Control Structures & Syntax in C

TAKEAWAYS:

• Master if-else ladders, switch-case, and loop syntax to solve branching/logic problems.
• Understand loop control (break, continue, goto) and their impact on program flow.
• Trace variable changes step-by-step in loops/conditions to debug efficiently.
• Apply control structures to real scenarios (e.g., eSewa’s transaction validation, Daraz’s order processing).
• Avoid common pitfalls like infinite loops or dangling else in nested conditions.

1. Basic Syntax Rules in C

C enforces strict syntax for readability and compiler compatibility. Key rules:

• Case sensitivity: int x; ≠ int X;.
• Semicolons: Every statement ends with ;.
• Braces {}: Required for blocks (e.g., loops, if bodies).
• Comments:

  // Single-line comment
  /* Multi-line
     comment */
  

2. Decision-Making Statements

A. if-else Ladder

Syntax:

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

Example: Validate a user’s eSewa PIN (3 attempts max).

int pin = 1234, attempts = 0;
while (attempts < 3) {
    int input;
    printf("Enter PIN: ");
    scanf("%d", &input);
    if (input == pin) {
        printf("Access granted!");
        break;
    } else {
        attempts++;
        printf("Wrong PIN. %d attempts left.\n", 3 - attempts);
    }
}

Trace Table:

Real-World Tie:

• eSewa App: Uses nested if-else to check transaction limits, PIN validity, and bank balance before processing payments.

B. switch-case

Syntax:

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

Example: Menu-driven program for a bank ATM.

int choice;
printf("1. Check Balance\n2. Withdraw\n3. Exit\n");
scanf("%d", &choice);
switch (choice) {
    case 1: printf("Balance: Rs. 5000\n"); break;
    case 2: printf("Enter amount: "); break;
    default: printf("Invalid choice.\n");
}

Visual Trace:

flowchart TD
    A["Start"] --> B["Input choice"]
    B --> C{"choice == 1"}
    C -->|"Yes"| D["Print Balance"]
    C -->|"No"| E{"choice == 2"}
    E -->|"Yes"| F["Prompt for amount"]
    E -->|"No"| G["Print 'Invalid'"]

Comparison Table:

Real-World Tie:

• Pathao Driver App: Uses switch-case to handle ride status updates (e.g., case 1: "Trip accepted", case 2: "Driver assigned").

3. Looping Constructs

A. for Loop

Syntax:

for (initialization; condition; increment) {
    // Loop body
}

Example: Calculate the total cost of a Daraz order with 5% discount if order > Rs. 2000.

float total = 0, price;
for (int i = 0; i < 3; i++) {
    printf("Enter item %d price: ", i+1);
    scanf("%f", &price);
    total += price;
}
if (total > 2000) total *= 0.95;
printf("Final total: Rs. %.2f\n", total);

Trace Table:

B. while Loop

Syntax:

while (condition) {
    // Loop body
}

Example: Simulate a traffic light cycle (red: 30s, green: 20s, yellow: 5s).

int time = 0;
while (time < 55) {
    if (time < 30) printf("Red\n");
    else if (time < 50) printf("Green\n");
    else printf("Yellow\n");
    time++;
    delay(1000); // Simulate 1-second delay
}

Visual Trace:

flowchart LR
    A["Start"] --> B["time = 0"]
    B --> C{"time < 30"}
    C -->|"Yes"| D["Print 'Red'"]
    C -->|"No"| E{"time < 50"}
    E -->|"Yes"| F["Print 'Green'"]
    E -->|"No"| G["Print 'Yellow'"]
    D --> H["time++"]
    F --> H
    G --> I["End (time >= 55)"]
    H --> C

C. do-while Loop

Syntax:

do {
    // Loop body
} while (condition);

Example: Validate user input for a Ncell recharge PIN (must be 4 digits).

int pin;
do {
    printf("Enter 4-digit PIN: ");
    scanf("%d", &pin);
} while (pin < 1000 || pin > 9999);
printf("Valid PIN!\n");

Comparison Table:

Real-World Tie:

• NTC Traffic Management System: Uses while loops to monitor real-time vehicle counts at intersections and trigger signals based on congestion.

