For loop in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple for loop with increment

What is the output of the following C++ code?

C++

for (int i = 0; i < 5; i++) {
    std::cout << i << " ";
}

A 0 1 2 3 4

B 1 2 3 4 5

C 0 1 2 3 4 5

D 1 2 3 4

💡 Hint

Remember the loop starts at 0 and runs while i is less than 5.

The loop starts at 0 and increments by 1 until it reaches 4. It stops before i equals 5, so it prints 0 1 2 3 4.

❓ Predict Output

intermediate

For loop with decrement

What will be printed by this C++ code?

C++

for (int i = 5; i > 0; i--) {
    std::cout << i << " ";
}

A 4 3 2 1 0

B 5 4 3 2 1

C 5 4 3 2 1 0

D 6 5 4 3 2 1

💡 Hint

The loop starts at 5 and decreases until it is greater than 0.

The loop prints from 5 down to 1. It stops before i reaches 0.

🧠 Conceptual

advanced

Understanding loop variable scope

What is the value of variable i after this loop finishes?

C++

int i = 10;
for (int i = 0; i < 3; i++) {
    // loop body
}
// What is i here?

A 3

B 0

C 10

D Undefined behavior

💡 Hint

Consider the scope of the variable declared inside the for loop.

The i declared inside the loop is a new variable local to the loop. The outer i remains unchanged at 10.

❓ Predict Output

advanced

Nested for loops output

What is the output of this nested for loop code?

C++

for (int i = 1; i <= 2; i++) {
    for (int j = 1; j <= 3; j++) {
        std::cout << i * j << " ";
    }
}

A 1 2 3 2 4 6

B 1 2 3 1 2 3

C 1 2 3 3 6 9

D 2 4 6 4 8 12

💡 Hint

Multiply i and j for each iteration and print in order.

For i=1, j=1 to 3 prints 1 2 3. For i=2, j=1 to 3 prints 2 4 6. Combined output is 1 2 3 2 4 6.

❓ Predict Output

expert

For loop with continue and break

What is the output of this code?

C++

for (int i = 0; i < 6; i++) {
    if (i == 2) continue;
    if (i == 4) break;
    std::cout << i << " ";
}

A 0 1 2 3

B 0 1 2 3 4

C 0 1 3 4

D 0 1 3

💡 Hint

Remember that continue skips the current iteration and break stops the loop.

When i is 2, continue skips printing. When i is 4, break stops the loop. So printed values are 0,1,3.

Practice

1.

What does a for loop in C++ do?

easy

A. Defines a new function

B. Runs code only once

C. Stops the program immediately

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

Solution

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

   A for loop is designed to repeat 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 behavior.

3. Final Answer:

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

4. Quick Check:

   For loop = repeat fixed times [OK]

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

Common Mistakes:

• Thinking for loops run only once
• Confusing for loops with if statements
• Believing for loops stop the program

2.

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

for (___; i < 5; i++) {
    // code
}

easy

A. int i = 0

B. i = 0

C. int i

D. int i == 0

Solution

1. Step 1: Check initialization syntax in for loop

   The initialization part must declare and initialize the loop variable, e.g., int i = 0.

2. Step 2: Compare options for correctness

   Only int i = 0 correctly declares and initializes the variable.

3. Final Answer:

   int i = 0 -> Option A

4. Quick Check:

   Initialization needs declaration and assignment [OK]

Hint: Initialization needs declaration and assignment [OK]

Common Mistakes:

• Using 'int i' without initialization
• Using 'int i == 0' which is a comparison
• Omitting the type declaration

3.

What is the output of this code?

for (int i = 1; i <= 3; i++) {
    std::cout << i * 2 << " ";
}

medium

A. 1 2 3

B. 2 4 6

C. 0 2 4

D. 2 3 4

Solution

1. Step 1: Identify loop iterations

   The loop runs with i = 1, 2, 3 because condition is i <= 3.

2. Step 2: Calculate output for each iteration

   Outputs are i*2: 2, 4, 6 printed with spaces.

3. Final Answer:

   2 4 6 -> Option B

4. Quick Check:

   Multiply i by 2 each loop [OK]

Hint: Multiply loop variable by 2 each iteration [OK]

Common Mistakes:

• Confusing loop range (off by one)
• Printing i instead of i*2
• Missing spaces in output

4.

Find the error in this for loop:

for (int i = 0; i > 5; i++) {
    std::cout << i << " ";
}

medium

A. Variable i is not declared

B. Missing semicolon after i++

C. The condition 'i > 5' is incorrect, loop never runs

D. The loop will run infinitely

Solution

1. Step 1: Analyze the loop condition

   The condition is i > 5 but i starts at 0, so it is false immediately.

2. Step 2: Understand loop execution

   Since condition is false at start, loop body never executes.

3. Final Answer:

   The condition 'i > 5' is incorrect, loop never runs -> Option C

4. Quick Check:

   Condition false at start means no loop runs [OK]

Hint: Check if loop condition is true initially [OK]

Common Mistakes:

• Assuming loop runs despite false condition
• Thinking missing semicolon causes error here
• Confusing variable declaration errors

5.

How can you use a for loop to print all even numbers from 2 to 10 inclusive?

Choose the correct code snippet:

hard

A. for (int i = 2; i <= 10; i += 2) { std::cout << i << ' '; }

B. for (int i = 1; i < 10; i++) { if (i % 2 == 0) std::cout << i << ' '; }

C. for (int i = 2; i < 10; i++) { std::cout << i * 2 << ' '; }

D. for (int i = 0; i <= 10; i += 3) { std::cout << i << ' '; }

Solution

1. Step 1: Understand the goal

   We want to print even numbers 2,4,6,8,10 inclusive.

2. Step 2: Check each option

   for (int i = 2; i <= 10; i += 2) { std::cout << i << ' '; } increments by 2 starting at 2, printing 2,4,6,8,10 correctly.
   for (int i = 1; i < 10; i++) { if (i % 2 == 0) std::cout << i << ' '; } prints 2,4,6,8 but misses 10.
   for (int i = 2; i < 10; i++) { std::cout << i * 2 << ' '; } prints 4,6,8,10,12,14,16,18 (wrong range).
   for (int i = 0; i <= 10; i += 3) { std::cout << i << ' '; } increments by 3, not even numbers.

3. Final Answer:

   for (int i = 2; i <= 10; i += 2) { std::cout << i << ' '; } -> Option A

4. Quick Check:

   Increment by 2 from 2 to 10 [OK]

Hint: Increment loop variable by 2 starting at 2 [OK]

Common Mistakes:

• Using wrong increment step
• Forgetting to include 10
• Using condition inside loop unnecessarily