A conditional expression is a special kind of expression with three operands. It allows you to use one expression's value to select one of two other expressions.
The conditional expression is the same as in the C language:
selector ? if-true-exp : if-false-exp
There are three subexpressions. The first, selector, is always computed first. If it is "true" (not zero and not null) then if-true-exp is computed next and its value becomes the value of the whole expression. Otherwise, if-false-exp is computed next and its value becomes the value of the whole expression.
For example, this expression produces the absolute value of x:
x > 0 ? x : -x
Each time the conditional expression is computed, exactly one of
if-true-exp and if-false-exp is computed; the other is ignored.
This is important when the expressions contain side effects. For example,
this conditional expression examines element i of either array
a or array b, and increments i.
x == y ? a[i++] : b[i++]
This is guaranteed to increment i exactly once, because each time
only one of the two increment expressions is executed,
and the other is not.
See section Arrays in awk,
for more information about arrays.
As a minor gawk extension,
you can continue a statement that uses `?:' simply
by putting a newline after either character.
However, you cannot put a newline in front
of either character without using backslash continuation
(see section awk Statements Versus Lines).
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