scanf

Learn to read input from the user with scanf.

Reading input with scanf

The scanf function reads input from the keyboard. The format specifier tells the type to read; the second argument must be the address of the variable. The & sign gives the address.

int age;
printf("Age: ");
scanf("%d", &age);
printf("Hello, you are %d!\n", age);

The screen first shows Age: , the user types a number and presses Enter. scanf writes this value to the memory address of age. Forgetting & is a common mistake; the program may crash.

The scanf return value

scanf returns the number of items successfully read. If the user types a letter instead of a number, the read fails and the return value is 0. So checking the input is a good habit.

int n;
int result = scanf("%d", &n);
if (result == 1) {
    printf("Read: %d\n", n);
} else {
    printf("Invalid input!\n");
}

You'll see the if statement in detail in the next section; for now it's enough to know that checking the scanf return value is possible.

Exercise Read an age input and print it

Write a C program. Declare int age;. Print a request line with printf, read the input with scanf("%d", &age);, then print the result with printf using the %d specifier. Put #include <stdio.h> at the top.

Show example solution

#include <stdio.h>

int main(void) {
    int age;
    printf("Age: ");
    scanf("%d", &age);
    printf("Your age: %d\n", age);
    return 0;
}

Test yourself

Add the address operator
```
scanf("%d", ____age);
```
Show answer
```
scanf("%d", &age);
```
What does scanf("%d", &n) do?
```
int n;
scanf("%d", &n);
```
- Prints the address of n
- Prints an integer to the screen
- Resets n to zero
- Reads an integer from the keyboard
Show answer

Reads an integer from the keyboard — scanf reads an integer from the keyboard into n.
Match the scanf pieces

%d

Reads an integer
&age

Gives the address
scanf

Input function
printf

Output function

Next lesson: Multiple inputs

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