Loops & Iteration: for, while, do-while and Loop Control

TAKEAWAYS:

• Loops automate repetitive tasks by executing a block of code while a condition is true or for a fixed number of iterations.
• while checks the condition before execution (entry-controlled), while do-while checks after (exit-controlled).
• Loop control statements (break, continue) alter the flow of loops; goto is rarely used but can exit nested loops.
• Nested loops solve multi-dimensional problems (e.g., printing patterns, matrix traversal).
• Infinite loops occur when the termination condition is never met; they must be avoided unless intentional (e.g., servers).
• Real-world loops appear in order processing (Daraz), billing cycles (Ncell), and transaction validation (eSewa).

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1. Introduction to Loops

Loops are control structures that allow a block of code to execute repeatedly based on a condition or a fixed count. They eliminate redundancy and improve efficiency.

Types of Loops in C

There are three primary loop constructs in C:

1. for loop: Used when the number of iterations is known.
2. while loop: Used when the loop must run until a condition becomes false.
3. do-while loop: Similar to while, but the condition is checked after execution (guarantees at least one iteration).

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2. The for Loop

The for loop is best suited for count-controlled iterations. Its syntax is:

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

How It Works

1. Initialization: Executed once at the start.
2. Condition: Checked before each iteration. If false, the loop exits.
3. Increment/Decrement: Executed after each iteration.

Example: Print Numbers 1 to 5

for (int i = 1; i <= 5; i++) {
    printf("%d ", i);
}

Output: 1 2 3 4 5

Trace Table

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3. The while Loop

The while loop is condition-controlled and executes as long as the condition is true.

while (condition) {
    // Loop body
}

Example: Sum of Digits in a Number

Problem: Calculate the sum of digits of 1234.

int num = 1234, sum = 0, rem;
while (num != 0) {
    rem = num % 10;       // Extract last digit
    sum += rem;           // Add to sum
    num /= 10;            // Remove last digit
}
printf("Sum = %d", sum); // Output: 10 (1+2+3+4)

Trace Table

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4. The do-while Loop

The do-while loop always executes at least once because the condition is checked after the loop body.

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

Example: Menu-Driven Program (eSewa-like)

int choice;
do {
    printf("\n1. Pay Bill\n2. Check Balance\n3. Exit\nEnter choice: ");
    scanf("%d", &choice);
    switch(choice) {
        case 1: printf("Paying bill...\n"); break;
        case 2: printf("Checking balance...\n"); break;
        case 3: printf("Exiting...\n"); break;
        default: printf("Invalid choice!\n");
    }
} while (choice != 3);

Output:

1. Pay Bill
2. Check Balance
3. Exit
Enter choice: 1
Paying bill...
Enter choice: 2
Checking balance...
Enter choice: 3
Exiting...

Trace Table

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5. Loop Control Statements

These statements modify the normal flow of loops.

a) break Statement

• Exits the loop immediately when encountered.
• Used in searching (e.g., finding an element in an array).

Example: Find if a Number is Prime

int num = 7, isPrime = 1;
for (int i = 2; i <= num/2; i++) {
    if (num % i == 0) {
        isPrime = 0;
        break; // Exit loop if divisor found
    }
}
printf("%d is %s", num, isPrime ? "prime" : "not prime");

Output: 7 is prime

Trace Table

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b) continue Statement

• Skips the current iteration and moves to the next.
• Used to skip unwanted values (e.g., even numbers).

Example: Print Odd Numbers from 1 to 10

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

Output: 1 3 5 7 9

Trace Table

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c) goto Statement

• Unconditionally jumps to a labeled statement.
• Rarely used (can make code hard to debug).
• Use case: Exiting nested loops.

Example: Exit Nested Loops on User Input

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        printf("(%d,%d) ", i, j);
        if (i == 2 && j == 2) goto exit; // Exit on (2,2)
    }
}
exit:
printf("\nExiting nested loops...");

Output:

(1,1) (1,2) (1,3)
(2,1) (2,2) Exiting nested loops...

