Re: AND OR

Another similarity between perl and shell (common to C as well), is that the second expression will not be evaluated at all if the first expression "answers the question". This matters when the second expression has side effects. Some trivial examples:

sub f0 { return 0 }
sub f1 { return 1 }

if (( my $x = f1() ) && ( my $y = f1() ))
#  true, and both variables are set to 1


if (( my $x = f0() ) && ( my $y = f1() ))
#  false, $x is set to 0, $y is undef


if (( my $x = f0() ) || ( my $y = f1() ))
#  true, $x is set to 0, $y is set to 1


if (( my $x = f1() ) || ( my $y = f1() ))
#  true, $x is set to 1, $y is undef
[download]

As in shell usage, this can be used to good effect, to perform (or skip) an action based on the outcome of previous action. (It can also be a trap for the unwary.)

Comment on Re: AND OR Download Code

Replies are listed 'Best First'.
Re^2: AND OR by davidrw (Prior) on May 14, 2006 at 14:19 UTC
(for keyword or further-search purposes) that behavior is dubbed "short-circuiting" And i'll toss in a good C example (avoid method calls on undef/null): `int x; if(ptr != NULL && ptr==5){ ... } if( objPtr != NULL && objPtr->foo() ){ ... } # extended to perl: my $obj = Foo::Bar->new(); if( $obj && $obj->blah() ){ ... }` [download] Update: added parens for `->foo()`	[reply] [d/l] [select]
Re^3: AND OR (C meth) by tye (Sage) on May 14, 2006 at 16:29 UTC
Heh, you hit a C pet peave of mine. This: `if( objPtr != NULL && objPtr->foo ){ ... }` [download] doesn't actually call any methods. You need `objPtr->foo()` for that. And the ANSI C committee should have made the "`&`" for getting a pointer to a method mandatory instead of optional and discouraged, as needing to do that is relatively rare, rarer than the mistake of not realizing that C requires the parens unlike several other quite similar languages. Further, this particular mistake usually results in compiler output that is quite difficult to diagnose, IME. - tye	[reply] [d/l] [select]


Pathologically Eclectic Rubbish Lister
	PerlMonks