Increment and Decrement

Big idea

i++ adds 1 to i, and i-- subtracts 1. The position of the ++ decides whether you get the value before or after the change.

The idea

These all add 1 to count:

count = count + 1;
count += 1;
count++;

+= works with any number and any arithmetic operator: score += 10;, lives -= 1;, price *= 2;, n /= 10;.

Before or after?

On a line by itself, i++ and ++i do the same thing. The difference only shows when you use the value in the same expression:

• i++ (post-increment): use the old value, then add 1.
• ++i (pre-increment): add 1 first, then use the new value.

#include <stdio.h>

int main(void) {
    int a = 5;
    int b = a++;
    printf("a=%d b=%d\n", a, b);

    int c = 5;
    int d = ++c;
    printf("c=%d d=%d\n", c, d);
    return 0;
}

a=6 b=5
c=6 d=6

Step through the first half:

The rule that keeps you safe

Don't change a variable and read it again in the same expression — i = i++; or printf("%d %d", i++, i); are undefined behaviour: C doesn't promise any particular result, and different compilers really do give different answers. One ++ per variable per line.

Watch it

Think of it like

A ticket machine at a deli. ticket++ gives you the current number and then advances the counter. ++ticket advances the counter first and gives you the new number.

🕷️ Spidey-sense: villains_caught++; — best line of code in the city. Pre or post, the count goes up either way.

Key terms

• Increment / decrement — Add 1 / subtract 1: ++ / --.
• Post-increment (i++) — Gives the old value, then adds 1.
• Pre-increment (++i) — Adds 1, then gives the new value.
• Compound assignment — +=, -=, *=, /=, %=.