2.1 Relational and Logical Operators

Programs do not only run statements in order. They often need to check whether a condition holds, then choose what to do next. In C, a condition’s result is an integer: 0 means false, and any nonzero value means true.

Relational operators

Relational operators compare two values. The result is 1 (true) or 0 (false), not another number.

Common C relational operators:

• <: less than
• >: greater than
• <=: less than or equal to
• >=: greater than or equal to
• ==: equal to
• !=: not equal to

Important: one equals sign = means assignment. Two equals signs == mean comparison.

int age = 17;

printf("%d\n", age < 18);     // 1
printf("%d\n", age == 18);    // 0
printf("%d\n", age != 18);    // 1

age = 17 stores 17 in the variable age. age == 18 checks whether the current value of age equals 18.

Logical operators

Logical operators combine multiple conditions. Their result is still 1 or 0.

• &&: the whole expression is true only when every condition is true.
• ||: the whole expression is true when at least one condition is true.
• !: turns true into false, and false into true.

Truth tables

A truth table lists every possible input combination to make the rules clear.

&& is true only when both sides are true.

|| is true when at least one side is true.

! acts on a single value and flips the result.

int age = 20;
int has_ticket = 1;

printf("%d\n", age >= 18 && has_ticket);    // 1
printf("%d\n", age < 18 || has_ticket);     // 1
printf("%d\n", !has_ticket);                // 0

It helps to read conditions like plain language: age >= 18 && has_ticket means “the age is at least 18, and the person has a ticket.”

Combining conditions

Unlike math notation, C does not support chaining comparisons. 0 < age < 18 does not cause an error in C, but it does not mean “age is between 0 and 18,” so write it as two conditions joined by &&.

int age = 16;

printf("%d\n", 0 < age && age < 18);    // 1

When a condition becomes complex, name each smaller condition first, then combine them.

int score = 85;
int attendance = 92;

int passed = score >= 60 && attendance >= 80;
int excellent = score >= 90 || attendance >= 95;

printf("%d\n", passed);       // 1
printf("%d\n", excellent);    // 0

Do not rush to squeeze every condition into one line. Naming smaller conditions makes the program easier to read and debug.