C User Input

Getting User Input

You already know printf() is used to print output. To do the opposite (read input) you use scanf(). Both functions live in the <stdio.h> header, so make sure you include it at the top of your file. When scanf() runs, the program pauses and waits for the user to type something and press Enter.

#include <stdio.h>

int main() {
  int age;
  printf("Enter your age: ");
  scanf("%d", &age);
  printf("You are %d years old.\n", age);
  return 0;
}

The first argument to scanf() is a format specifier that tells C what kind of data to expect. This is the same idea you saw with printf(). Use %d for an integer, %f for a float, and %c for a single character. The second argument is the address of the variable where the value should be stored.

Reading More Than One Value

You can ask for several values in a single scanf() call by listing multiple format specifiers and multiple addresses. When the user types the values separated by spaces (or Enter), C fills each variable in order.

#include <stdio.h>

int main() {
  int first, second;
  printf("Enter two numbers: ");
  scanf("%d %d", &first, &second);
  printf("The sum is %d\n", first + second);
  return 0;
}

Reading a Word of Text

To read text you use a char array (a string) with the %s specifier. Because the array name already refers to a memory address, you do NOT put an & before it. Keep in mind that %s reads only up to the first space, so it captures a single word, not a full sentence.

#include <stdio.h>

int main() {
  char firstName[30];
  printf("Enter your first name: ");
  scanf("%s", firstName);
  printf("Hello, %s!\n", firstName);
  return 0;
}