For and while loops in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a for loop in a smart contract

What is the output of this Solidity code snippet when the function sumUpToFive() is called?

Blockchain / Solidity

pragma solidity ^0.8.0;
contract LoopTest {
    function sumUpToFive() public pure returns (uint) {
        uint sum = 0;
        for (uint i = 1; i <= 5; i++) {
            sum += i;
        }
        return sum;
    }
}

A 0

B 10

C 5

D 15

💡 Hint

Add numbers from 1 to 5 using a loop.

The for loop adds numbers 1 through 5 to sum. 1+2+3+4+5 equals 15.

❓ Predict Output

intermediate

While loop behavior in a blockchain contract

What will be the value of counter after calling countDown() in this Solidity contract?

Blockchain / Solidity

pragma solidity ^0.8.0;
contract LoopCountdown {
    function countDown() public pure returns (uint) {
        uint counter = 5;
        while (counter > 0) {
            counter--;
        }
        return counter;
    }
}

A 1

B 0

C 5

D Undefined behavior

💡 Hint

The loop decreases counter until it is no longer greater than zero.

The while loop decreases counter from 5 down to 0, then stops and returns 0.

🔧 Debug

advanced

Identify the error in this for loop

This Solidity function is intended to sum numbers from 1 to 3, but it causes a compilation error. What is the cause?

Blockchain / Solidity

pragma solidity ^0.8.0;
contract LoopError {
    function sumThree() public pure returns (uint) {
        uint sum = 0;
        for (uint i = 1; i < 4; i++)
            sum += i;
        return sum;
    }
}

A Missing semicolon after sum += i

B Return statement inside the loop

C Variable i is not declared

D Incorrect loop condition syntax

💡 Hint

Check punctuation after statements inside the loop.

The line 'sum += i' is missing a semicolon, causing a syntax error.

❓ Predict Output

advanced

Output of nested loops in a smart contract

What is the output of the nestedSum() function below?

Blockchain / Solidity

pragma solidity ^0.8.0;
contract NestedLoops {
    function nestedSum() public pure returns (uint) {
        uint total = 0;
        for (uint i = 1; i <= 2; i++) {
            for (uint j = 1; j <= 3; j++) {
                total += i * j;
            }
        }
        return total;
    }
}

A 9

B 12

C 18

D 6

💡 Hint

Multiply i and j for each pair and add all results.

The sums are: (1*1 + 1*2 + 1*3) + (2*1 + 2*2 + 2*3) = 6 + 12 = 18.

🧠 Conceptual

expert

Gas cost implications of loops in blockchain

Which statement best describes the gas cost behavior of loops in Ethereum smart contracts?

A Gas cost increases linearly with the number of loop iterations

B Loops do not consume gas in smart contracts

C Gas cost decreases as loops run longer due to optimization

D Gas cost is fixed regardless of loop iterations

💡 Hint

Think about how Ethereum charges for computation steps.

Each loop iteration executes instructions, increasing gas cost linearly with iterations.

Practice

1. Which statement best describes a for loop in blockchain programming?

easy

A. It repeats code while a condition is true.

B. It runs only once and then stops.

C. It repeats code a fixed number of times.

D. It stops the program immediately.

Solution

1. Step 1: Understand the for loop purpose

   A for loop is designed to repeat code a set number of times, often counting through a range or list.

2. Step 2: Compare with other loop types

   A while loop repeats while a condition is true, not a fixed count. The other options do not describe for loops correctly.

3. Final Answer:

   It repeats code a fixed number of times. -> Option C

4. Quick Check:

   for loop = fixed repeats [OK]

Hint: For loops count repeats; while loops check conditions [OK]

Common Mistakes:

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

2. Which of the following is the correct syntax for a while loop in blockchain programming?

easy

A. while (condition) { // code }

B. while condition: // code

C. while condition { // code }

D. while (condition): // code

Solution

1. Step 1: Identify correct syntax style

   In blockchain programming languages like Solidity, the while loop uses parentheses for the condition and curly braces for the code block.

2. Step 2: Check each option

   while (condition) { // code } matches the correct syntax: while (condition) { // code }. Others miss parentheses or use incorrect punctuation.

3. Final Answer:

   while (condition) { // code } -> Option A

4. Quick Check:

   Correct while syntax uses parentheses and braces [OK]

Hint: Remember: while(condition) { code } uses parentheses and braces [OK]

Common Mistakes:

• Omitting parentheses around condition
• Using colon instead of braces
• Mixing syntax from other languages

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

uint sum = 0;
for (uint i = 1; i <= 3; i++) {
  sum += i;
}
return sum;

medium

A. 6

B. 3

C. 0

D. Error

Solution

1. Step 1: Analyze the loop iterations

   The loop runs with i = 1, 2, 3. Each time, sum adds i: sum = 0+1, then 1+2, then 3+3.

2. Step 2: Calculate the final sum

   Sum after loop: 0+1=1, 1+2=3, 3+3=6.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   1+2+3 = 6 [OK]

Hint: Add numbers from 1 to 3 to get sum quickly [OK]

Common Mistakes:

• Stopping loop early
• Adding wrong values
• Confusing sum initialization

4. Identify the error in this code snippet:

uint i = 0;
while (i < 3)
  i++;

medium

A. Condition should be i <= 3

B. Missing braces {} for while loop body

C. Variable i should start at 1

D. Incorrect increment operator

Solution

1. Step 1: Check loop body syntax

   The while loop lacks braces {}, so only the next statement is inside the loop.

2. Step 2: Understand impact of missing braces

   Without braces, the loop runs only the single statement i++;. This is valid but can cause confusion or errors if more statements are added.

3. Final Answer:

   Missing braces {} for while loop body -> Option B

4. Quick Check:

   Always use braces for clarity in loops [OK]

Hint: Use braces {} to clearly define loop body [OK]

Common Mistakes:

• Assuming multiple statements run without braces
• Changing variable start unnecessarily
• Altering loop condition wrongly

5. You want to create a loop that counts down from 5 to 1 and stores each number in an array. Which code snippet correctly uses a for loop to do this in blockchain programming?

A) for (uint i = 5; i > 0; i--) { arr.push(i); }
B) for (uint i = 1; i <= 5; i++) { arr.push(i); }
C) for (uint i = 5; i >= 1; i++) { arr.push(i); }
D) for (uint i = 5; i > 0; i++) { arr.push(i); }

hard

A. Infinite loop due to i++ with i > 0

B. Counts up from 1 to 5

C. Infinite loop due to i++ with i >= 1

D. Counts down from 5 to 1 correctly

Solution

1. Step 1: Analyze each loop's counting direction

   Counts down from 5 to 1 correctly counts down from 5 to 1 using i-- and condition i > 0. Counts up from 1 to 5 counts up. Options C and D increase i but have conditions that cause infinite loops.

2. Step 2: Confirm correct loop behavior

   Counts down from 5 to 1 correctly correctly pushes 5,4,3,2,1 into the array and stops when i reaches 0.

3. Final Answer:

   Counts down from 5 to 1 correctly -> Option D

4. Quick Check:

   Decrement i with i-- and condition i > 0 for countdown [OK]

Hint: Use i-- and i > 0 to count down safely [OK]

Common Mistakes:

• Using i++ when counting down
• Wrong loop condition causing infinite loops
• Confusing counting up vs down