Input and Output

A program becomes much more useful when it can communicate with the user.

For example, we may want to:

• Display a message on the screen

• Ask the user for their age

• Take two numbers and calculate their sum

In C, these operations are called Input and Output (I/O).

What is Output?

Output means displaying information from the program to the user.

The most common function for output in C is printf().

#include <stdio.h>

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

    return 0;
}

Output:

Hello, World!

We can also print numbers and variables:

int age = 20;

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

Output:

Age = 20

Here, %d tells printf() that we want to display an integer.

What is Input?

Input means taking data from the user and using it in the program.

The most commonly used function for taking input in C is scanf().

For example:

int age;

scanf("%d", &age);

Here, the user enters an integer, and that value is stored in the age variable.

The & is related to the memory address of the variable. We'll understand exactly why it is needed when we learn pointers.

Simple Input and Output Example

Let's take the user's age and display it:

#include <stdio.h>

int main()
{
    int age;

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

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

    return 0;
}

If the user enters:

20

The output will be:

Enter your age: 20
Your age is 20

So the flow is:

User enters data
       ↓
     scanf()
       ↓
   Variable
       ↓
    printf()
       ↓
   Screen output

Common Format Specifiers

printf() and scanf() use format specifiers to work with different data types.

For example:

int age = 20;
float price = 99.5;
char grade = 'A';

printf("%d\n", age);
printf("%f\n", price);
printf("%c\n", grade);

In Simple Words

Input is data received by the program, while output is information produced by the program.

In C, you'll mainly use:

• printf() → Output

• scanf() → Input

These two functions will appear again and again throughout your C programs, so getting comfortable with them is important.