Variables: Boxes for Your Data

What is a variable?

A variable is a named place in the computer's memory where you keep a value, so you can use it later.

Think of it like this: A variable is like a labelled box. The label is the name (age), and inside the box is the value (25). In C every box also has a type: a box made for whole numbers can only hold whole numbers.

Creating a variable

To create a variable you write its type, its name, and a starting value:

#include <stdio.h>

int main() {
    int age = 25;
    printf("My age is %d\n", age);
    return 0;
}

Output

My age is 25

• int is the type: a whole number (an integer).
• age is the name we chose.
• = 25 puts the value 25 into the box.

Printing a variable: %d

To print a variable, printf uses a placeholder. %d means "put a whole number here". After the text, add a comma and the variable. When the program runs, %d is replaced by the value.

You can have several placeholders. They are filled in order, left to right:

#include <stdio.h>

int main() {
    int apples = 3;
    int oranges = 5;
    printf("%d apples and %d oranges\n", apples, oranges);
    printf("Total: %d\n", apples + oranges);
    return 0;
}

Output

3 apples and 5 oranges
Total: 8

Changing a variable

A variable can change – that's why it is called variable. Use = again to put a new value in. The old value is thrown away:

#include <stdio.h>

int main() {
    int score = 10;
    printf("Score: %d\n", score);

    score = 50;               // no "int" this time: the box already exists
    printf("Score: %d\n", score);

    score = score + 5;        // take the old value, add 5, store it back
    printf("Score: %d\n", score);
    return 0;
}

Output

Score: 10
Score: 50
Score: 55

Remember: You write the type (int) only once, when you create the variable. After that you just use its name.

Common mistake: Using a variable before giving it a value. int x; printf("%d", x); prints a random leftover number from memory. Always give your variables a starting value.