C Increment & Decrement

In this page:

Prefix Increment
Postfix Increment
Prefix Decrement
Postfix Decrement
Mixing in Expressions

Prefix Increment

Prefix increment (++x) adds 1 to the variable's value first, and the expression it's part of then uses that already-updated value -- so y = ++x; gives y the value of x after the increase.

Example: Prefix Increment

#include <stdio.h>
int main() {
	int x = 5;
	int y = ++x;
	printf("%d %d", x, y);
	return 0;
}

Postfix Increment

Postfix increment (x++) uses the variable's current value in the surrounding expression first, and only applies the +1 increase afterward -- so y = x++; gives y the value x had *before* it was incremented.

Example: Postfix Increment

#include <stdio.h>
int main() {
	int x = 5;
	int y = x++;
	printf("%d %d", x, y);
	return 0;
}

Prefix Decrement

Prefix decrement (--x) subtracts 1 first and the updated value is what gets used in the expression, mirroring prefix increment but decreasing instead of increasing the value.

Example: Prefix Decrement

#include <stdio.h>
int main() {
	int x = 5;
	int y = --x;
	printf("%d %d", x, y);
	return 0;
}

Postfix Decrement

Postfix decrement (x--) uses the current value in the expression first, then decreases the variable afterward -- mirroring postfix increment's timing but subtracting instead of adding.

Example: Postfix Decrement

#include <stdio.h>
int main() {
	int x = 5;
	int y = x--;
	printf("%d %d", x, y);
	return 0;
}

Mixing in Expressions

Mixing several increment/decrement operators inside one complex expression (like x++ + ++x) leads to undefined or compiler-dependent behavior in C -- it's best practice to keep such operators in their own separate statement.

Example: Mixing in Expressions

#include <stdio.h>
int main() {
	int x = 5;
	x++;
	++x;
	printf("%d", x);
	return 0;
}

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