Logical Defined-Or
Although it has no direct equivalent in C, Perl's // operator is related to its C-style "or". In fact, it's exactly the same as ||, except that it tests the left hand side's definedness instead of its truth. Thus, EXPR1 // EXPR2 returns the value of EXPR1 if it's defined, otherwise, the value of EXPR2 is returned. (EXPR1 is evaluated in scalar context, EXPR2 in the context of // itself). Usually, this is the same result as defined(EXPR1) ? EXPR1 : EXPR2 (except that the ternary-operator form can be used as a lvalue, while EXPR1 // EXPR2 cannot). This is very useful for providing default values for variables. If you actually want to test if at least one of $x and $y is defined, use defined($x // $y) .
The ||, // and && operators return the last value evaluated (unlike C's || and &&, which return 0 or 1). Thus, a reasonably portable way to find out the home directory might be:
1 2 3 4 | $home = $ENV{HOME}// $ENV{LOGDIR}// (getpwuid($<))[7]// die "You're homeless!\n"; |
In particular, this means that you shouldn't use this for selecting between two aggregates for assignment:
1 2 3 | @a = @b || @c; # this is wrong@a = scalar(@b) || @c; # really meant this@a = @b ? @b : @c; # this works fine, though |
As alternatives to && and || when used for control flow, Perl provides the and and or operators (see below). The short-circuit behavior is identical. The precedence of "and" and "or" is much lower, however, so that you can safely use them after a list operator without the need for parentheses:
1 2 | unlink "alpha", "beta", "gamma"or gripe(), next LINE; |
With the C-style operators that would have been written like this:
1 2 | unlink("alpha", "beta", "gamma")|| (gripe(), next LINE); |
It would be even more readable to write that this way:
1 2 3 4 | unless(unlink("alpha", "beta", "gamma")) { gripe(); next LINE;} |
Using "or" for assignment is unlikely to do what you want; see below.
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