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3.16. Conditional Operator
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, $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!\n",
$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.