What are compound assignment operators and how do increment/decrement work?

a ●= b is shorthand for a = a ● b; ++/-- add or subtract 1, and prefix updates before the value is used while postfix updates after.

Compound assignment:

c = c + b;   →   c += b;
c = c >> 2;  →   c >>= 2;
c = c & a;   →   c &= a;

Increment/decrement:

// or ++c
c = c + 1;   →   c++;
// or --c
c = c - 1;   →   c--;

Prefix vs postfix - the critical difference:

int b = 5;
// a = 5, b = 6 (use THEN increment)
int a = b++;
// a = 6, b = 6 (increment THEN use)
int a = ++b;

Example in expressions:

int x = 5;
// Prints 5, then x becomes 6
printf("%d\n", x++);
// x becomes 7, then prints 7
printf("%d\n", ++x);

Warning: Avoid multiple ++/-- on same variable in one expression:

// UNDEFINED BEHAVIOR! Don't do this.
int a = i++ + ++i;

Go deeper:

• Increment and decrement operators — Wikipedia — prefix vs postfix ++/-- semantics.