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 usually a bool: false means false, and true means true.

Relational operators

Relational operators compare two values. The result is true or 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;

std::cout << (age < 18) << std::endl;     // 1
std::cout << (age == 18) << std::endl;    // 0
std::cout << (age != 18) << std::endl;    // 1

By default, std::cout prints true as 1 and false as 0. You can print the words true and false by using std::boolalpha.

std::cout << std::boolalpha << (age < 18) << std::endl;   // 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 true or false.

• &&: 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;
bool has_ticket = true;

std::cout << std::boolalpha;
std::cout << (age >= 18 && has_ticket) << std::endl;    // true
std::cout << (age < 18 || has_ticket) << std::endl;     // true
std::cout << (!has_ticket) << std::endl;                // false

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 chained comparisons with mathematical meaning. An expression such as 0 < age < 18 may compile, but it does not mean "age is between 0 and 18," so write it as two conditions joined by &&.

int age = 16;

std::cout << (0 < age && age < 18) << std::endl;    // 1

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

int score = 85;
int attendance = 92;

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

std::cout << std::boolalpha;
std::cout << passed << std::endl;       // true
std::cout << excellent << std::endl;    // false

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