L-05: Nested Loops, Loop Control, Patterns

Lecture 5 #

Recall: Basic Loops #

While Loop (Review) #

#include <stdio.h>

int main() {
    int i = 1;
    while (i <= 5) {
        printf("%d ", i);
        i++;
    }
    return 0;
}

Output: 1 2 3 4 5

For Loop #

More compact way to write loops when we know the number of iterations.

#include <stdio.h>

int main() {
    int i;
    for (i = 1; i <= 5; i++) {
        printf("%d ", i);
    }
    return 0;
}

Output: 1 2 3 4 5

For Loop Structure #

for (initialization; condition; update) {
    // code to execute
}

Example:

for (int i = 0; i < 10; i++) {
    printf("%d\n", i);
}

Do-While Loop #

Executes at least once, then checks condition.

#include <stdio.h>

int main() {
    int i = 1;
    do {
        printf("%d ", i);
        i++;
    } while (i <= 5);
    return 0;
}

Output: 1 2 3 4 5

Difference: While vs Do-While #

// While loop - may not execute at all
int i = 10;
while (i < 5) {
    printf("%d ", i);  // Never prints
    i++;
}

// Do-while loop - executes at least once
int j = 10;
do {
    printf("%d ", j);  // Prints 10
    j++;
} while (j < 5);

Nested Loops #

Loop inside another loop.

#include <stdio.h>

int main() {
    int i, j;
    for (i = 1; i <= 3; i++) {
        for (j = 1; j <= 3; j++) {
            printf("(%d,%d) ", i, j);
        }
        printf("\n");
    }
    return 0;
}

Output of Nested Loop #

(1,1) (1,2) (1,3) 
(2,1) (2,2) (2,3) 
(3,1) (3,2) (3,3)

Inner loop completes all iterations for each iteration of outer loop.

Pattern Printing #

Pattern 1: Rectangle #

int rows = 4, cols = 5;
for (int i = 1; i <= rows; i++) {
    for (int j = 1; j <= cols; j++) {
        printf("* ");
    }
    printf("\n");
}

Output:

* * * * * 
* * * * * 
* * * * * 
* * * * *

Pattern 2: Right Triangle #

int n = 5;
for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= i; j++) {
        printf("* ");
    }
    printf("\n");
}

Output:

* 
* * 
* * * 
* * * * 
* * * * *

Pattern 3: Number Triangle #

int n = 4;
for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= i; j++) {
        printf("%d ", j);
    }
    printf("\n");
}

Output:

1 
1 2 
1 2 3 
1 2 3 4

Loop Control: Break #

Break statement exits the loop immediately.

#include <stdio.h>

int main() {
    for (int i = 1; i <= 10; i++) {
        if (i == 6) {
            break;  // Exit loop when i is 6
        }
        printf("%d ", i);
    }
    return 0;
}

Output: 1 2 3 4 5

Loop Control: Continue #

Continue statement skips current iteration and goes to next.

#include <stdio.h>

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

Output: 1 3 5 7 9

Break in Nested Loops #

Break only exits the innermost loop.

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        if (j == 2) {
            break;  // Only exits inner loop
        }
        printf("(%d,%d) ", i, j);
    }
    printf("\n");
}

Output:

(1,1) 
(2,1) 
(3,1)

Multiplication Table #

#include <stdio.h>

int main() {
    int n = 5;
    printf("Multiplication Table for %d:\n", n);
    
    for (int i = 1; i <= 10; i++) {
        printf("%d x %d = %d\n", n, i, n * i);
    }
    return 0;
}

Finding Prime Numbers #

#include <stdio.h>

int main() {
    int n = 29;
    int isPrime = 1;  // Assume prime
    
    for (int i = 2; i < n; i++) {
        if (n % i == 0) {
            isPrime = 0;  // Not prime
            break;
        }
    }
    
    if (isPrime)
        printf("%d is prime\n", n);
    else
        printf("%d is not prime\n", n);
    return 0;
}

Sum of Digits #

#include <stdio.h>

int main() {
    int num = 12345;
    int sum = 0;
    
    while (num > 0) {
        sum += num % 10;  // Add last digit
        num /= 10;        // Remove last digit
    }
    
    printf("Sum of digits: %d\n", sum);
    return 0;
}

Exercise: Star Patterns #

Write programs to print these patterns:

