Unit 3: Decision Control and Loops

This unit covers the core logic of C programming: how to make decisions and how to repeat actions.

Decision Control Structures

if Statement

The if statement executes a block of code *only if* a given condition is true (non-zero).

Syntax:

if (condition) { // Code to execute if condition is true } 

If there is only one statement, the braces { } are optional, but it is good practice to always use them.

if-else Statement

The if-else statement provides an alternative block of code to execute if the condition is false (zero).

Syntax:

if (condition) { // Code to execute if condition is true } else { // Code to execute if condition is false } 

This can be extended into an if-else-if ladder to check multiple conditions:

if (condition1) { // ... } else if (condition2) { // ... } else { // ... if all else fails } 

switch Statement

The switch statement is an alternative to a long if-else-if ladder. It checks a single integer or character variable against a list of constant values (cases).

Syntax:

switch (variable) { case value1: // Code for value1 break; // IMPORTANT! case value2: // Code for value2 break; default: // Code if no other case matches break; } 

Loop Structures

Loops are used to repeat a block of code multiple times.

for Loop

The for loop is ideal when you know exactly how many times you want to loop (e.g., 10 times, or 'n' times).

Syntax:

// (1. Initialization; 2. Condition; 4. Update) for (int i = 0; i < 10; i++) { // 3. Loop Body (executes if condition is true) } 

Execution Order: 1 → 2 → 3 → 4 → 2 → 3 → 4 ... until 2 is false.

while Loop

The while loop is an entry-controlled loop. It is ideal when you don't know how many times you need to loop, but you know the condition to *stop*.

The condition is checked *before* the loop body executes.

Syntax:

int i = 0; // 1. Initialization while (i < 10) { // 2. Condition // 3. Loop Body i++; // 4. Update } 

If the condition is false the first time, the loop body never runs.

do-while Loop

The do-while loop is an exit-controlled loop. It is the only loop in C that is guaranteed to execute at least once.

The condition is checked *after* the loop body executes.

Syntax:

int i = 0; // 1. Initialization do { // 2. Loop Body i++; // 3. Update } while (i < 10); // 4. Condition (note the semicolon!) 

Loop and Switch Control

break Statement

The break statement has two uses:

1. Inside a switch block, it stops execution and exits the block.
2. Inside a loop (for, while, do-while), it immediately terminates the loop and execution continues at the next statement *after* the loop.

continue Statement

The continue statement is used only inside loops.

It immediately stops the *current iteration* and sends execution back to the top of the loop to start the next iteration.

• In a for loop, it jumps to the *update* statement (e.g., i++).
• In a while loop, it jumps to the *condition* check.

Example: Print only odd numbers from 1 to 10.

for (int i = 1; i <= 10; i++) { if (i % 2 == 0) { continue; // Skip this iteration if 'i' is even } printf("%d ", i); // This line is skipped for even numbers } // Output: 1 3 5 7 9 

goto Statement

The goto statement provides an unconditional jump to a labeled statement elsewhere in the *same function*.

Syntax:

if (x < 0) { goto error_handler; } // ... more code ... error_handler: printf("An error occurred.\n"); 

Code Examples (from Practicals)

Practical #2: Leap Year (if-else)

#include <stdio.h> int main() { int year; printf("Enter a year: "); scanf("%d", &year); // A year is a leap year if it's divisible by 4,  // UNLESS it's divisible by 100 but NOT by 400. if ((year % 4 == 0 && year % 100 != 0) || (year % 400 == 0)) { printf("%d is a leap year.\n", year); } else { printf("%d is not a leap year.\n", year); } return 0; } 

Practical #4: Prime Number (for loop)

#include <stdio.h> int main() { int n, i, is_prime = 1; // 1 means true printf("Enter a positive integer: "); scanf("%d", &n); if (n <= 1) { is_prime = 0; // 0 and 1 are not prime } // Check for factors from 2 up to n/2 for (i = 2; i <= n / 2; i++) { if (n % i == 0) { is_prime = 0; // Found a factor, not prime break; // Exit the loop early } } if (is_prime == 1) { printf("%d is a prime number.\n", n); } else { printf("%d is not a prime number.\n", n); } return 0; } 

Practical #5: Factorial (while loop)

#include <stdio.h> int main() { int n; long long factorial = 1; // Use long long for larger numbers int i; printf("Enter a positive integer: "); scanf("%d", &n); i = n; // Initialization while (i > 1) { // Condition factorial = factorial * i; i--; // Update } printf("Factorial of %d = %lld\n", n, factorial); return 0; } 

Unit 3: Exam Quick Tips