Comparisons, Logic & Precedence
beginner11 min readLesson 12 of 148
Boolean results in C, short-circuit && and ||, !, and the precedence rules that decide what an expression means.
Comparisons yield 0 or 1
== != < <= > >= produce int: 1 (true) or 0 (false). C has no separate
boolean type in everyday use — any nonzero value is "true".
The classic bug: = assigns, == compares. if (x = 5) assigns 5 to x
and is always true. Compilers warn; turn warnings on (this course always does).
Logical operators, short-circuit
int a = 0, b = 5;
a && b // 0 — a is false, b never evaluated (short-circuit)
b || a // 1 — b is true, a never evaluated
!b // 0 — not true is false
Short-circuiting is a feature: p != NULL && p->value > 0 is safe — the second
operand only runs when p is non-NULL.
Precedence (the rules that matter now)
!, unary-— highest* / %+ -< <= > >=== !=&&||=— lowest
x + y == 3 && a < b parses as ((x + y) == 3) && (a < b). When in doubt,
parenthesize for humans.