Control Structures: Selection (if, switch, nested conditions)

TAKEAWAYS:

• Learn how if, else if, and else statements execute code blocks conditionally based on boolean expressions.
• Understand the switch statement for multi-way branching and its efficiency compared to nested if-else.
• Master nested conditions to handle complex decision-making scenarios.
• Recognize logical operators (&&, ||, !) and their role in combining conditions.
• Apply selection structures to real-world problems like loan eligibility, discount calculations, and user input validation.
• Avoid common pitfalls like missing braces, redundant conditions, and infinite loops due to incorrect logic.

1. Introduction to Control Structures (Selection)

Control structures allow a program to make decisions and execute different blocks of code based on conditions. In C, selection structures include:

• Single if statement
• if-else statement
• else if ladder
• switch-case statement
• Nested conditions

These structures enable dynamic execution paths, making programs adapt to user input or data.

2. The if Statement

The simplest selection structure checks a condition and executes a block if true.

Syntax

if (condition) {
    // Code to execute if condition is true
}

How It Works

• The condition is evaluated as a boolean (true/false).
• If true, the block inside {} runs; otherwise, it skips.

Example: Check if a number is positive

flowchart TD
    A["Start"] --> B{"Is number > 0?"}
    B -->|"Yes"| C["Print 'Positive'"]
    B -->|"No"| D["Print 'Non-positive'"]
    C --> E["End"]
    D --> E

Code Example

#include <stdio.h>
int main() {
    int num = 5;
    if (num > 0) {
        printf("%d is positive.\n", num);
    }
    return 0;
}

Output:

5 is positive.

Trace Table

3. The if-else Statement

Adds an alternative block when the condition is false.

Syntax

if (condition) {
    // Code if true
} else {
    // Code if false
}

Example: Check even/odd

flowchart TD
    A["Start"] --> B{"Is number % 2 == 0?"}
    B -->|"Yes"| C["Print 'Even'"]
    B -->|"No"| D["Print 'Odd'"]
    C --> E["End"]
    D --> E

Code Example

#include <stdio.h>
int main() {
    int num = 7;
    if (num % 2 == 0) {
        printf("%d is even.\n", num);
    } else {
        printf("%d is odd.\n", num);
    }
    return 0;
}

Output:

7 is odd.

Trace Table

4. else if Ladder

Handles multiple conditions sequentially.

Syntax

if (condition1) {
    // Code if condition1 is true
} else if (condition2) {
    // Code if condition2 is true
} else {
    // Default code
}

Example: Grade classification

flowchart TD
    A["Start"] --> B{"Marks >= 80?"}
    B -->|"Yes"| C["Print 'A'"]
    B -->|"No"| D{"Marks >= 60?"}
    D -->|"Yes"| E["Print 'B'"]
    D -->|"No"| F{"Marks >= 40?"}
    F -->|"Yes"| G["Print 'C'"]
    F -->|"No"| H["Print 'Fail'"]
    C --> I["End"]
    E --> I
    G --> I
    H --> I

Code Example

#include <stdio.h>
int main() {
    int marks = 75;
    if (marks >= 80) {
        printf("Grade: A\n");
    } else if (marks >= 60) {
        printf("Grade: B\n");
    } else if (marks >= 40) {
        printf("Grade: C\n");
    } else {
        printf("Grade: Fail\n");
    }
    return 0;
}

Output:

Grade: B

Trace Table

5. The switch-case Statement

Efficient for multi-way branching (e.g., menu-driven programs).

Syntax

switch (expression) {
    case value1:
        // Code if expression == value1
        break;
    case value2:
        // Code if expression == value2
        break;
    default:
        // Default code
}

Key Points

• expression must be an integer, char, or enum.
• break exits the switch; without it, execution "falls through."
• default runs if no case matches.

Example: Day of the week

flowchart TD
    A["Start"] --> B["Input day (1-7)"]
    B --> C{"day == 1?"}
    C -->|"Yes"| D["Print 'Monday'"]
    C -->|"No"| E{"day == 2?"}
    E -->|"Yes"| F["Print 'Tuesday'"]
    E -->|"No"| G{"day == 3?"}
    G -->|"Yes"| H["Print 'Wednesday'"]
    G -->|"No"| I{"day == 4?"}
    I -->|"Yes"| J["Print 'Thursday'"]
    I -->|"No"| K{"day == 5?"}
    K -->|"Yes"| L["Print 'Friday'"]
    K -->|"No"| M{"day == 6?"}
    M -->|"Yes"| N["Print 'Saturday'"]
    M -->|"No"| O["Print 'Sunday'"]
    D --> P["End"]
    F --> P
    H --> P
    J --> P
    L --> P
    N --> P
    O --> P

Code Example

#include <stdio.h>
int main() {
    int day = 3;
    switch (day) {
        case 1:
            printf("Monday\n");
            break;
        case 2:
            printf("Tuesday\n");
            break;
        case 3:
            printf("Wednesday\n");
            break;
        default:
            printf("Invalid day\n");
    }
    return 0;
}

Output:

Wednesday

Trace Table

6. Nested Conditions

Combines multiple if-else or switch statements for complex logic.

