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Input and Output

Read numbers and text from the keyboard with scanf and format output with printf.

printf writes formatted output, and its partner scanf reads formatted input. Both live in stdio.h.

Reading a number

scanf needs the address of the variable to fill, which you get with &. You will learn more about addresses in the pointers lesson.

C
Open in playground
#include <stdio.h>

int main(void) {
    int age;
    printf("Enter your age: ");
    scanf("%d", &age);
    printf("In ten years you will be %d.\n", age + 10);
    return 0;
}

A sample run, where the user typed 19:

text
Enter your age: 19
In ten years you will be 29.

Reading several values

One scanf call can read several values separated by spaces. It returns how many values it read successfully, which is a simple way to check the input.

C
Open in playground
#include <stdio.h>

int main(void) {
    double length, width;
    printf("Enter length and width: ");
    if (scanf("%lf %lf", &length, &width) != 2) {
        printf("Please enter two numbers.\n");
        return 1;
    }
    printf("Area = %.2f\n", length * width);
    return 0;
}

Format specifiers for printf

C
Open in playground
#include <stdio.h>

int main(void) {
    int qty = 3;
    double price = 49.5;
    char grade = 'A';

    printf("[%d]\n", qty);
    printf("[%5d]\n", qty);      /* right-aligned in 5 spaces */
    printf("[%-5d]\n", qty);     /* left-aligned in 5 spaces */
    printf("[%05d]\n", qty);     /* padded with zeros */
    printf("[%.2f]\n", price);
    printf("[%8.1f]\n", price);
    printf("[%c] [%s]\n", grade, "text");
    return 0;
}
Output
[3]
[    3]
[3    ]
[00003]
[49.50]
[    49.5]
[A] [text]

GATE-style practice questions

GATE-styleMCQ · one correct answer

What does the following program print? (The _ characters are not printed; they are only to show where the output starts and ends.)

#include <stdio.h>
int main(void) {
    printf("_%5.2f_%-4d_%04d_\n", 3.14159, 42, 42);
    return 0;
}
Show answer and explanationAnswer: B

%5.2f prints 3.14 (4 characters) right-aligned in width 5, adding one space in front. %-4d prints 42 left-aligned in width 4, adding two spaces after. %04d pads with zeros to width 4: 0042.

GATE-styleNAT · numerical answer

The call scanf("%d %d", &a, &b) is given the input 12 abc. What value does scanf return?

Show answer and explanationAnswer: 1

scanf returns the number of items successfully read and stored. 12 is read into a, then reading an int from "abc" fails, so it stops and returns 1.