Increment and decrement operators increase or decrease the value of a variable by one. An assignment operator can do the same thing, so the increment operators add no power to the awk language; however, they are convenient abbreviations for very common operations.
The operator used for adding one is written ‘++’. It can be used to increment
a variable either before or after taking its value.
To pre-increment a variable
v, write ‘++v’. This adds
one to the value of
v—that new value is also the value of the
expression. (The assignment expression ‘v += 1’ is completely
Writing the ‘++’ after the variable specifies post-increment. This
increments the variable value just the same; the difference is that the
value of the increment expression itself is the variable's old
value. Thus, if
foo has the value four, then the expression ‘foo++’
has the value four, but it changes the value of
foo to five.
In other words, the operator returns the old value of the variable,
but with the side effect of incrementing it.
The post-increment ‘foo++’ is nearly the same as writing ‘(foo
+= 1) - 1’. It is not perfectly equivalent because all numbers in
awk are floating-point—in floating-point, ‘foo + 1 - 1’ does
not necessarily equal
foo. But the difference is minute as
long as you stick to numbers that are fairly small (less than 10e12).
Fields and array elements are incremented just like variables. (Use ‘$(i++)’ when you want to do a field reference and a variable increment at the same time. The parentheses are necessary because of the precedence of the field reference operator ‘$’.)
The decrement operator ‘--’ works just like ‘++’, except that it subtracts one instead of adding it. As with ‘++’, it can be used before the lvalue to pre-decrement or after it to post-decrement. Following is a summary of increment and decrement expressions:
Operator Evaluation Order
Doctor, doctor! It hurts when I do this!
What happens for something like the following?
b = 6 print b += b++
Or something even stranger?
b = 6 b += ++b + b++ print b
In other words, when do the various side effects prescribed by the postfix operators (‘b++’) take effect? When side effects happen is implementation defined. In other words, it is up to the particular version of awk. The result for the first example may be 12 or 13, and for the second, it may be 22 or 23.
In short, doing things like this is not recommended and definitely not anything that you can rely upon for portability. You should avoid such things in your own programs.