Reading Input with scanf

Asking the user for a value

So far our programs always print the same thing. To make them interactive, we read what the user types. In C we do this with scanf (say "scan-eff"). It works like printf in reverse:

#include <stdio.h>

int main() {
    int age;
    printf("How old are you? ");
    scanf("%d", &age);
    printf("Next year you will be %d.\n", age + 1);
    return 0;
}

Input (what the user types)

14

Output

How old are you? Next year you will be 15.

• int age; makes an empty box for the number.
• "%d" tells scanf to read a whole number.
• &age tells scanf where to put it: in the box age.

What is the &?

The & means "the address of". scanf needs to know where the box is in memory, so it can put the number inside. For now, just remember: with scanf, put & before the variable name. Module 9 explains addresses properly.

Think of it like this: Giving scanf the variable without & is like asking a delivery driver to bring a parcel but not telling them your address.

Common mistake: Forgetting the &: scanf("%d", age); compiles, but crashes or does something strange when it runs.

Reading several values

One scanf can read several values. The user can separate them with spaces or new lines:

#include <stdio.h>

int main() {
    int a, b;
    printf("Type two numbers: ");
    scanf("%d %d", &a, &b);
    printf("%d + %d = %d\n", a, b, a + b);
    return 0;
}

Input (what the user types)

7 5

Output

Type two numbers: 7 + 5 = 12

Tip: int a, b; creates two int variables in one line.

Reading decimals and characters

Use the same placeholders as printf, with one exception: to read a double you write %lf ("long float"):

#include <stdio.h>

int main() {
    double price;
    char size;
    scanf("%lf", &price);
    scanf(" %c", &size);
    printf("Size %c costs %.2f\n", size, price);
    return 0;
}

Input (what the user types)

4.5
M

Output

Size M costs 4.50

The space in " %c" tells scanf to skip any spaces or Enter key presses left over from earlier input, and read the next real character.

Remember: scanf("%d", &x) reads an int, %lf reads a double, " %c" reads a char. Don't forget the &!