As in C, ?
: is the only trinary
operator. It's often called the conditional operator because it works
much like an if-then-else, except that, since it's an expression and
not a statement, it can be safely embedded within other expressions
and functions calls. As a trinary operator, its two parts separate
three expressions:
COND
?THEN
:ELSE
If the condition COND
is true, only
the THEN
expression is evaluated, and the
value of that expression becomes the value of the entire expression.
Otherwise, only the ELSE
expression is
evaluated, and its value becomes the value of the entire
expression.
Scalar or list context propagates downward into the second or third argument, whichever is selected. (The first argument is always in scalar context, since it's a conditional.)
$a = $ok ? $b : $c; # get a scalar @a = $ok ? @b : @c; # get an array $a = $ok ? @b : @c; # get a count of an array's elements
You'll often see the conditional operator embedded in
lists of values to format with printf
, since nobody
wants to replicate the whole statement just to switch between two
related values.
printf "I have %d camel%s. ", $n, $n == 1 ? "" : "s";
Conveniently, the precedence of ?
: is higher
than a comma but lower than most operators you'd use inside (such as
==
in this example), so you don't usually have to
parenthesize anything. But you can add parentheses for clarity if you
like. For conditional operators nested within the
THEN
parts of other conditional operators,
we suggest that you put in line breaks and indent as if they were
ordinary if
statements:
$leapyear = $year % 4 == 0 ? $year % 100 == 0 ? $year % 400 == 0 ? 1 : 0 : 1 : 0;
For conditionals nested within the
ELSE
parts of earlier conditionals, you can
do a similar thing:
$leapyear = $year % 4 ? 0 : $year % 100 ? 1 : $year % 400 ? 0 : 1;
but it's usually better to line up all the
COND
and THEN
parts vertically:
$leapyear = $year % 4 ? 0 : $year % 100 ? 1 : $year % 400 ? 0 : 1;
Lining up the question marks and colons can make sense of even fairly cluttered structures:
printf "Yes, I like my %s book! ", $i18n eq "french" ? "chameau" : $i18n eq "german" ? "Kamel" : $i18n eq "japanese" ? "x{99F1}x{99DD}" : "camel"
You can assign to the conditional operator[5] if both the second and third arguments are legal lvalues (meaning that you can assign to them), and both are scalars or both are lists (otherwise, Perl won't know which context to supply to the right side of the assignment):
($a_or_b ? $a : $b) = $c; # sets either $a or $b to have the value of $c
Bear in mind that the conditional operator binds more tightly
than the various assignment operators. Usually this is what you want
(see the $leapyear
assignments above, for example),
but you can't have it the other way without using parentheses. Using
embedded assignments without parentheses will get you into trouble,
and you might not get a parse error because the conditional operator
can be parsed as an lvalue. For example, you might write this:
$a % 2 ? $a += 10 : $a += 2 # WRONG
But that would be parsed like this:
(($a % 2) ? ($a += 10) : $a) += 2
[5] This is not necessarily guaranteed to contribute to the readability of your program. But it can be used to create some cool entries in an Obfuscated Perl contest.