For loop in C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a nested for loop

What is the output of this C code snippet?

C

#include <stdio.h>

int main() {
    for (int i = 1; i <= 3; i++) {
        for (int j = 1; j <= 2; j++) {
            printf("%d%d ", i, j);
        }
    }
    return 0;
}

A Error: Syntax error

B 12 11 22 21 32 31

C 11 21 31 12 22 32

D 11 12 21 22 31 32

💡 Hint

Think about how the inner loop runs completely for each iteration of the outer loop.

The outer loop runs i from 1 to 3. For each i, the inner loop runs j from 1 to 2, printing i and j together. So the output is pairs: 11 12 21 22 31 32.

❓ Predict Output

intermediate

Value of variable after for loop

What is the value of variable sum after running this code?

C

#include <stdio.h>

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

A 10

B 15

C 5

D 0

💡 Hint

Add numbers from 0 to 4.

The loop adds 0+1+2+3+4 = 10 to sum.

🔧 Debug

advanced

Identify the error in the for loop

What error does this code produce when compiled or run?

C

#include <stdio.h>

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

A No error, prints: 0 1 2 3 4

B Compilation error: missing braces

C Runtime error: segmentation fault

D No output

💡 Hint

Check if braces are required for single statement loops.

The for loop has one statement without braces, which is valid in C. The code prints numbers 0 to 4 with spaces.

❓ Predict Output

advanced

Output of a for loop with continue

What is the output of this code?

C

#include <stdio.h>

int main() {
    for (int i = 0; i < 5; i++) {
        if (i == 2) continue;
        printf("%d", i);
    }
    return 0;
}

A 01234

B 0134

C 024

D Error: continue outside loop

💡 Hint

The continue skips printing when i is 2.

When i is 2, continue skips the printf. So output is 0 1 3 4 without spaces.

🧠 Conceptual

expert

Number of iterations in a complex for loop

How many times will the loop body execute in this code?

C

#include <stdio.h>

int main() {
    int count = 0;
    for (int i = 1; i < 10; i *= 3) {
        count++;
    }
    printf("%d", count);
    return 0;
}

A 2

B 4

C 3

D 5

💡 Hint

Look at how i changes each time: 1, 3, 9, then stops.

i starts at 1, then 3, then 9. Next would be 27 which is not less than 10, so loop stops. So 3 iterations.

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