Input and Output in C

Input is data provided to a program while it is running. For example, a user can enter their name, age, marks, or other information.

Enter your age: 20

Here, 20 is input provided by the user.

Output is information produced by a program and displayed on a screen or sent to another output device.

Hello World

A C program commonly displays output using printf().

C provides standard input/output facilities through the <stdio.h> header.

#include <stdio.h>

This header provides declarations for functions such as printf(), scanf(), getchar(), and putchar().

The printf() function is used to display formatted output.

#include <stdio.h>

int main()
{
    printf("Hello World");

    return 0;
}

Output:

Hello World

Text can be printed by placing it inside double quotation marks.

printf("Welcome to C Programming");

Output:

Welcome to C Programming

The %d conversion specifier can be used to display an int.

int age = 20;

printf("Age = %d", age);

Output:

Age = 20

Floating-point values can be displayed using %f.

float price = 99.50f;

printf("Price = %f", price);

The %c conversion specifier can be used to display a character.

char grade = 'A';

printf("Grade = %c", grade);

Output:

Grade = A

The %s conversion specifier can be used to display a null-terminated character string.

char name[] = "Rahul";

printf("Name = %s", name);

The escape sequence \n moves the cursor to the next line.

printf("Hello\nWorld");

Output:

Hello
World

A single printf() call can display multiple values.

int age = 20;
int marks = 85;

printf("Age = %d, Marks = %d", age, marks);

The scanf() function is used to read formatted input from the standard input stream, normally the keyboard.

int age;

scanf("%d", &age);

Use %d with scanf() to read an int.

int age;

printf("Enter age: ");
scanf("%d", &age);

The value entered by the user is stored in age.

For a typical scalar variable, scanf() needs the address where the input value should be stored. The address-of operator & provides that address.

int age;

scanf("%d", &age);

Use %f in scanf() when reading a float.

float price;

printf("Enter price: ");
scanf("%f", &price);

Use %lf in scanf() to read a double.

double salary;

printf("Enter salary: ");
scanf("%lf", &salary);

The %c conversion specification can be used to read a character.

char grade;

printf("Enter grade: ");
scanf(" %c", &grade);

The leading space before %c tells scanf() to skip leading whitespace.

A character array can be used to store a string.

char name[20];

printf("Enter name: ");
scanf("%19s", name);

The array name already represents the address of its first element, so & is not used here.

With %s, scanf() normally reads characters until whitespace is encountered.

char name[30];

scanf("%29s", name);

For example, entering Rahul works, but a multi-word name would be split at the whitespace.

#include <stdio.h>

int main()
{
    int age;

    printf("Enter your age: ");
    scanf("%d", &age);

    printf("Your age is %d", age);

    return 0;
}

A program can receive input and then display the result.

#include <stdio.h>

int main()
{
    int number;

    printf("Enter a number: ");
    scanf("%d", &number);

    printf("You entered: %d", number);

    return 0;
}

Multiple values can be read using multiple conversion specifications.

int a, b;

printf("Enter two numbers: ");
scanf("%d %d", &a, &b);

The user can enter the values separated by whitespace.

#include <stdio.h>

int main()
{
    int a, b, sum;

    printf("Enter first number: ");
    scanf("%d", &a);

    printf("Enter second number: ");
    scanf("%d", &b);

    sum = a + b;

    printf("Sum = %d", sum);

    return 0;
}

#include <stdio.h>

int main()
{
    float price, quantity;

    printf("Enter price: ");
    scanf("%f", &price);

    printf("Enter quantity: ");
    scanf("%f", &quantity);

    printf("Total = %.2f", price * quantity);

    return 0;
}

The putchar() function writes a single character to standard output.

#include <stdio.h>

int main()
{
    putchar('A');

    return 0;
}

Output:

A

The getchar() function reads one character from standard input.

#include <stdio.h>

int main()
{
    char ch;

    ch = getchar();

    printf("You entered: %c", ch);

    return 0;
}

The printf() function supports width and precision to control formatted output.

float price = 99.5678;

printf("%.2f", price);

Output:

99.57

scanf() returns the number of input items successfully assigned. This can be used to check whether the expected input was received.

int age;

if (scanf("%d", &age) == 1)
{
    printf("Valid input");
}

• Forgetting #include <stdio.h>.
• Using the wrong format specifier.
• Forgetting & when it is required by scanf().
• Using & incorrectly with a character array used as a string.
• Forgetting quotation marks in printf().
• Using an input width that does not safely fit the destination array.

#include <stdio.h>

int main()
{
    char name[30];
    int age;
    float marks;

    printf("Enter your name: ");
    scanf("%29s", name);

    printf("Enter your age: ");
    scanf("%d", &age);

    printf("Enter your marks: ");
    scanf("%f", &marks);

    printf("\n--- Student Details ---\n");
    printf("Name = %s\n", name);
    printf("Age = %d\n", age);
    printf("Marks = %.2f\n", marks);

    return 0;
}

This program demonstrates basic keyboard input and formatted output using different data types.