Relational and Logical Operators

Programs do not only run statements in order. They often need to check whether a condition is true, then choose what to do next. A condition usually produces a Boolean value: true or false.

Relational operators

Relational operators compare two values. The result of a comparison is a Boolean value, 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", (int)(age < 18)); // true
printf("%d\n", (int)(age == 18)); // false
printf("%d\n", (int)(age != 18)); // true

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 a Boolean value.

• &&: 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.

int age = 20;
bool hasTicket = true;

printf("%d\n", (int)(age >= 18 && hasTicket)); // true
printf("%d\n", (int)(age < 18 || hasTicket)); // true
printf("%d\n", (int)(!hasTicket)); // false

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

Combining conditions

C does not support mathematical chained-comparison syntax; join two comparisons with &&:

int age = 16;
printf("%d\n", (int)(age > 0 && age < 18)); // true

This is equivalent to:

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

When a condition becomes complex, use parentheses or meaningful variable names to keep the logic readable.

int score = 85;
int attendance = 92;

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

printf("%d\n", (int)(passed)); // true
printf("%d\n", (int)(excellent)); // false

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