Programming Essentials in C - Lecture 5

Introduction

• Loops are programming constructs that repeat a block of code either a specific number of times or while a certain condition is true.
• They improve efficiency, reduce code duplication, and enhance readability.
• C provides three loop constructs: while, do–while, and for.

while Loop

• Executes a block of code as long as the specified condition is true.
• The condition is checked before the loop body is executed.
• Known as an entry-verified loop.

Example

char letter = 'A';

// Print the uppercase alphabet from A to Z
while (letter <= 'Z') {
  printf("%c ", letter);
  letter++;
}

do-while Loop

• Executes the block of code once, then checks the condition.
• Repeats the loop as long as the condition is true.

Example

int i = 6;

// This loop runs once even though the condition is false
do {
  printf("i = %d\n", i);
  i++;
} while (i < 5);

printf("End of the loop\n");

Difference Between while and do-while

Feature	while Loop	do-while Loop
Condition Check	Before execution of the loop body	After execution of the loop body
Execution Guarantee	May not execute if the condition is false initially	Always executes at least once

for Loop

• Ideal when the number of iterations is known in advance.
• Combines initialization, condition check, and update in a single line.

for Loop Components

• Initialization: Runs once before the loop starts.
• Condition: Determines if the loop continues.
• Update: Executes after each loop iteration.

Syntax:

for (Initialization; Condition; Update) {
  // Loop body
}

Example 1: Basic for Loop

for (int i = 0; i < 5; i++) {
  printf("i = %d\n", i);
}

Example 2: Initialization Before Loop

int i = 0;

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

Example 3: Multiple Counters

• Multiple variables can be initialized and updated in a for loop.
• Only one condition is allowed to control the loop.

for (int i = 0, j = 10; i <= 5; i++, j--) {
  printf("i = %d, j = %d\n", i, j);
}

/*
Output:
  i = 0, j = 10
  i = 1, j = 9
  i = 2, j = 8
  i = 3, j = 7
  i = 4, j = 6
  i = 5, j = 5
*/

Example 4: Factorial Calculation

int x = 5, factorial = 1;

for (int i = 1; i <= x; i++) {
  factorial *= i;
}

printf("%d! = %d\n", x, factorial);  // Output: 5! = 120

Nested Loops

• A loop placed inside another loop is called a nested loop.
• Outer loop controls the number of complete iterations of the inner loop.
• Any type of loop (while, do-while, or for) can be nested within each other.

Example: Multiplication Table

// Print a 10x10 multiplication table
for (int i = 1; i <= 10; i++) {
  for (int j = 1; j <= 10; j++) {
    // %-4d => adds right padding the output to be always 4 characters long
    printf("%-4d", i * j);
  }

  printf("\n");
}

Output:

Example: Infinite Loop

while (1) {
  printf("hello\n");  // Runs indefinitely
}

• Infinite loops run endlessly unless interrupted manually or by a break statement.

Break and Continue

break Statement

• Immediately exits the loop when encountered.

Example

int i = 0, limit = 10;

while (1) {
  if (i == limit) break;  // Exit loop when i reaches limit

  printf("%d\n", i);
  i++;
}

printf("i reached the limit\n");

continue Statement

• Skips the current iteration and proceeds to the next one.

Example

for (int i = 1; i <= 10; i++) {
  if (i % 2 != 0) continue;  // Skip odd numbers

  printf("%d\n", i);  // Print only even numbers
}