Integer Division

Division in C++ has a catch. Try to guess what this statement prints:

std::cout << 7 / 2 << "\n";

This prints:

3

Not 3.5, but 3. When you divide two whole numbers, the result is a whole number too: C++ throws away the decimal part. It doesn't round, it just cuts it off, so 19 / 10 is 1, and 2 / 3 is 0. This is called integer division.

Integer division works like division with a remainder from primary school: how many times one number fits into another. The remainder is given by the % operator (the percent sign, which programmers call modulo):

std::cout << 7 / 2 << "\n";
std::cout << 7 % 2 << "\n";
std::cout << 10 % 5 << "\n";

This prints:

3
1
0

Two fits into seven three times (3 × 2 = 6), and 1 is left over. When the division works out exactly, the remainder is 0. That's how you recognize even numbers: an even number divided by two leaves a remainder of 0, an odd number leaves 1. The % operator works only with whole numbers.

If you want a decimal result, at least one of the numbers has to be a decimal number:

std::cout << 7.0 / 2 << "\n";
std::cout << 7 / 2.0 << "\n";
std::cout << 10.0 / 3 << "\n";

This prints:

3.5
3.5
3.33333

std::cout prints decimal numbers with at most 6 significant digits, which is why you see 3.33333.

A typical mistake is to think that storing the result in a double is enough:

double half = 7 / 2;
std::cout << half << "\n";

This prints:

3

C++ calculates 7 / 2 first. Both numbers are whole, so the result is 3, and only then is that 3 stored in half. The type of the variable on the left of = doesn't matter.

And what if you divide two variables of type int? Convert one of them to a decimal number with static_cast<double>(...):

int points = 7;
int games = 2;
std::cout << static_cast<double>(points) / games << "\n";

This prints:

3.5

static_cast<double>(points) takes the value of points as a decimal number. The variable itself stays an int.