Unit 2: Control Statements

• Decision making statements: if, if else, else if ladder, switch statements.
• Loop control statements: while loop, for loop and do-while loop.
• Jump Control statements: break, continue and goto.

Control Statements allow a programmer to:

• Make decisions based on conditions
• Repeat a block of code multiple times
• Skip, stop or jump to another part of the program
• Control the logical flow and structure of the program

These are divided into 3 major categories:

1. Decision Making
2. Loop Control
3. Jump Control

1. Decision Making Statements

Decision making statements help the program choose what to do based on some condition (true/false).

If Statement

Executes a block of code if a condition is true.

When condition is true: The block of code inside it executes.
When condition is false: Nothing happens.

Syntax:

if (condition) {
    // Code to be executed if the condition is true
}

Example

#include <stdio.h>

int main() {
    int n;
    printf("Enter a number: ");
    scanf("%d", &n);
    
    if (n % 5 == 0) {
        printf("Divisible by 5");
    }
    return 0;
}

Output:

Divisible by 5

If-Else Statement

Executes one block of code if a condition is true and another block if the condition is false.

Used when: Only two possibilities exist:

1. Condition is true → Run if block
2. Condition is false → Run else block

Syntax:

if (condition) {
    // Code to be executed if the condition is true
} else {
    // Code to be executed if the condition is false
}

Example

int x = 3;
if (x > 5) {
    printf("x is greater than 5");
} else {
    printf("x is not greater than 5");
}

Example

Check if number is even or odd

#include <stdio.h>

int main() {
    int a;
    scanf("%d", &a);
    
    if (a % 2 == 0) {
        printf("Even");
    } else {
        printf("Odd");
    }
    return 0;
}

If-Else-If Ladder

Tests multiple conditions sequentially and executes the block of code corresponding to the first true condition. Used when Multiple conditions needs to checked.

Syntax:

if (condition1) {
    // Code to be executed if condition1 is true
} else if (condition2) {
    // Code to be executed if condition2 is true
} else {
    // Code to be executed if none of the conditions are true
}

Example

int x = 5;
if (x > 10) {
    printf("x is greater than 10");
} else if (x > 5) {
    printf("x is greater than 5");
} else {
    printf("x is not greater than 10 or 5");
}

Example

Find grade based on marks

#include <stdio.h>

int main() {
    int marks;
    printf("Enter your marks: ");
    scanf("%d", &marks);
    
    if (marks >= 90) {
        printf("Grade A\n");
    } else if (marks >= 75) {
        printf("Grade B\n");
    } else if (marks >= 50) {
        printf("Grade C\n");
    } else {
        printf("Fail\n");
    }
    return 0;
}

Output:

Enter your marks: 80
Grade B

Nested If Statements

Places an if statement inside another if statement.

Syntax:

if (condition1) {
    // Code to be executed if condition1 is true
    if (condition2) {
        // Code to be executed if condition2 is true
    }
}

Example

Check if number is divisible by 2 and 4.

#include <stdio.h>

int main() {
    int a;
    printf("Enter a number: ");
    scanf("%d", &a);
    
    if (a % 2 == 0) {
        if (a % 4 == 0) {
            printf("It is divisible by 2 and 4");
        } else {
            printf("It is only divisible by 2");
        }
    } else {
        printf("It is not divisible by 2 and 4");
    }
    return 0;
}

Output:

Enter a number: 24
It is divisible by 2 and 4

Switch Statement

Performs arithmetic operations based on user choice.

Program Example:

#include <stdio.h>

int main() {
    int choice;
    int num1, num2, result;
    
    printf("Enter 2 numbers: ");
    scanf("%d %d", &num1, &num2);
    
    printf("Choose an operation:\n");
    printf("1. Addition\n");
    printf("2. Subtraction\n");
    printf("3. Multiplication\n");
    printf("4. Division\n");
    printf("Enter your choice: ");
    scanf("%d", &choice);
    
    switch(choice) {
        case 1:
            result = num1 + num2;
            printf("%d\n", result);
            break;
            
        case 2:
            result = num1 - num2;
            printf("%d\n", result);
            break;
            
        case 3:
            result = num1 * num2;
            printf("%d\n", result);
            break;
            
        case 4:
            if (num2 == 0) {
                printf("Division by zero is not possible\n");
            } else {
                result = num1 / num2;
                printf("%d\n", result);
            }
            break;
            
        default:
            printf("Invalid choice\n");
    }
    return 0;
}

2. Iterative & Looping Statements

Iterative and loop statements mean repetition statements. In this, the action is repeated over and again.

There are 2 types of iterative statements:

1. Entry Controlled Loop
2. Exit Controlled Loop

Entry Controlled Loop

In the entry controlled loop, the conditions are tested before the start of the loop execution. If the conditions are not satisfied, the body of the loop won't be executed.

Example

for loop, while loop

Exit Controlled Loop

In the exit controlled loop, the test is performed at the end of the body of the loop, and the body is executed unconditionally for the first time.

