لطفا منتظر باشید ...
3.20. Logical and, or, not, and xor
As lower precedence alternatives to &&, ||, and !, Perl
provides the and, or, and not operators. The behavior of these
operators is identical--in particular, and and or short-circuit like
their counterparts, which makes them useful not only for logical
expressions but also for control flow.Since the precedence of these operators is much lower than the ones
borrowed from C, you can safely use them after a list operator without
the need for parentheses:
unlink "alpha", "beta", "gamma"
or gripe(), next LINE;
With the C-style operators you''d have to write it like this:
unlink("alpha", "beta", "gamma")
|| (gripe(), next LINE);
But you can''t just up and replace all instances of
|| with or. Suppose you change
this:
$xyz = $x || $y || $z;
to this:
$xyz = $x or $y or $z; # WRONG
That wouldn''t do the same thing at all! The precedence of the
assignment is higher than or but lower than
||, so it would always assign $x
to $xyz, and then do the ors.
To get the same effect as ||, you''d have to write:
$xyz = ( $x or $y or $z );
The moral of the story is that you still must learn precedence (or use
parentheses) no matter which variety of logical operators you use.There is also a logical xor operator that has no
exact counterpart in C or Perl, since the only other exclusive-OR
operator ((^)) works on bits. The
xor operator can''t short-circuit, since both sides
must be evaluated. The best equivalent for $a xor
$b is perhaps !$a != !$b. One could also
write !$a ^ !$b or even $a ? !$b :
!!$b, of course. The point is that both
$a and $b have to evaluate to
true or false in a Boolean context, and the existing bitwise operator
doesn''t provide a Boolean context without help.