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6. Nested Loops

Nested loops are loops inside loops. Used for:

• Multi-dimensional problems (e.g., matrices, patterns).
• Simulating real-world scenarios (e.g., Daraz order processing).

Example: Print a Right-Angled Triangle Pattern

for (int i = 1; i <= 4; i++) {       // Rows
    for (int j = 1; j <= i; j++) {   // Columns
        printf("* ");
    }
    printf("\n");
}

Output:

*
* *
* * *
* * * *

Trace Table (Step-by-Step Execution)

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7. Infinite Loops

An infinite loop runs forever unless manually terminated. Causes:

• Missing update in loop variable (e.g., for (;;) or while(1)).
• Condition never becomes false.

Example: Infinite Loop (Server Simulation)

while (1) { // Runs forever (like a web server)
    printf("Server running...\n");
    sleep(1); // Simulate delay
}

Output (until manually stopped):

Server running...
Server running...
...

How to Avoid Infinite Loops

• Ensure the loop condition can become false.
• Use break in critical sections.

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8. Real-World Applications of Loops

Loops are ubiquitous in software. Here’s how Nepalese and global companies use them:

a) eSewa (Nepal) – Transaction Validation

• Loop Idea: while loop checks if a transaction is valid before processing.
• How?

  while (!isTransactionValid(userInput)) {
      printf("Invalid! Retry...\n");
      // Re-prompt for input
  }
  

• Visual:

  flowchart TD
    A["Start Transaction"] --> B["Check Validity"]
    B -->|"Valid"| C["Process Payment"]
    B -->|"Invalid"| D["Show Error"]
    D --> B

b) Daraz (Nepal) – Order Processing Queue

• Loop Idea: for loop processes each order in a queue.
• How?

  for (int i = 0; i < totalOrders; i++) {
      processOrder(orders[i]);
      updateInventory(orders[i]);
  }
  

• Visual (Order Queue as a Queue Data Structure):

Daraz Order Processing Queue (FIFO) (Image: This Image was created by User:Vegpuff. If you are using the, CC BY-SA 3.0, via Wikimedia Commons)

[Order 1] → [Order 2] → [Order 3] → [Order 4] → NULL

### **c) Ncell (Nepal) – Billing Cycle**
- **Loop Idea**: `do-while` ensures **at least one billing cycle** runs.
- **How?**
```c
do {
    calculateBill(customer);
    sendBill(customer);
    updateCustomerStatus(customer);
} while (hasMoreCustomers());

d) Google (Global) – Page Ranking (PageRank Algorithm)

• Loop Idea: while loop iteratively updates page ranks until convergence.
• How?

  while (!hasConverged()) {
      updatePageRanks();
      checkConvergence();
  }
  

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9. Common Mistakes & Debugging

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10. Exam Tip

What Examiners Look For

1. Correct Syntax: Ensure for, while, and do-while are used properly.
2. Trace Tables: Always show step-by-step execution (like the examples above).
3. Real-World Connection: Relate loops to Nepali software (e.g., Daraz orders, eSewa transactions).
4. Loop Control: Know when to use break, continue, and goto.
5. Nested Loops: Be able to draw patterns or explain matrix traversal.
6. Infinite Loops: Identify and fix them in given code snippets.

High-Scoring Answers Include

✅ Flowcharts (for complex logic). ✅ Trace tables (showing variable changes). ✅ Real-world examples (eSewa, Daraz, Ncell). ✅ Error analysis (debugging infinite loops).

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Practice Questions (Self-Check)

1. Write a for loop to print the multiplication table of 5.
2. Use a while loop to reverse a number (e.g., 123 → 321).
3. Explain why while (1) creates an infinite loop.
4. Modify the following loop to print even numbers only:

   for (int i = 1; i <= 10; i++) printf("%d ", i);
   

5. Debug: What’s wrong with this loop?

   for (int i = 10; i >= 1; i--) {
       printf("%d ", i);
   }
   

   (Hint: Check the condition.)

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Final Summary

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Remember: Loops are the backbone of automation in programming. Master them, and you’ll write efficient code for any problem! 🚀