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The GNU Awk User's Guide

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6.12 Conditional Expressions

A conditional expression is a special kind of expression that has 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, as shown here:

 
selector ? if-true-exp : if-false-exp

There are three subexpressions. The first, selector, is always computed first. If it is "true" (not zero or 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, the following expression produces the absolute value of x:

 
x >= 0 ? x : -x

Each time the conditional expression is computed, only one of if-true-exp and if-false-exp is used; the other is ignored. This is important when the expressions have 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, a statement that uses `?:' can be continued simply by putting a newline after either character. However, putting a newline in front of either character does not work without using backslash continuation (see section awk Statements Versus Lines). If `--posix' is specified (see section Command-Line Options), then this extension is disabled.


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