Input and Output in C

Programs become meaningful only when they can take input from the user and display output.

In C, this interaction is mainly done using two functions:

• printf() – for output
• scanf() – for input

Both functions belong to the standard input/output library.

Standard Input and Output

C uses standard streams for communication:

• Standard Input – keyboard
• Standard Output – screen

To use input and output functions, we must include:

#include <stdio.h>

The printf() Function

The printf() function is used to display output on the screen.

printf("Hello, World!");

It can also display values of variables using format specifiers.

• %d – integer
• %f – float
• %c – character
• %s – string

int age = 20;
printf("Age is %d", age);

The scanf() Function

The scanf() function is used to read input from the user.

It requires:

• A format specifier
• The address of the variable (using &)

int age;
scanf("%d", &age);

The & symbol tells C where to store the input value in memory.

Complete Example: Input and Output

#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;
}

This program:

• Reads two numbers from the user
• Adds them
• Displays the result

Common Mistakes Beginners Make

• Forgetting & in scanf()
• Using wrong format specifier
• Not including <stdio.h>

These mistakes often cause runtime errors or unexpected results.

Mini Practice

• Write a program to read and print your age
• Read two numbers and display their product
• Read a character and display it

Quick Quiz

Q1. Which header file is needed for input/output?

stdio.h

Q2. Which function is used to display output?

printf()

Q3. Why is & used in scanf?

To store input at the variable's memory address

Q4. Which format specifier is used for float?

%f

Q5. Can scanf read multiple values?

Yes, it can read multiple values in one statement