If–else statement in C - Mini Project: Build & Apply

If-else statement

📖 Scenario: You are creating a simple program to check if a person is old enough to vote. Voting age is 18 years or older.

🎯 Goal: Build a program that uses an if-else statement to decide if a person can vote based on their age.

📋 What You'll Learn

Create an integer variable called age with the value 20

Create an integer variable called voting_age and set it to 18

Use an if-else statement to check if age is greater than or equal to voting_age

Print "You are eligible to vote." if the condition is true

Print "You are not eligible to vote." if the condition is false

💡 Why This Matters

🌍 Real World

Checking age eligibility is common in many real-life situations like voting, driving, or drinking alcohol.

💼 Career

Understanding if-else statements is essential for any programming job because it helps programs make decisions.

1

Create the age variable

Create an integer variable called age and set it to 20.

C

// Your code here

Hint

Use int age = 20; inside the main function.

2

Add the voting age variable

Create an integer variable called voting_age and set it to 18.

C

#include 

int main() {
    int age = 20;
    // Your code here
    return 0;
}

Hint

Declare int voting_age = 18; after age.

3

Write the if-else statement

Use an if-else statement to check if age is greater than or equal to voting_age. Inside the if, write a comment // Eligible. Inside the else, write a comment // Not eligible.

C

#include 

int main() {
    int age = 20;
    int voting_age = 18;
    // Use if-else to check eligibility
    // Your code here
    return 0;
}

Hint

Use if (age >= voting_age) {} and else {} blocks with comments inside.

4

Print the eligibility message

Inside the if block, write printf("You are eligible to vote.\n");. Inside the else block, write printf("You are not eligible to vote.\n");. Then run the program to see the output.

C

#include 

int main() {
    int age = 20;
    int voting_age = 18;
    if (age >= voting_age) {
        // Print eligible message
        // Your code here
    } else {
        // Print not eligible message
        // Your code here
    }
    return 0;
}

Hint

Use printf inside each block to show the correct message.

Practice

1. What does an if-else statement do in C?

easy

A. It declares a variable.

B. It repeats a block of code multiple times.

C. It defines a new function.

D. It chooses between two actions based on a condition.

Solution

1. Step 1: Understand the purpose of if-else

   An if-else statement lets the program pick one of two paths depending on whether a condition is true or false.

2. Step 2: Match the description to the options

   Only It chooses between two actions based on a condition. describes choosing between two actions based on a condition, which is exactly what if-else does.

3. Final Answer:

   It chooses between two actions based on a condition. -> Option D

4. Quick Check:

   If-else = choose between two paths [OK]

Hint: If you want two choices, use if-else [OK]

Common Mistakes:

• Confusing if-else with loops
• Thinking if-else declares variables
• Mixing if-else with function definitions

2. Which of the following is the correct syntax for an if-else statement in C?

easy

A. if (condition) then { /* code */ } else { /* code */ }

B. if condition then { /* code */ } else { /* code */ }

C. if (condition) { /* code */ } else { /* code */ }

D. if condition { /* code */ } else { /* code */ }

Solution

1. Step 1: Recall C syntax rules for if-else

   In C, conditions must be inside parentheses and braces {} are used for code blocks.

2. Step 2: Check each option

   Only if (condition) { /* code */ } else { /* code */ } uses parentheses around the condition and braces correctly without extra keywords like 'then'.

3. Final Answer:

   if (condition) { /* code */ } else { /* code */ } -> Option C

4. Quick Check:

   Correct syntax uses parentheses and braces [OK]

Hint: Remember: if(condition) with braces {} [OK]

Common Mistakes:

• Omitting parentheses around condition
• Using 'then' keyword (not in C)
• Skipping braces for multiple statements

3. What will be the output of this C code?

#include 
int main() {
    int x = 10;
    if (x > 5) {
        printf("Greater\n");
    } else {
        printf("Smaller\n");
    }
    return 0;
}

medium

A. Greater

B. Smaller

C. 10

D. No output

Solution

1. Step 1: Evaluate the condition x > 5

   Variable x is 10, which is greater than 5, so the condition is true.

2. Step 2: Determine which block runs

   Since the condition is true, the code inside the if block runs, printing "Greater".

3. Final Answer:

   Greater -> Option A

4. Quick Check:

   10 > 5 is true, so prints "Greater" [OK]

Hint: Check if condition is true or false to pick output [OK]

Common Mistakes:

• Confusing which block runs on true condition
• Ignoring the newline character in printf
• Assuming else block runs when condition is true

4. Find the error in this if-else code snippet:

int a = 3;
if a > 0
    printf("Positive\n");
else
    printf("Non-positive\n");

medium

A. Missing semicolon after if statement

B. Missing parentheses around condition in if statement

C. Missing braces around printf statements

D. Incorrect else keyword

Solution

1. Step 1: Check syntax of if condition

   In C, the condition in an if statement must be inside parentheses (). Here, it is missing.

2. Step 2: Verify other parts

   Semicolons after printf are present, braces are optional for single statements, and else keyword is correct.

3. Final Answer:

   Missing parentheses around condition in if statement -> Option B

4. Quick Check:

   if condition needs parentheses ( ) [OK]

Hint: Always put condition inside parentheses ( ) [OK]

Common Mistakes:

• Forgetting parentheses in if condition
• Thinking braces are always required for single statements
• Misplacing else keyword

5. You want to write a program that prints "Even" if a number is even, and "Odd" if it is odd. Which if-else code correctly does this in C?

hard

A. if (num % 2 == 0) { printf("Even\n"); } else { printf("Odd\n"); }

B. if (num / 2 == 0) { printf("Even\n"); } else { printf("Odd\n"); }

C. if (num % 2 = 0) { printf("Even\n"); } else { printf("Odd\n"); }

D. if (num % 2 != 0) { printf("Even\n"); } else { printf("Odd\n"); }

Solution

1. Step 1: Understand how to check even numbers

   A number is even if dividing by 2 leaves remainder 0, which is checked by num % 2 == 0.

2. Step 2: Analyze each option

   if (num % 2 == 0) { printf("Even\n"); } else { printf("Odd\n"); } correctly uses modulus operator and equality check. if (num / 2 == 0) { printf("Even\n"); } else { printf("Odd\n"); } uses division instead of modulus. if (num % 2 = 0) { printf("Even\n"); } else { printf("Odd\n"); } uses assignment (=) instead of comparison (==). if (num % 2 != 0) { printf("Even\n"); } else { printf("Odd\n"); } reverses the logic.

3. Final Answer:

   if (num % 2 == 0) { printf("Even\n"); } else { printf("Odd\n"); } -> Option A

4. Quick Check:

   Use modulus == 0 to check even [OK]

Hint: Use num % 2 == 0 to test even numbers [OK]

Common Mistakes:

• Using division instead of modulus
• Using = instead of == in condition
• Reversing even and odd logic