Unit 4: Input / Output Statements in C | B. Sc Year: III

🧭 Introduction

In programming, a program becomes meaningful only when it can receive data from the user and display results.
This is handled using input/output (I/O) statements.

In Unit 4, you will learn:

• How to take input from the keyboard

• How to display output neatly on the screen

• How to handle characters, numbers, and strings

• How formatted input/output works in C

This unit is highly practical, and many exam questions directly ask programs from this section.

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🔑 Key Concepts (Explained Simply)

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1️⃣ Single Character Input and Output

C provides special functions to read and write one character at a time.

🔹 Input: getchar()

• Reads one character from the keyboard

🔹 Output: putchar()

• Displays one character on the screen

Example:

char ch;
ch = getchar();
putchar(ch);

✅ Useful when working with single characters instead of full strings.

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2️⃣ Formatted Input Using scanf()

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

Syntax:

scanf("format specifier", &variable);

Common Format Specifiers:

Data Type	Format
int	%d
float	%f
char	%c

Example:

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

⚠️ Remember: & (address operator) is mandatory in scanf().

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3️⃣ Formatted Output Using printf()

The printf() function is used to display output.

Syntax:

printf("message", variable);

Example:

int x = 10;
printf("Value of x = %d", x);

✅ Allows formatted and aligned output.

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4️⃣ String Input and Output (gets() and puts())

🔹 gets() – String Input

• Reads a line of text

• Stores it in a character array

char name[50];
gets(name);

⚠️ Note: gets() is unsafe, but commonly asked in exams.

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🔹 puts() – String Output

• Displays a string

• Automatically moves to the next line

puts(name);

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5️⃣ Escape Sequence Characters

Escape sequences are special characters for formatting output.

Escape	Meaning
\n	New line
\t	Tab
\"	Double quotes
\\	Backslash

Example:

printf("Name:\tRam\nRoll no:\t10");

✅ Very commonly asked in exams.

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6️⃣ Difference Between scanf() and gets()

scanf()	gets()
Used for numbers & words	Used for full strings
Stops at space	Reads entire line
Safer	Unsafe
Requires format specifier	No format specifier

💡 In real programs, fgets() is preferred—but exams focus on gets().

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🧩 Step‑by‑Step Exam Programs

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✅ Program 1: Read and Display a Character

#include <stdio.h>

int main() {
    char ch;
    printf("Enter a character: ");
    ch = getchar();
    printf("You entered: ");
    putchar(ch);
    return 0;
}

✅ Reads a single character and displays it using putchar().

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✅ Program 2: Read Two Floating Numbers and Display Sum

#include <stdio.h>

int main() {
    float a, b, sum;
    printf("Enter two numbers: ");
    scanf("%f %f", &a, &b);
    sum = a + b;
    printf("Sum = %.2f", sum);
    return 0;
}

✅ "%.2f" prints output up to two decimal places.

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✅ Program 3: Use of gets() and puts()

#include <stdio.h>

int main() {
    char name[50];
    printf("Enter your name: ");
    gets(name);
    puts("Your name is:");
    puts(name);
    return 0;
}

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✅ Program 4: Read Roll Number, Name, Address (Formatted Output)

#include <stdio.h>

int main() {
    char name[30], address[50];
    int roll;

    printf("Enter name: ");
    gets(name);

    printf("Enter address: ");
    gets(address);

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

    printf("\nName:\t%s\nAddress:\t%s\nRoll No:\t%d",
           name, address, roll);

    return 0;
}

✅ Demonstrates escape sequences and string handling.

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📝 Important Exam Questions

1. Explain scanf() and printf() with examples

2. Write a program using getchar() and putchar()

3. Write a program using gets() and puts()

4. What are escape sequences? Explain any four

5. Differentiate scanf() and gets()

6. Write a program to read two numbers and find their sum

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🎯 Exam Tips / Key Points

• Always include:

  • #include <stdio.h>

  • main() function

  • return 0;

• Use correct format specifiers

• Write comments if asked

• Print values with proper formatting

• Programs from this unit are high scoring

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✅ Short Summary

In Unit 4, you learned how C programs interact with users using input/output statements. You studied:

• Character input/output

• Formatted input/output

• String input/output

• Escape sequences

• Exam-ready C programs

This unit is essential for writing interactive C programs and will be repeatedly used in upcoming units.