Predecrement

Predecrement is a unary operator, typically written as --x, that decreases the value of its operand by one and yields the decremented value as the result of the expression. The operation also updates the value stored in the operand’s location. The operand must be a modifiable lvalue in most programming languages.

In use, predecrement reduces the variable before its value is used in the surrounding expression. For example, int a = 5; int b = --a; results in a == 4 and b == 4. The side effect (the decrement) occurs before the value is produced for the assignment or further computation.

Predecrement is contrasted with postdecrement, which is written as x--. Postdecrement yields the original value of x and then decrements it. In many languages, this means --x and x-- evaluate to different results even though both modify the same variable.

Common contexts for predecrement include loop counters, iterator advancement, and index or pointer calculations where the updated value is needed immediately.

Not all languages provide a predecrement operator. Some languages, such as Python, do not have -- and instead use explicit subtraction assignments, e.g., x -= 1. In languages that do support it, applying predecrement to a non-modifiable expression (such as a constant) is invalid and will typically produce a compile-time error.