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3.14. C-Style Logical (Short-Circuit) Operators

Like C, Perl provides the && (logical AND) and || (logical OR) operators. They evaluate from left to right (with && having slightly higher precedence than ||) testing the truth of the statement. These operators are known as short-circuit operators because they determine the truth of the statement by evaluating the fewest number of operands possible. For example, if the left operand of an && operator is false, the right operand is never evaluated because the result of the operator is false regardless of the value of the right operand.

Example Name Result
$a && $b And $a if $a is false, $b otherwise
$a || $b Or

$a if $a is true, $b otherwise

Such short circuits not only save time, but are frequently used to control the flow of evaluation. For example, an oft-appearing idiom in Perl programs is:

open(FILE, "somefile") || die "Can't open somefile: $!\n";
In this case, Perl first evaluates the open function. If the value is true (because somefile was successfully opened), the execution of the die function is unnecessary, and so is skipped. You can read this literally as "Open some file or die!"

The && and || operators differ from C's in that, rather than returning 0 or 1, they return the last value evaluated. In the case of ||, this has the delightful result that you can select the first of a series of scalar values that happens to be true. Thus, a reasonably portable way to find out the user's home directory might be:

$home = $ENV{HOME} 
     || $ENV{LOGDIR} 
     || (getpwuid($<))[7] 
     || die "You're homeless!\n";
On the other hand, since the left argument is always evaluated in scalar context, you can't use || for selecting between two aggregates for assignment:
@a = @b || @c;             # This doesn't do the right thing
@a = scalar(@b) || @c;     # because it really means this.
@a = @b ? @b : @c;         # This works fine, though.
Perl also provides lower precedence and and or operators that some people find more readable and don't force you to use parentheses on list operators. They also short-circuit. See Table 3-1 for a complete list.



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