Operators and Expressions

Teaching C++ to do math, compare values, and make decisions

Big Idea: An operator is a symbol that tells C++ to do something with values, like + to add or < to compare. When you join values and operators together, you get an expression — and every expression works out to a single result.

By the end you will be able to:

use arithmetic operators + - * / %,
compare values with == != < > <= >=,
combine conditions with && || !,
predict how an expression is evaluated,
explain why 7 / 2 is 3 but 7.0 / 2 is 3.5.

1. Arithmetic operators

These do everyday math. The only surprising one is % (called modulo): it gives the remainder after division. For example 7 % 3 is 1, because 3 goes into 7 twice with 1 left over.

#include <iostream>
using namespace std;

int main() {
    int a = 7, b = 3;
    cout << a + b << endl;   // 10
    cout << a - b << endl;   // 4
    cout << a * b << endl;   // 21
    cout << a / b << endl;   // 2   (integer division)
    cout << a % b << endl;   // 1   (remainder)
    return 0;
}

Output:

Notice 7 / 3 gave 2, not 2.33. We explain that next.

2. Integer division vs floating division

This trips up almost every beginner. In C++, the type of the numbers decides the kind of division:

If both sides are whole numbers (int), C++ does integer division and throws away the fractional part. So 7 / 2 is 3, not 3.5.
If at least one side is a double (a number with a decimal point), C++ keeps the decimals. So 7.0 / 2 is 3.5.

#include <iostream>
using namespace std;

int main() {
    cout << 7 / 2   << endl;   // 3     both are int
    cout << 7.0 / 2 << endl;   // 3.5   one is double
    cout << 7 / 2.0 << endl;   // 3.5   one is double
    double x = 7;
    cout << x / 2   << endl;   // 3.5   x is double
    return 0;
}

Output:

3
3.5
3.5
3.5

To force a decimal answer, make one value a double — for example write 7.0 instead of 7.

Operators and Expressions