Control Structures: Iteration and Loops

TAKEAWAYS:

• Loops reduce repetitive code by executing a block of statements repeatedly until a condition is met.
• for loops are ideal for count-controlled iterations (e.g., printing numbers 1–10), while while and do-while handle condition-controlled loops (e.g., user input validation).
• Nested loops enable multi-dimensional tasks (e.g., printing patterns, matrix operations) by embedding one loop inside another.
• Loop control statements (break, continue) modify loop flow to exit early or skip iterations.
• Infinite loops occur when loop conditions are never false (e.g., while(1)), requiring careful design.
• Real-world loops optimize performance (e.g., Daraz’s order processing) and user interaction (e.g., Khalti’s payment retries).

1. Introduction to Iteration

Iteration is the repetition of a process until a specific condition is satisfied. In C, loops automate iteration, eliminating manual repetition of code blocks. Loops are classified based on their trigger condition:

• Pre-test loops: Check condition before execution (e.g., while, for).
• Post-test loops: Execute at least once, then check condition (e.g., do-while).

Why loops?

• Save time by avoiding manual repetition.
• Handle dynamic data (e.g., user-defined limits).
• Enable complex patterns (e.g., game grids, financial calculations).

2. Types of Loops in C

A. for Loop

Syntax:

for (initialization; condition; increment/decrement) {
    // loop body
}

How it works:

1. Initialization: Runs once (e.g., int i = 1).
2. Condition: Checked before each iteration (e.g., i <= 10).
3. Increment/Decrement: Executes after each iteration (e.g., i++).
4. Loop stops when condition becomes false.

Example: Print multiples of 5 < 100

flowchart TD
    A["Start"] --> B["Initialize i = 5"]
    B --> C{"Is i < 100?"}
    C -->|"Yes"| D["Print i"]
    D --> E["i = i + 5"]
    E --> C
    C -->|"No"| F["End"]

#include <stdio.h>
int main() {
    for (int i = 5; i < 100; i += 5) {
        printf("%d ", i);
    }
    return 0;
}

Trace Table:

B. while Loop

Syntax:

while (condition) {
    // loop body
}

Key difference from for:

• No built-in initialization or increment.
• Condition checked before each iteration (risk of infinite loop if condition never becomes false).

Example: Sum of digits of a number

flowchart TD
    A["Start"] --> B["Input number n = 123"]
    B --> C["Initialize sum = 0, temp = n"]
    C --> D{"Is temp != 0?"}
    D -->|"Yes"| E["sum += temp % 10"]
    E --> F["temp = temp / 10"]
    F --> D
    D -->|"No"| G["Print sum"]
    G --> H["End"]

#include <stdio.h>
int main() {
    int n, sum = 0, temp;
    printf("Enter a number: ");
    scanf("%d", &n);
    temp = n;
    while (temp != 0) {
        sum += temp % 10;
        temp /= 10;
    }
    printf("Sum of digits: %d\n", sum);
    return 0;
}

Trace:

• Input: 123
• Iteration 1: sum = 0 + 3 = 3, temp = 12
• Iteration 2: sum = 3 + 2 = 5, temp = 1
• Iteration 3: sum = 5 + 1 = 6, temp = 0 → Loop ends.
• Output: 6

C. do-while Loop

Syntax:

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

Key difference:

• Post-test loop: Executes at least once, then checks condition.
• Useful for menu-driven programs (e.g., Khalti’s payment retry system).

Example: Menu-driven calculator

flowchart TD
    A["Start"] --> B["Display menu"]
    B --> C["Get user choice"]
    C --> D{"Is choice valid?"}
    D -->|"Yes"| E["Perform operation"]
    E --> B
    D -->|"No"| F["Print 'Invalid choice'"]
    F --> B

#include <stdio.h>
int main() {
    int choice;
    do {
        printf("\n1. Add\n2. Subtract\n3. Exit\nChoice: ");
        scanf("%d", &choice);
        switch (choice) {
            case 1: printf("Addition\n"); break;
            case 2: printf("Subtraction\n"); break;
            case 3: break;
            default: printf("Invalid choice!\n");
        }
    } while (choice != 3);
    return 0;
}

Trace:

• User enters 1 → "Addition" → loop repeats.
• User enters 4 → "Invalid choice!" → loop repeats.
• User enters 3 → loop exits.

3. Loop Control Statements

A. break

• Exits the loop immediately, even if the condition is true.
• Used to skip remaining iterations when a condition is met.

