Control Statements in C Language

1. What are Control Statements?

Control statements are statements used to change the normal flow of execution of a program.

Normal Flow of a Program

Normally, a C program executes statements:

• One after another
• From top to bottom
• Each statement is executed exactly once, unless the flow is changed

#include <stdio.h>

int main()
{
    printf("First\n");
    printf("Second\n");
    printf("Third\n");

    return 0;
}

First
Second
Third

2. Types of Control Statements

There are three major types of control statements in C:

3. Selection Statements in C

Selection statements are used when a program has to make a decision based on a condition.

1. Simple if
2. Multiple if
3. if...else
4. Nested if
5. else...if ladder
6. switch...case

4. Simple if Statement

A simple if statement is used to execute a group of statements only when a given condition is true.

Syntax

if (condition)
{
    statements;
}

How it Works

1. The condition is checked.
2. If the condition is true, the statements inside the if block execute.
3. If the condition is false, the if block is skipped.
4. The program continues with the next statement.

5. Example: Check Whether a Number is Natural

Problem

Write an algorithm and draw a flowchart to determine whether the value of a variable is a natural number.

Logic

For this example, we will consider positive integers greater than 0 as natural numbers.

Therefore, the condition is:

x > 0

Algorithm

1. Start
2. Declare an integer variable x
3. Read the value of x
4. Check whether x > 0
5. If true, display “x is a natural number”
6. Stop

Flowchart

C Program

#include <stdio.h>

int main()
{
    int x = 12;

    if (x > 0)
    {
        printf("%d is a natural number", x);
    }

    printf("\nRest of the program");

    return 0;
}

Output

12 is a natural number
Rest of the program

6. Example 1: Voting Eligibility

Question

Write a C program to accept age from the user and display “Eligible for voting” when the age entered is greater than 18.

Hint

age > 18

Program

#include <stdio.h>

int main()
{
    int age;

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

    if (age > 18)
    {
        printf("Eligible for voting");
    }

    return 0;
}

Sample Output

Enter your age: 25
Eligible for voting

7. Example 2: Even or Odd

Question

Write a C program to determine whether the number entered by the user is even or odd.

Hint

Use the modulus operator %.

x % 2 == 0   → Even
x % 2 != 0   → Odd

Simple if Approach

#include <stdio.h>

int main()
{
    int x;

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

    if (x % 2 == 0)
    {
        printf("%d is even", x);
    }

    if (x % 2 != 0)
    {
        printf("%d is odd", x);
    }

    return 0;
}

Sample Output

Enter a number: 24
24 is even

8. Example 3: Positive, Negative or Zero

Question

Write C code to determine whether the number entered by the user is greater than, less than, or equal to zero.

Conditions

Program Using Multiple if Statements

#include <stdio.h>

int main()
{
    int a;

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

    if (a > 0)
    {
        printf("Positive number");
    }

    if (a < 0)
    {
        printf("Negative number");
    }

    if (a == 0)
    {
        printf("Zero");
    }

    return 0;
}

Sample Outputs

Enter a number: 15
Positive number

Enter a number: -8
Negative number

Enter a number: 0
Zero

9. Example 4: Number in Words

Question

Write a C program to accept any number out of 2, 4, 6, 8 from the user and display the input number in words.

Example

Enter any number from 2, 4, 6, 8: 4
FOUR

Approach

Since the program has to check several possible values, we can use if...else if.

Program

#include <stdio.h>

int main()
{
    int num;

    printf("Enter any number from 2, 4, 6, 8: ");
    scanf("%d", &num);

    if (num == 2)
    {
        printf("TWO");
    }
    else if (num == 4)
    {
        printf("FOUR");
    }
    else if (num == 6)
    {
        printf("SIX");
    }
    else if (num == 8)
    {
        printf("EIGHT");
    }
    else
    {
        printf("Invalid number");
    }

    return 0;
}

Sample Output

Enter any number from 2, 4, 6, 8: 4
FOUR

10. Important Operators Used in Conditions

11. Practice Questions

12. Quick Revision

• Control statements change the normal flow of execution.
• Selection statements are used for decision-making.
• Simple if executes a block when a condition is true.
• Multiple if uses more than one independent condition.
• if…else selects between two alternatives.
• Nested if means an if statement inside another if statement.
• else…if is useful when there are multiple alternatives.
• switch…case is useful for selecting one option from multiple fixed choices.
• == is used for comparison.
• = is used for assignment.
• % gives the remainder.