Variables (Integers)

In programming we often want to store values so we can use them later. The simplest example: numbers. In C, before storing something, we have to tell the language what kind of value we're storing. This is called a type

#include <stdio.h>

int main(void) {
    int age = 18;
    int x = 1;

    printf("%d\n", age);
    printf("%d\n", x);

    return 0;
}

Output

18
1

int is the type for whole numbers (1, 2, 100, -20, 0). C cares about types: it won't let us store a number in a variable without telling it what kind of number it is

Notice the new thing: %d inside printf. This is a format specifier. printf doesn't know on its own how to display an int, we have to tell it: "here's an int, please display it"

• %d → for an int
• \n → new line, same as before

The format specifier (%d) is replaced with the value (the variable that comes after the comma)

We can mix format specifiers with regular text

#include <stdio.h>

int main(void) {
    int age = 60;
    printf("My age is %d\n", age);
    return 0;
}

Output

My age is 60

We can use several specifiers on a single line

#include <stdio.h>

int main(void) {
    int a = 10;
    int b = 20;
    printf("a = %d, b = %d\n", a, b);
    return 0;
}

Output

a = 10, b = 20

The first %d is replaced with a, the second with b, in order

We can do arithmetic, just like any regular calculation

#include <stdio.h>

int main(void) {
    int a = 2;
    int b = 6;
    int c = a + b;
    printf("%d\n", c);
    return 0;
}

Prints 8. Same rule: the right side of = is computed first, then stored on the left

The classic increment by 1 also has a shortcut in C

#include <stdio.h>

int main(void) {
    int n = 10;
    n++;
    printf("%d\n", n);
    return 0;
}

Prints 11. n++ is the same thing as n = n + 1

A little surprise. Let's trace what happens

#include <stdio.h>

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

Prints 3, not 3.5. Why? Because a / b with two ints returns an int: C throws away the decimal part. We'll see how to keep decimals in the next lesson, with float

Mission: New Operator's File

At the computing center, every new operator gets a file with two numbers: their age and the system access level assigned by the administrator.

Write a program that, inside main

• declares an int named age (any whole number you like)
• declares an int named access_level (any whole number you like)
• prints age on one line, with the label Age:
• prints access_level on a separate line, with the label Access:

Example

For an age of 45 and an access_level of 3, your program would print something like

Age: 45
Access: 3