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Postfix and prefix ++ and --

Increment (++) and decrement (--) expressions also have values, and like assignment expressions, can be hidden away in inconspicuous places. These two operators are slightly more complicated than assignments because they exist in two forms, postfix (for example, my_var++) and prefix (for example, ++my_var).

Postfix and prefix forms have subtly different meanings. Take the following example:

int my_int = 3;
printf ("%d\n", my_int++);

The increment operator is hidden in the parameter list of the printf call. The variable my_int has a value before the ++ operator acts on it (3) and afterwards (4).

Which value is passed to printf? Is my_int incremented before or after the printf call? This is where the two forms of the operator (postfix and prefix) come into play.

If the increment or decrement operator is used as a prefix, the operation is performed before the function call. If the operator is used as a postfix, the operation is performed after the function call.

In the example above, then, the value passed to printf is 3, and when the printf function returns, the value of my_int is incremented to 4. The alternative is to write

int my_int = 3;
printf ("%d\n", ++my_int);

in which case the value 4 is passed to printf.

The same remarks apply to the decrement operator as to the increment operator.