Example

do-while loop

---

While Loop

A while loop is the most straightforward looping structure.

Syntax:

while (condition) {
    // Statements
}

Steps:

1. The condition is checked first
2. If the condition is true, the loop runs
3. If false, the loop doesn't execute even once

Example

Print 1 to 10 numbers**

#include <stdio.h>

int main() {
    int i = 1;
    while (i <= 10) {
        printf("%d\n", i);
        i++;
    }
    return 0;
}

Output:

1
2
3
4
5
6
7
8
9
10

Example

Factorial of a number

#include <stdio.h>

int main() {
    int a, i = 1;
    int factorial = 1;
    
    printf("Enter a number: ");
    scanf("%d", &a);
    
    while (i <= a) {
        factorial = factorial * i;
        i++;
    }
    
    printf("Factorial of the given number is: %d", factorial);
    return 0;
}

Output:

Enter a number: 5
Factorial of the given number is: 120

Example

Fibonacci Series

#include <stdio.h>

int main() {
    int a = 0, b = 1, next, n;
    int i = 1;
    
    printf("Enter number of terms to print: ");
    scanf("%d", &n);
    
    while (i <= n) {
        printf("%d ", a);
        next = a + b;
        a = b;
        b = next;
        i++;
    }
    return 0;
}

Output:

Enter number of terms to print: 5

0 1 1 2 3

Do-While Loop

Similar to the while loop except that the condition is always executed after the body of a loop. It is called an exit controlled loop.

Syntax:

do {
    // Statements
} while (condition);

Example

Print first 100 natural numbers

#include <stdio.h>

int main() {
    int i = 1;
    do {
        printf("%d\n", i);
        i++;
    } while (i <= 100);
    return 0;
}

Output:

1
2
3

Example

Prints 2 table**

#include <stdio.h>

int main() {
    int i = 1;
    
    do {
        printf("%d\n", i * 2);
        i++;
    } while (i <= 10);
    return 0;
}

Output:

2
4
6
8
10
12
14
16
18
20

Differences b/w While Loop and Do-While Loop

While Loop	Do-While Loop
Top tested loop	Bottom tested loop
Condition is first tested. If condition is true, then the block is executed	It executes the body once, after it checks the condition. If it is true, the body is executed until the condition becomes false
Loop won't be executed if the condition is false	Loop is executed at least once even if the condition is false

For Loop

A for loop is an entry controlled loop. It is used when you know exactly how many times the loop should run. For loop has 3 parts:

1. Initialization
2. Condition
3. Update (written in one line)

It is the most commonly used loop in C.

Note

I found a use case for this. Processing images from 1 to x or a custom value.

Syntax:

for (initialization; condition; update) {
    // Statements
}

Explanation of each part:

1. Initialization: Executed only once before the loop starts Example: int i = 1;
2. Condition: Checking is written here
   Example: i <= 10
3. Update: Runs after the loop body in each iteration
   Example: i++ or i = i + 1

Example

Print 1 to 10 numbers**

#include <stdio.h>

int main() {
    int i;
    for (i = 1; i <= 10; i++) {
        printf("%d\n", i);
    }
    return 0;
}

Output:

1
2
3
4
5
6
7
8
9
10

3. Jumping/Unconditional Branching Statements

There are 3 types:

1. Break
2. Continue
3. Goto

Break Statement

A break statement terminates the execution of the loop, and the control is transferred to the statement immediately following the loop. The break statement is used to terminate a loop or to exit from a switch.

When to use:

• It can be used within for, while, do-while loops
• In case of nested loops, it terminates the control of the inner loop only

Syntax:

break;

Example

Breaks when i = 5.

#include <stdio.h>

int main() {
    int i;
    for (i = 1; i <= 10; i++) {
        printf("%d\n", i);
        if (i == 5) 
      {
      break;
      }
    }
    return 0;
}

Output:

1
2
3
4
5

Continue Statement

It is used to bypass the remainder of the current pass through a loop. The loop doesn't terminate when a continue statement is used; instead, the remaining loop statements are skipped and the computation proceeds directly to the next pass through the loop.

Syntax:

continue;

Example

#include <stdio.h>

int main() {
    int i;
    for (i = 1; i <= 10; i++) {
        if (i >= 5) {
            continue;
        }
        printf("%d\n", i);
    }
    return 0;
}

Output:

1
2
3
4

Goto Statement

The goto statement is used to jump from one line of code to another line without checking any conditions.

Syntax:

Forward Jump:

goto label;
// statements
label:
// statements

Backward Jump:

label:
// statements
goto label;

Example

#include <stdio.h>

int main() {
    int a, b;
    scanf("%d %d", &a, &b);
    
    if (a == b)
        goto equal;
    else {
        printf("A and B are not equal");
        exit(0);
    }
    
equal:
    printf("A and B are equal");
    return 0;
}

Output: