scanf
- C course
- Beginner
- Lesson 31/116
- 4 min
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.
Practice 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.
Starter code
#include <stdio.h>
int main(void) {
// Declare age, read input and print it
return 0;
}Sample solution
#include <stdio.h>
int main(void) {
int age;
printf("Age: ");
scanf("%d", &age);
printf("Your age: %d\n", age);
return 0;
}Expected output
Age: Your age: 25
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Add the address operator
scanf("%d", ____age);scanf("%d", &age);Explanation: & gives the address of age.
2 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 Correct answer
Explanation: scanf reads an integer from the keyboard into n.
3 Match the scanf pieces
| Term | Meaning |
|---|---|
%d |
Reads an integer |
&age |
Gives the address |
scanf |
Input function |
printf |
Output function |