Variables (Floats)

For numbers with decimals (like 3.14 or 1.75), int is not enough. C has two types for decimal numbers: float and double

#include <stdio.h>

int main(void) {
    float pi = 3.14f;
    double price = 9.99;

    printf("%f\n", pi);
    printf("%f\n", price);

    return 0;
}

Output

3.140000
9.990000

Two new format specifiers

• %f → for a float or double
• (you'll also see %lf in some C books, mostly for scanf, not for printf. %f works fine for both with printf)

By default, %f prints 6 decimals. If we want fewer, we can tell printf how many digits we want

#include <stdio.h>

int main(void) {
    double pi = 3.14159;
    printf("%.2f\n", pi);
    printf("%.4f\n", pi);
    return 0;
}

Output

3.14
3.1416

The .2 and .4 between % and f mean "this many decimals". A small trick that makes the output much cleaner

What is the difference between float and double?

• float uses less memory but keeps fewer correct digits
• double uses more memory but keeps more precise values

In practice, use double unless you have a strong reason not to. double is the default type for decimal numbers in C

Notice the f at the end of 3.14f in the float example. This tells C "treat this as a float, not a double". Without the f, the value 3.14 would be a double. For double variables, you don't need any suffix

Now, the famous trap from the previous lesson, fixed

#include <stdio.h>

int main(void) {
    int a = 7;
    int b = 2;
    double result = a / b;
    printf("%f\n", result);
    return 0;
}

You'd expect 3.5. But the output is 3.000000. Why?

Because a / b is computed first, with both a and b being ints. The result is 3 (integer division). Only after that, 3 is stored in result as 3.0. By that point, the .5 is already lost forever

To get the real answer, at least one of the operands must be a decimal

#include <stdio.h>

int main(void) {
    double a = 7;
    int b = 2;
    printf("%f\n", a / b);
    return 0;
}

Or we can use a cast to force one of them to be a double

#include <stdio.h>

int main(void) {
    int a = 7;
    int b = 2;
    printf("%f\n", (double) a / b);
    return 0;
}

(double) in front of a says "treat this as a double for this operation". This is called a cast, and it's a tool you'll use a lot in C

Mission: Machine Room Log

The computing center's machine room has a thermometer and a hygrometer connected to a teletype. Every shift, the operator reads the two values and logs them, rounded to 2 decimal places.

Write a program that, inside main

• declares a double named temperature
• declares a double named humidity
• prints temperature on one line, with the label Temperature: , rounded to 2 decimals
• prints humidity on a separate line, with the label Humidity: , rounded to 2 decimals

Example

For a temperature of 21.6 and a humidity of 45.5, your program would print something like

Temperature: 21.60
Humidity: 45.50