Relational operators

1. Less than and greater than: Use < and >. For a = 5 and b = 8, a < b gives 1.
2. Inclusive comparisons: Use <= and >= when equality is also allowed.
3. Equality: Use == to test equality and != to test inequality. a == b gives 0 for 5 and 8.

Logical operators

1. AND (&&): True only when both conditions are true. Example: age >= 18 && has_id.
2. OR (: |)|True when at least one condition is true. Example: day == 6 || day == 7.
3. NOT (!): Reverses the truth value. !0 gives 1 and !5 gives 0.

Short-circuit evaluation

For &&, C skips the right condition when the left condition is false. For ||, it skips the right condition when the left condition is true. For example, d != 0 && n % d == 0 checks d first. When d is zero, the remainder operation is skipped, avoiding division by zero in that expression.

Important points

Zero is treated as false; a non-zero value is treated as true. Write x > 0 && x < 10 to test a range. Do not write 0 < x < 10, because C first produces 0 or 1 from the first comparison and then compares that result with 10. Use == for comparison, not =.

Reference table

Key points

• Logical operators normalise their result to 0 or 1.
• Short-circuiting can prevent evaluating an unsafe expression.