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.