🎯 Unit 3: Decision Making and Branching

Welcome back, coder! 🚀
This time, we’re learning how to make our C programs think and choose. Yep — like teaching your code how to make decisions.

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🤔 Introduction to Decision Making

Decision making structures let programs choose different paths depending on conditions.
Think of it like asking: “If it’s raining, should I take an umbrella? If not, sunglasses!” 😎☔

C gives us:

1. ✅ Simple if
2. ✅ if-else
3. ✅ Nested if
4. ✅ else-if ladder
5. ✅ switch-case

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🔹 The if Statement

Syntax

if (condition) {
    // Executes if condition is true
}

Flow:

Condition → True → Run block
          → False → Skip block

Example

#include <stdio.h>

int main() {
    int age = 20;
    if (age >= 18) {
        printf("You are eligible to vote.\n");
    }
    printf("Thank you!\n");
    return 0;
}

💡 Tip: if doesn’t need an else. It can stand alone.

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🔸 The if-else Statement

Gives us two paths — one for true, one for false.

if (condition) {
    // True path
} else {
    // False path
}

Example: Even or Odd

Code

#include <stdio.h>

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

    if (number % 2 == 0) {
        printf("%d is even.\n", number);
    } else {
        printf("%d is odd.\n", number);
    }
    return 0;

}

Sample Run

Enter a number: 7
7 is odd.

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🔹 Nested if Statements

One if inside another.

if (condition1) {
    if (condition2) {
        // Both true
    } else {
        // Only condition1 true
    }
} else {
    // condition1 false
}

Example: Driving Eligibility

if (age >= 18) {
    if (hasLicense == 'y') {
        printf("You can drive!\n");
    } else {
        printf("You need a license.\n");
    }
} else {
    printf("You must be 18+.\n");
}

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🔸 The else-if Ladder

Perfect for checking multiple conditions.

if (marks >= 90) grade = 'A';
else if (marks >= 80) grade = 'B';
else if (marks >= 70) grade = 'C';
else grade = 'F';

Example: Grade Calculator

if (marks < 0 || marks > 100) {
    printf("Invalid!\n");
} else if (marks >= 90) {
    printf("Grade A\n");
} else if (marks >= 80) {
    printf("Grade B\n");
} else {
    printf("Grade C or below\n");
}

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🔹 The switch-case Statement

Efficient multi-way branching.

switch (expression) {
    case value1: // do something
        break;
    case value2:
        break;
    default:
        // fallback
}

Rules

• Expression must be int/char
• Case values = constants only
• break prevents fall-through
• default is optional but useful

Example: Calculator

switch(op) {
    case '+': result = a + b; break;
    case '-': result = a - b; break;
    case '*': result = a * b; break;
    case '/':
        if (b != 0) result = a / b;
        else printf("Division by 0!\n");
        break;
    default: printf("Invalid operator\n");
}

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⚖️ if-else vs switch

Feature	if-else	switch
Condition type	Any	int/char only
Multiple checks	✅ Flexible	❌ Only equality
Floating point	✅ Yes	❌ No
Range checking	✅ Easy	❌ Hard
Performance	Slower for many checks	Faster for many cases
Best for	Complex conditions	Menu-driven apps

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🧪 Practical Mini Projects

Leap Year Checker

if (year % 400 == 0) leap = 1;
else if (year % 100 == 0) leap = 0;
else if (year % 4 == 0) leap = 1;
else leap = 0;

Triangle Validator

Checks if sides form a triangle and its type (Equilateral, Isosceles, Scalene).

Student Grade System

Use if-else ladder for grades + switch for remarks. Perfect combo!

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🎓 Quick Quiz

Which statement is faster for checking 10 fixed integer values?

• if-else ladder
• switch-case ✅
• Nested if
• None of the above

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👏 That’s it for Unit 3!
Now your code can decide, branch, and react like a pro.
Next stop: Unit 4: Loops 🔄