Example: Loan eligibility

flowchart TD
    A["Start"] --> B{"Income >= 30000?"}
    B -->|"Yes"| C{"Credit score >= 700?"}
    C -->|"Yes"| D["Approve loan"]
    C -->|"No"| E["Reject loan"]
    B -->|"No"| F["Reject loan"]
    D --> G["End"]
    E --> G
    F --> G

Code Example

#include <stdio.h>
int main() {
    int income = 35000, credit_score = 750;
    if (income >= 30000) {
        if (credit_score >= 700) {
            printf("Loan approved!\n");
        } else {
            printf("Loan rejected: Low credit score.\n");
        }
    } else {
        printf("Loan rejected: Insufficient income.\n");
    }
    return 0;
}

Output:

Loan approved!

Trace Table

7. Logical Operators

Combine conditions using:

• && (AND): Both must be true.
• || (OR): At least one must be true.
• ! (NOT): Inverts the condition.

Example: Check if a number is between 10 and 20

if (num > 10 && num < 20) {
    printf("Number is between 10 and 20.\n");
}

Truth Table for Logical Operators

8. Comparison Table: if-else vs switch

9. Common Pitfalls

1. Missing braces: Causes syntax errors.

   if (x > 0) printf("Positive"); // Error: No braces!
   

2. Infinite loops: Incorrect conditions (e.g., while (1)).
3. Redundant conditions: Overlapping else if clauses.
4. Forgotten break: Fall-through in switch.

10. Real-World Applications

In the Real World

1. eSewa/Khalti Payment Gateways

   • Idea Used: switch-case for transaction types (e.g., bill payment, recharge, transfer).
   • How: When a user selects "Electricity Bill," the system uses a switch to route the payment to the correct utility provider (NESA, NEA, etc.).
   • Worked Example:

     switch (transaction_type) {
         case BILL_PAYMENT:
             if (provider == ELECTRICITY) { /* Route to NESA */ }
             break;
         case RECHARGE:
             /* Route to NTC/Ncell */ break;
         default: /* Error */ break;
     }
     

2. Daraz Order Processing

   • Idea Used: Nested if-else for order validation.
   • How: Daraz checks:
     • Is the user logged in? (if (logged_in))
     • Is the item in stock? (else if (stock > 0))
     • Is the payment successful? (else if (payment_confirmed))
   • Worked Example:

     if (user_logged_in) {
         if (item_stock > 0) {
             if (payment_successful) {
                 printf("Order confirmed!\n");
             } else {
                 printf("Payment failed.\n");
             }
         } else {
             printf("Out of stock.\n");
         }
     } else {
         printf("Login required.\n");
     }
     

3. Pathao Ride Booking

   • Idea Used: switch for ride type selection.
   • How: When a user taps "Car," "Bike," or "Van," Pathao uses a switch to:
     • Show available drivers.
     • Calculate fare dynamically.
     • Apply discounts (e.g., early morning rides).
   • Worked Example:

     switch (ride_type) {
         case CAR: fare = 150 + (distance * 2); break;
         case BIKE: fare = 100 + (distance * 1.5); break;
         case VAN: fare = 200 + (distance * 3); break;
         default: printf("Invalid ride type.\n");
     }
     

11. Exam Tips

1. Understand Syntax: Know the exact structure of if, else if, else, and switch.
2. Trace Logic: Always draw flowcharts or trace tables for nested conditions.
3. Practice Errors: Write code with common mistakes (missing braces, incorrect operators) and fix them.
4. Real-World Mapping: Relate selection structures to scenarios like:
   • Bank loans: if (income >= threshold) { approve(); }
   • NEPSE trading: switch (order_type) { case BUY; case SELL; }
   • NTC/Ncell tariffs: if (minutes_used > 100) { apply_discount(); }
5. Avoid Overcomplicating: Use switch only for fixed values; prefer if-else for ranges.
6. Memory Layout: For nested conditions, visualize the decision tree (like the Daraz order example above).

Example Exam Question:

Write a C program that takes a student's marks (out of 100) and prints:
- "A" if marks >= 80,
- "B" if marks >= 60,
- "C" if marks >= 40,
- "Fail" otherwise.
Also, handle invalid input (marks < 0 or > 100) by printing "Invalid marks."

Expected Solution:

#include <stdio.h>
int main() {
    int marks;
    printf("Enter marks: ");
    scanf("%d", &marks);
    if (marks < 0 || marks > 100) {
        printf("Invalid marks.\n");
    } else if (marks >= 80) {
        printf("Grade: A\n");
    } else if (marks >= 60) {
        printf("Grade: B\n");
    } else if (marks >= 40) {
        printf("Grade: C\n");
    } else {
        printf("Grade: Fail\n");
    }
    return 0;
}