 3, 4, and 5, as it would if executed
> as a C++ or Java program. The actual output that I get (v5.16.2), however,
> is 4, 4, and 5. This leads me to believe that operand evaluation is either
> non-deterministic and compiler-dependent, or it's simply broken.
>
> The Perl documentation discusses operator evaluation, but doesn't directly
> address operand evaluation order. Does anyone know of a good resource for
> this information?
>
> Thanks,
> Ralph
>
>
>
> sub doit {
> return ++$g ;
> }
>
> $g = 0;
> my $sum = $g + doit() + doit();
> print "$sum \n";
>
> $g = 0;
> $sum = doit() + $g + doit();
> print "$sum \n";
>
> $g = 0;
> $sum = doit() + doit() + $g;
> print "$sum \n";
http://perldoc.perl.org/perlop.html#Auto-increment-and-Auto-decrement
In particular:
----
Note that just as in C, Perl doesn't define *when* the variable is
incremented or decremented. You just know it will be done sometime before
or after the value is returned. This also means that modifying a variable
twice in the same statement will lead to undefined behavior.
----
For what it's worth, here's some hackery to see the current order:
use 5.010;
sub TIESCALAR {
my $class = shift;
return bless {val => 0}, $class;
}
sub FETCH {
my $self = shift;
my $val = $self->{val};
warn "FETCH called, returning $val";
$val
}
sub STORE {
warn "STORE called with $_[1]";
my $self = shift;
$self->{val} = shift;
}
tie my $g, __PACKAGE__;
sub doit {
say " doit: will run ++\$g now:";
my $ret = ++$g;
say " doit: \$g set to $ret, returning that";
return $ret;
}
say "First:";
$g = 0;
say "^\$g = 0^";
my $sum = $g + doit() + doit();
print "$sum \n\n\n";
say "Second:";
$g = 0;
say "^\$g = 0^";
$sum = doit() + $g + doit();
print "$sum \n\n\n";
say "Third:";
$g = 0;
say "^\$g = 0^";
$sum = doit() + doit() + $g;
print "$sum \n\n\n";
As to why THAT specific order, well, you're on the wrong mailing list :D
Try perl5-porters if you're interested, or dig into pp_hot.c in the perl
source.