4. Loop Control Statements

Example: Process a queue of Kathmandu traffic routes, skipping blocked roads.

char routes[] = {'A', 'B', 'C', 'D'};
for (int i = 0; i < 4; i++) {
    if (routes[i] == 'B') continue; // Road B is blocked
    printf("Take route %c\n", routes[i]);
}

Output:

Take route A
Take route C
Take route D

5. Nested Control Structures

Combine loops/conditions for complex logic. Example: Bank Loan Eligibility Check.

int income, age;
printf("Enter income (Rs.): ");
scanf("%d", &income);
printf("Enter age: ");
scanf("%d", &age);
if (income >= 50000) {
    if (age >= 21 && age <= 60) {
        printf("Loan approved!\n");
    } else {
        printf("Age not eligible.\n");
    }
} else {
    printf("Income too low.\n");
}

Visual Trace:

flowchart TD
    A["Start"] --> B["Check income >= 50000"]
    B -->|"No"| C["Print 'Income too low'"]
    B -->|"Yes"| D["Check age 21-60"]
    D -->|"No"| E["Print 'Age not eligible'"]
    D -->|"Yes"| F["Print 'Loan approved!'"]

In the Real World

1. eSewa Transaction Validation:

   • Uses if-else ladders to check:
     • User login credentials (if (username == stored_user && password == stored_pass)).
     • Transaction amount (else if (amount > balance) { reject; }).
     • OTP verification (else if (otp == generated_otp) { process; }).

2. Daraz Order Processing:

   • for loops iterate through cart items to calculate subtotal, tax, and shipping cost.
   • switch-case handles payment methods:

     switch (payment_method) {
         case 1: // Khalti
             verify_khalti_payment();
             break;
         case 2: // Credit Card
             verify_card_details();
             break;
     }
     

3. Pathao Driver App:

   • while loops monitor GPS coordinates to update ride progress:

     while (distance_to_destination > 0.1) {
         update_location();
         distance_to_destination = calculate_distance();
     }
     

Exam Tip

1. Syntax Precision:

   • Memorize the exact syntax for else-if ladders (no colon : in C!).
   • Example: ❌ else if (x > 10): → ✅ else if (x > 10) { ... }.

2. Trace Tables:

   • For every loop/condition, create a table showing variable changes per iteration (as above). Examiners love step-by-step traces!

3. Common Pitfalls:

   • Infinite Loops: Always ensure loop conditions can become false (e.g., i++ in for).
   • Dangling else: Use braces {} to avoid ambiguity in nested if-else.

     // Ambiguous without braces:
     if (x > 5)
         if (y > 10)
             printf("A");
         else
             printf("B"); // Does this else belong to x or y?
     

4. Real-World Scenarios:

   • Link your answers to apps like eSewa (validation), Daraz (order processing), or NTC (traffic signals). Examiners reward contextual understanding.

5. Code Structure:

   • Use meaningful variable names (e.g., max_attempts instead of x).
   • Indent blocks clearly to match logical hierarchy.

Practice Question: Write a program to simulate a Khalti payment gateway that:

1. Asks for amount and merchant ID.
2. Validates the merchant ID (must be 8 digits).
3. Uses a do-while loop to re-prompt if invalid.
4. Simulates payment success/failure with a random number (e.g., rand() % 2).

Solution Trace:

flowchart LR
    A["Start"] --> B["Input amount, merchant_id"]
    B --> C{"merchant_id is 8 digits?"}
    C -->|"No"| D["Print 'Invalid ID'"]
    D --> B
    C -->|"Yes"| E["Generate random success/failure"]
    E -->|"Success"| F["Print 'Payment done!'"]
    E -->|"Failure"| G["Print 'Payment failed'"]