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5.14 Operator Precedence (How Operators Nest)
"Operator precedence" determines how operators are grouped when
different operators appear close by in one expression. For example,
`*' has higher precedence than `+'; thus, `a + b * c' means to multiply
`b' and `c', and then add `a' to the product (i.e., `a + (b * c)').
The normal precedence of the operators can be overruled by using
parentheses. Think of the precedence rules as saying where the
parentheses are assumed to be. In fact, it is wise to always use
parentheses whenever there is an unusual combination of operators,
because other people who read the program may not remember what the
precedence is in this case. Even experienced programmers occasionally
forget the exact rules, which leads to mistakes. Explicit parentheses
help prevent any such mistakes.
When operators of equal precedence are used together, the leftmost
operator groups first, except for the assignment, conditional, and
exponentiation operators, which group in the opposite order. Thus, `a
- b + c' groups as `(a - b) + c' and `a = b = c' groups as `a = (b =
Normally the precedence of prefix unary operators does not matter,
because there is only one way to interpret them: innermost first.
Thus, `$++i' means `$(++i)' and `++$x' means `++($x)'. However, when
another operator follows the operand, then the precedence of the unary
operators can matter. `$x^2' means `($x)^2', but `-x^2' means
`-(x^2)', because `-' has lower precedence than `^', whereas `$' has
higher precedence. Also, operators cannot be combined in a way that
violates the precedence rules; for example, `$$0++--' is not a valid
expression because the first `$' has higher precedence than the `++';
to avoid the problem the expression can be rewritten as `$($0++)--'.
This table presents `awk''s operators, in order of highest to lowest
Exponentiation. These operators group right-to-left.
`+ - !'
Unary plus, minus, logical "not."
`* / %'
Multiplication, division, remainder.
No special symbol is used to indicate concatenation. The operands
are simply written side by side ( Concatenation).
`< <= == !='
`> >= >> | |&'
Relational and redirection. The relational operators and the
redirections have the same precedence level. Characters such as
`>' serve both as relationals and as redirections; the context
distinguishes between the two meanings.
Note that the I/O redirection operators in `print' and `printf'
statements belong to the statement level, not to expressions. The
redirection does not produce an expression that could be the
operand of another operator. As a result, it does not make sense
to use a redirection operator near another operator of lower
precedence without parentheses. Such combinations (for example,
`print foo > a ? b : c'), result in syntax errors. The correct
way to write this statement is `print foo > (a ? b : c)'.
Conditional. This operator groups right-to-left.
`= += -= *='
`/= %= ^= **='
Assignment. These operators group right to left.
NOTE: The `|&', `**', and `**=' operators are not specified by
POSIX. For maximum portability, do not use them.
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