Input and Output in C

Until now, all our C programs displayed fixed messages on the screen. However, real-world applications need to interact with users by accepting input and displaying meaningful output.

C provides two important functions for this purpose:

• printf() → Displays output.
• scanf() → Accepts input from the user.

In this lesson, you'll learn how to use these functions effectively and write interactive C programs.

🎯 Learning Objectives

After completing this lesson, you will be able to:

• Understand Input and Output in C.
• Use printf() correctly.
• Use scanf() to receive user input.
• Learn format specifiers.
• Understand escape sequences.
• Write interactive C programs.
• Avoid common beginner mistakes.

📖 What is Input?

Input is the data provided by the user to a program.

Examples:

• Name
• Age
• Marks
• Salary
• City

Example

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Enter your age:
20

Here, 20 is the input.

📖 What is Output?

Output is the information displayed by the program.

Example

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Welcome to KauraX

The message shown on the screen is the output.

Input and Output Functions

printf()

printf() stands for Print Formatted.

It is used to display information on the screen.

Syntax

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printf("Message");

Example

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#include <stdio.h>

int main()
{
    printf("Welcome to KauraX");

    return 0;
}

Output

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Welcome to KauraX

Printing Variables

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#include <stdio.h>

int main()
{
    int age = 20;

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

    return 0;
}

Output

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Age = 20

Multiple printf()

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#include <stdio.h>

int main()
{
    printf("Welcome\n");

    printf("To\n");

    printf("KauraX");

    return 0;
}

Output

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Welcome
To
KauraX

Escape Sequences

Escape sequences are special characters beginning with a backslash (\).

Example

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printf("Name\tAge\n");
printf("Rahul\t20");

Output

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Name    Age
Rahul   20

scanf()

scanf() is used to read input from the keyboard.

Syntax

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scanf("format", &variable);

Example

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#include <stdio.h>

int main()
{
    int age;

    printf("Enter your age: ");

    scanf("%d", &age);

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

    return 0;
}

Output

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Enter your age: 20
Your age is 20

Why '&' is Used?

The & (address-of operator) tells scanf() where the variable is stored in memory.

Example

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scanf("%d", &age);

Without &, the compiler cannot correctly store the input.

Reading Multiple Values

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#include <stdio.h>

int main()
{
    int a, b;

    printf("Enter two numbers: ");

    scanf("%d %d", &a, &b);

    printf("First = %d\n", a);

    printf("Second = %d", b);

    return 0;
}

Example Output

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Enter two numbers: 15 25

First = 15

Second = 25

Reading Float Values

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#include <stdio.h>

int main()
{
    float cgpa;

    printf("Enter CGPA: ");

    scanf("%f", &cgpa);

    printf("CGPA = %.2f", cgpa);

    return 0;
}

Reading Character Input

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#include <stdio.h>

int main()
{
    char grade;

    printf("Enter Grade: ");

    scanf(" %c", &grade);

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

    return 0;
}

Notice the space before %c.

It prevents scanf() from reading a leftover newline character.

Reading Strings

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#include <stdio.h>

int main()
{
    char name[30];

    printf("Enter Name: ");

    scanf("%s", name);

    printf("Welcome %s", name);

    return 0;
}

Output

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Enter Name: KauraX

Welcome KauraX

Format Specifiers

Complete Example

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#include <stdio.h>

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

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

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

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

    printf("\n----- Student Details -----\n");

    printf("Name : %s\n", name);

    printf("Age  : %d\n", age);

    printf("CGPA : %.2f\n", cgpa);

    return 0;
}

Example Output

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Enter your name: Rahul
Enter your age: 20
Enter your CGPA: 8.65

----- Student Details -----

Name : Rahul
Age  : 20
CGPA : 8.65

Common Beginner Mistakes

Forgetting '&'

Wrong

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scanf("%d", age);

Correct

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scanf("%d", &age);

Wrong Format Specifier

Wrong

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float marks;

scanf("%d", &marks);

Correct

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scanf("%f", &marks);

Missing Quotes

Wrong

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printf(Welcome);

Correct

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printf("Welcome");

Using %s for Integer

Wrong

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scanf("%s", &age);

Correct

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scanf("%d", &age);

Best Practices

• Display a meaningful prompt before taking input.
• Use the correct format specifier.
• Always initialize variables when appropriate.
• Validate user input in larger applications.
• Keep the output neat and readable.

Real-World Example

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#include <stdio.h>

int main()
{
    char student[30];
    int courseCount;

    printf("Welcome to KauraX\n\n");

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

    printf("Enter completed courses: ");
    scanf("%d", &courseCount);

    printf("\nHello %s!\n", student);
    printf("You have completed %d courses on KauraX.\n", courseCount);

    return 0;
}

🧠 Interview Questions

1. What is printf()?
2. What is scanf()?
3. Why is & used in scanf()?
4. What is a format specifier?
5. Differentiate between input and output.
6. What are escape sequences?
7. Why is %c often written as " %c" in scanf()?

✍ Practice Questions

1. Read and print your name.
2. Read two numbers and display them.
3. Read your age and CGPA.
4. Display data in a formatted table.
5. Create a simple student information program.

📚 Lesson Summary

In this lesson, you learned:

• Input and Output in C.
• printf() and scanf().
• Escape sequences.
• Format specifiers.
• Reading integers, floats, characters, and strings.
• Common mistakes.
• Best practices.

Interactive programs become possible using input and output functions, making your applications much more useful.

🚀 What's Next?

In the next lesson, Operators in C, you'll learn:

• Arithmetic Operators
• Relational Operators
• Logical Operators
• Assignment Operators
• Increment & Decrement Operators
• Bitwise Operators
• Operator Precedence
• Practical Programs
• Interview Questions

Happy Coding with KauraX 💙