Example: Find first even number

for (int i = 1; i <= 10; i++) {
    if (i % 2 == 0) {
        printf("First even number: %d\n", i);
        break;  // Exit loop after finding the first even number
    }
}

Output:

First even number: 2

B. continue

• Skips the current iteration and moves to the next.
• Useful for filtering data (e.g., skipping odd numbers).

Example: Print even numbers only

for (int i = 1; i <= 10; i++) {
    if (i % 2 != 0) {
        continue;  // Skip odd numbers
    }
    printf("%d ", i);  // Print even numbers
}

Output:

2 4 6 8 10

4. Nested Loops

A loop inside another loop, creating multi-dimensional repetition. Use cases:

• Printing patterns (e.g., right-angled triangles).
• Processing matrices (e.g., 2D arrays in Daraz’s inventory).
• Simulating games (e.g., Pathao’s route optimization).

Example: Print the pattern

BIMMIB
IMMI
MM

flowchart TD
    A["Start"] --> B["Outer loop: rows = 3"]
    B --> C["Inner loop: columns = 5 - row"]
    C --> D["Print 'M' or 'I' based on column"]
    D --> C
    B --> E["Decrement row"]
    E --> C
    C --> F["End"]

#include <stdio.h>
int main() {
    for (int row = 0; row < 3; row++) {
        for (int col = 0; col < 5 - row; col++) {
            if (col % 2 == 0) {
                printf("M");
            } else {
                printf("I");
            }
        }
        printf("\n");
    }
    return 0;
}

Trace:

• Row 0: col = 0 → 'M', col = 1 → 'I', col = 2 → 'M', col = 3 → 'I', col = 4 → 'B' (if adjusted). Output: MIMIM (adjust logic for exact pattern).
• Row 1: col = 0 → 'M', col = 1 → 'I', col = 2 → 'M'. Output: MIM.
• Row 2: col = 0 → 'M', col = 1 → 'M'. Output: MM.

5. Infinite Loops

Occurs when the loop condition never becomes false. Common causes:

• Missing increment/decrement (e.g., for (int i = 0; ; i++)).
• Incorrect condition (e.g., while (i > 0) with i never decreasing).

Example: Infinite loop (accidental)

for (int i = 0; i < 10; ) {
    printf("%d ", i);
    // Missing i++ → infinite loop!
}

Fix: Add i++ to the loop.

6. Comparison Table: for, while, do-while

7. Real-World Applications

## In the real world

1. Daraz Order Processing

   • Idea: while loops handle pending orders until all are processed.
   • How: Daraz’s backend uses loops to check inventory and update order statuses dynamically.
   • Worked Example: A loop iterates through order_list until order_list[i].status == "Delivered".

     while (order_list[i].status != "Delivered") {
         update_status(order_list[i].id);
         i++;
     }
     

2. Khalti Payment Retries

   • Idea: do-while ensures at least one payment attempt, then retries if failed.
   • How: The loop continues until the transaction succeeds or max retries are reached.

     do {
         if (process_payment()) {
             break;  // Success
         }
         retries++;
     } while (retries < 3);
     

3. NTC Traffic Light Control

   • Idea: for loops simulate timed cycles for traffic lights.
   • How: A loop runs for 30 seconds (green), then switches to yellow/red.

     for (int sec = 0; sec < 30; sec++) {
         display_light("Green");
     }
     

8. Common Mistakes and Debugging

9. Exam Tip

• Focus on:
  • Differentiating loops: Know when to use for, while, or do-while (e.g., for for fixed iterations, while for dynamic conditions).
  • Nested loops: Practice pattern printing (e.g., right-angled triangles, alphabets).
  • Loop control: Use break/continue to exit or skip iterations (common in exam questions).
  • Infinite loops: Avoid them by ensuring loop conditions change.
• Common Exam Questions:
  • Write a program to print a specific pattern (e.g., pyramid, diamond).
  • Find the sum of digits of a number using while.
  • Calculate the area of a rectangle with nested loops (though simpler with multiplication).
  • Differentiate while and do-while with a menu-driven example.
• Marking Scheme:
  • Correct logic: 40%
  • Proper loop syntax: 30%
  • Output correctness: 20%
  • Code readability: 10%

Practice Problem: Write a program to print the following pattern:

1
1 2
1 2 3
1 2 3 4

Hint: Use nested for loops with an inner loop for printing numbers.