Increment and decrement operators are unary operators that add or subtract one from their operand, respectively. They are commonly implemented in imperative programming languages. C-like languages feature two versions (pre- and post-) of each operator with slightly different semantics.
Contents
In languages syntactically derived from B (including C and its various derivatives), the increment operator is written as ++ and the decrement operator is written as --.
The increment operator increases the value of its operand by 1. The operand must have an arithmetic or pointer data type, and must refer to a modifiable data object. Similarly, the decrement operator decreases the value of its modifiable arithmetic operand by 1. Pointers values are increased (or decreased) by an amount that makes them point to the next (or previous) element adjacent in memory.
In languages that support both versions of the operators, the pre-increment and pre-decrement operators increment (or decrement) their operand by 1, and the value of the expression is the resulting incremented (or decremented) value. In contrast, the post-increment and post-decrement operators increase (or decrease) the value of their operand by 1, but the value of the expression is the operand's original value prior to the increment (or decrement) operation. In languages where increment/decrement is not an expression (e.g. Go), only one version is needed (in the case of Go, post operators only).
Since the increment/decrement operator modifies its operand, use of such an operand more than once within the same expression can produce undefined results. For example, in expressions such as x - ++x, it is not clear in what sequence the subtraction and increment operations should be performed. Such expressions generally invoke undefined behavior, and should be avoided.
Examples
The following C code fragment illustrates the difference between the pre and post increment and decrement operators:
The post-increment operator is commonly used with array subscripts. For example:
Likewise, the post-decrement operator is commonly used with pointers:
Note that these examples also work in other C-like languages, such as C++, Java, and C#.
Supporting languages
The following list, though not complete or all-inclusive, lists some of the major programming languages that support the ++/-- increment/decrement operators.
(Apple's Swift once supported these operators, but support was removed as of version 3.)
History
The concept was introduced in the B programming language circa 1969 by Ken Thompson.
Thompson went a step further by inventing the ++ and -- operators, which increment or decrement; their prefix or postfix position determines whether the alteration occurs before or after noting the value of the operand. They were not in the earliest versions of B, but appeared along the way. People often guess that they were created to use the auto-increment and auto-decrement address modes provided by the DEC PDP-11 on which C and Unix first became popular. This is historically impossible, since there was no PDP-11 when B was developed. The PDP-7, however, did have a few `auto-increment' memory cells, with the property that an indirect memory reference through them incremented the cell. This feature probably suggested such operators to Thompson; the generalization to make them both prefix and postfix was his own. Indeed, the auto-increment cells were not used directly in implementation of the operators, and a stronger motivation for the innovation was probably his observation that the translation of ++x was smaller than that of x=x+1.