For loop in C - Mini Project: Build & Apply

Counting Even Numbers with a For Loop in C

📖 Scenario: You have a list of numbers representing daily sales counts in a small shop. You want to find out how many days had an even number of sales.

🎯 Goal: Build a C program that counts how many numbers in a given list are even using a for loop.

📋 What You'll Learn

Create an integer array called sales with the exact values: 12, 7, 9, 20, 15

Create an integer variable called count_even and set it to 0

Use a for loop with an integer variable i to iterate over the sales array

Inside the loop, use an if statement to check if the current number is even

Increase count_even by 1 for each even number found

Print the final value of count_even using printf

💡 Why This Matters

🌍 Real World

Counting specific types of data in a list is common in many real-world tasks, like analyzing sales, temperatures, or survey results.

💼 Career

Understanding loops and conditions is essential for programming jobs, as they help automate repetitive tasks and data analysis.

1

Create the sales data array

Create an integer array called sales with these exact values: 12, 7, 9, 20, 15.

C

// Your code here

Hint

Use curly braces {} to list the numbers inside the array.

2

Create a counter variable

Create an integer variable called count_even and set it to 0.

C

int sales[] = {12, 7, 9, 20, 15};
// Your code here

Hint

This variable will keep track of how many even numbers you find.

3

Use a for loop to count even numbers

Use a for loop with an integer variable i to iterate over the sales array. Inside the loop, use an if statement to check if sales[i] is even (divisible by 2). If it is even, increase count_even by 1.

C

int sales[] = {12, 7, 9, 20, 15};
int count_even = 0;
// Your code here

Hint

Remember, use i < 5 because the array has 5 elements indexed 0 to 4.

4

Print the count of even numbers

Write a printf statement to print the value of count_even with the exact text: "Number of even sales days: %d\n".

C

int sales[] = {12, 7, 9, 20, 15};
int count_even = 0;
for (int i = 0; i < 5; i++) {
    if (sales[i] % 2 == 0) {
        count_even++;
    }
}
// Your code here

Hint

Use printf with %d to show the number stored in count_even.

Practice

1. What does a for loop in C primarily do?

easy

A. Repeats a block of code a fixed number of times

B. Defines a new variable

C. Stops the program execution

D. Declares a function

Solution

1. Step 1: Understand the purpose of a for loop

   A for loop is designed to repeat a block of code multiple times based on a condition.

2. Step 2: Identify the correct description

   Among the options, only repeating code a fixed number of times matches the for loop's purpose.

3. Final Answer:

   Repeats a block of code a fixed number of times -> Option A

4. Quick Check:

   For loop = repeat code [OK]

Hint: For loops repeat code a set number of times [OK]

Common Mistakes:

• Confusing for loop with variable declaration
• Thinking for loop stops program
• Mixing for loop with function declaration

2. Which of the following is the correct syntax for a for loop in C?

easy

A. for i = 0 to 5 { }

B. for (int i = 0; i++ < 5;) { }

C. for (i < 5; i = 0; i++) { }

D. for (int i = 0; i < 5; i++) { }

Solution

1. Step 1: Review standard for loop syntax in C

   The correct syntax is: for (initialization; condition; update) { body }

2. Step 2: Check each option

   for (int i = 0; i < 5; i++) { } matches the correct syntax with initialization, condition, and update in proper order.

3. Final Answer:

   for (int i = 0; i < 5; i++) { } -> Option D

4. Quick Check:

   Syntax = for(init; cond; update) [OK]

Hint: For loop syntax: for(init; condition; update) [OK]

Common Mistakes:

• Using 'for i = 0 to 5' which is not C syntax
• Swapping condition and initialization
• Missing semicolons inside for parentheses

3. What is the output of this code?

for(int i = 1; i <= 3; i++) {
    printf("%d ", i);
}

medium

A. 3 2 1

B. 0 1 2 3

C. 1 2 3

D. 1 2 3 4

Solution

1. Step 1: Analyze the loop range

   The loop starts at i=1 and runs while i is less than or equal to 3, incrementing i by 1 each time.

2. Step 2: Determine printed values

   It prints i each time: 1, then 2, then 3, then stops.

3. Final Answer:

   1 2 3 -> Option C

4. Quick Check:

   Loop prints 1 to 3 [OK]

Hint: Check loop start, end, and increments carefully [OK]

Common Mistakes:

• Starting loop at 0 instead of 1
• Including an extra number beyond 3
• Printing in reverse order

4. Identify the error in this for loop:

for(int i = 0; i < 5; i)
    printf("%d ", i);

medium

A. Missing semicolon after printf statement

B. Missing increment operator in for loop update

C. Incorrect loop condition syntax

D. Variable i not declared

Solution

1. Step 1: Examine the for loop update part

   The update section is 'i' without increment operator, so i never changes.

2. Step 2: Understand the effect of missing increment

   Without 'i++' or similar, the loop runs infinitely or behaves incorrectly.

3. Final Answer:

   Missing increment operator in for loop update -> Option B

4. Quick Check:

   Update must change loop variable [OK]

Hint: For loop update must change variable (e.g., i++) [OK]

Common Mistakes:

• Forgetting to increment loop variable
• Confusing semicolon placement
• Assuming variable auto-increments

5. How can you use a for loop to print only even numbers from 2 to 10 inclusive in C?

hard

A. for(int i = 2; i <= 10; i += 2) { printf("%d ", i); }

B. for(int i = 0; i <= 10; i++) { if(i % 2 == 0) printf("%d ", i); }

C. for(int i = 2; i < 10; i++) { if(i % 2 == 0) printf("%d ", i); }

D. for(int i = 0; i <= 10; i += 3) { printf("%d ", i); }

Solution

1. Step 1: Understand the goal

   We want to print even numbers from 2 to 10 inclusive.

2. Step 2: Check each option

   for(int i = 2; i <= 10; i += 2) { printf("%d ", i); } starts at 2 and increments by 2, printing 2,4,6,8,10 exactly.

3. Final Answer:

   for(int i = 2; i <= 10; i += 2) { printf("%d ", i); } -> Option A

4. Quick Check:

   Increment by 2 prints evens directly [OK]

Hint: Increment by 2 to print evens directly [OK]

Common Mistakes:

• Using wrong increment step
• Off-by-one errors in loop condition
• Printing odd numbers by mistake