Pattern A:

*****
****
***
**
*

Pattern B:

    *
   ***
  *****
 *******
*********

Exercise: Number Patterns #

Pattern C:

1
22
333
4444
55555

Pattern D:

12345
1234
123
12
1

Nested While Loops #

#include <stdio.h>

int main() {
    int i = 1;
    while (i <= 3) {
        int j = 1;
        while (j <= 4) {
            printf("%d ", j);
            j++;
        }
        printf("\n");
        i++;
    }
    return 0;
}

Output:

1 2 3 4 
1 2 3 4 
1 2 3 4 

#include <stdio.h>

int main() {
    int choice;
    do {
        printf("\n--- Menu ---\n");
        printf("1. Print Numbers 1-10\n");
        printf("2. Print Even Numbers\n");
        printf("3. Exit\n");
        printf("Enter choice: ");
        scanf("%d", &choice);
        
        switch (choice) {
            case 1:
                for (int i = 1; i <= 10; i++) {
                    printf("%d ", i);
                }
                printf("\n");
                break;
            case 2:
                for (int i = 2; i <= 20; i += 2) {
                    printf("%d ", i);
                }
                printf("\n");
                break;
            case 3:
                printf("Goodbye!\n");
                break;
            default:
                printf("Invalid choice!\n");
        }
    } while (choice != 3);
    
    return 0;
}

Exercise: Enhanced Calculator #

Create a calculator that:

  • Shows menu repeatedly until user chooses to exit
  • Performs basic operations (+, -, *, /, %)
  • Uses do-while for menu loop
  • Uses switch case for operations
  • Validates input (no division by zero)

Exercise: Pattern Challenge #

Create this diamond pattern using nested loops:

    *
   ***
  *****
 *******
*********
 *******
  *****
   ***
    *

Exercise: Perfect Numbers #

Write a program to find all perfect numbers up to 1000.

  • A perfect number equals sum of its proper divisors
  • Example: 6 = 1 + 2 + 3 (divisors of 6 except 6)
  • Use nested loops: outer for numbers, inner for divisors

Common Loop Mistakes #

Infinite Loop #

int i = 1;
while (i <= 10) {
    printf("%d ", i);
    // Missing i++ - infinite loop!
}

Off-by-One Error #

// Wrong: prints 0 to 9 (10 numbers)
for (int i = 0; i <= 10; i++) {
    printf("%d ", i);
}

// Correct: prints 1 to 10 (10 numbers)
for (int i = 1; i <= 10; i++) {
    printf("%d ", i);
}

Loop Efficiency Tips #

Reduce Work in Inner Loop #

// Less efficient
for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
        int temp = i * i;  // Calculated n times!
        printf("%d ", temp + j);
    }
}

// More efficient
for (int i = 0; i < n; i++) {
    int temp = i * i;  // Calculated once
    for (int j = 0; j < n; j++) {
        printf("%d ", temp + j);
    }
}

Exercise: Number System Converter #

Write a program that:

  • Takes a decimal number as input
  • Converts it to binary using loops
  • Displays the binary representation
  • Use do-while loop for conversion process

Exercise: Grade Statistics #

Create a program that:

  • Reads marks for 10 students using loops
  • Calculates average using nested logic
  • Counts how many students got each grade (A, B, C, D, F)
  • Uses multiple nested loops for processing

Advanced: Loop Optimization #

Early Exit #

// Finding if number exists in array
int found = 0;
for (int i = 0; i < n && !found; i++) {
    if (arr[i] == target) {
        found = 1;
        printf("Found at position %d\n", i);
    }
}

Reading #

Next: Arrays, Strings, Functions

Textbooks: #

  • Chapter 4 & 5: Computer Science: A Structured Programming Approach Using C - Behrouz A. Forouzan, Richard F. Gilberg

Online Resources: #

Interactive Learning: #

Video Resources: #

  • C Programming Loops: Search “C programming nested loops” on YouTube
  • Pattern Programming: Look for “C star pattern programs” tutorials

Documentation: #

Calendar August 27, 2025