Getting a Decimal Result

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

printf("%f\n", 7 / 2.0);
printf("%.1f\n", 7.0 / 2);

This prints:

3.500000
3.5

The result is a double, so you print it with %f or %.1f. With %d, it would be a compile error, as you know.

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

double half = 7 / 2;
printf("%f\n", half);

This prints:

3.000000

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 change how the right side is calculated.

The same thing happens with variables. A player scored 17 points in 4 games. What's the average?

int points = 17;
int games = 4;
double average = points / games;
printf("Average: %.2f\n", average);

This prints:

Average: 4.00

That's wrong: the right answer is 4.25. You can't add .0 to a variable, but you can convert its value to a double. Write the type in parentheses in front of it:

int points = 17;
int games = 4;
double average = (double)points / games;
printf("Average: %.2f\n", average);

This prints:

Average: 4.25

(double)points takes the value of points as a decimal number, 17.0. And a decimal number divided by a whole number gives a decimal result. The variable points itself stays an int. This kind of conversion is called a cast.

Watch where you put it. (double)(points / games) doesn't help: the parentheses make C divide first, and only the result, 4, is converted to 4.0.