Gas usage testing in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Gas usage of simple storage update

Consider a smart contract function that updates a stored number. What is the gas usage behavior when calling this function multiple times with the same value?

Blockchain / Solidity

contract SimpleStorage {
    uint256 public storedData;

    function set(uint256 x) public {
        storedData = x;
    }
}

A Gas cost is zero for calls with the same value as the current storedData.

B Gas cost is the same for every call regardless of the value.

C Gas cost is high only on the first update; subsequent calls with the same value cost less gas.

D Gas cost increases with each call regardless of the value.

💡 Hint

Think about how Ethereum refunds gas when storage is not changed.

When the stored value changes, gas is spent to update storage. If the value is the same, less gas is used because no storage slot is modified.

❓ Predict Output

intermediate

Gas cost of adding vs removing elements in a mapping

Given a mapping in a smart contract, what happens to gas usage when you add a new key-value pair versus when you delete a key?

Blockchain / Solidity

contract MapExample {
    mapping(address => uint256) public balances;

    function addBalance(address user, uint256 amount) public {
        balances[user] = amount;
    }

    function removeBalance(address user) public {
        delete balances[user];
    }
}

A Deleting a key costs more gas than adding one.

B Adding and deleting keys cost the same gas with no refunds.

C Neither adding nor deleting keys affects gas usage.

D Adding a new key costs more gas; deleting a key refunds some gas.

💡 Hint

Consider how storage refunds work in Ethereum.

Adding a new key writes to storage and costs gas. Deleting a key frees storage and refunds gas to the caller.

🔧 Debug

advanced

Unexpected high gas usage in loop

This contract function loops over an array and updates a mapping. Why does it consume unexpectedly high gas?

Blockchain / Solidity

contract LoopGas {
    mapping(uint256 => uint256) public data;
    uint256[] public keys;

    function updateData() public {
        for (uint i = 0; i < keys.length; i++) {
            data[keys[i]] = i;
        }
    }
}

A The mapping update inside the loop is free and does not affect gas.

B The loop condition uses <= which causes an out-of-bounds access and wastes gas.

C The keys array is empty, so the loop does not run and gas is wasted.

D The function is missing a payable modifier causing high gas.

💡 Hint

Check the loop boundary carefully.

Using <= in the loop condition causes the loop to run one extra time, accessing keys[keys.length], which is out-of-bounds and wastes gas.

📝 Syntax

advanced

Gas optimization with unchecked arithmetic

Which option correctly uses unchecked arithmetic to save gas in Solidity 0.8+ without causing overflow errors?

Blockchain / Solidity

function sum(uint256 a, uint256 b) public pure returns (uint256) {
    // Fill in the blank
}

A unchecked { return a + b; }

B return a + b; // no unchecked block

C unchecked return a + b;

D return unchecked(a + b);

💡 Hint

The unchecked block disables overflow checks inside it.

In Solidity 0.8+, arithmetic operations check for overflow by default. Wrapping the operation in an unchecked block disables this check and saves gas.

🚀 Application

expert

Estimating gas cost for dynamic array resizing

You have a function that appends elements to a dynamic array in a smart contract. How does the gas cost behave as the array grows, and which option best describes this behavior?

Blockchain / Solidity

contract DynamicArray {
    uint256[] public numbers;

    function addNumber(uint256 num) public {
        numbers.push(num);
    }
}

A Gas cost per push remains roughly constant regardless of array size.

B Gas cost per push increases linearly as the array grows.

C Gas cost per push decreases as the array grows due to storage optimization.

D Gas cost per push is zero after the first element is added.

💡 Hint

Think about how storage slots are allocated for dynamic arrays.

Each push writes to a new storage slot, so gas cost per push stays roughly constant regardless of array length.

Practice

1. What is the main purpose of gas usage testing in blockchain smart contracts?

easy

A. To check if the smart contract code has syntax errors

B. To increase the gas price for faster transactions

C. To verify the correctness of smart contract logic

D. To measure how much gas a function consumes when executed

Solution

1. Step 1: Understand gas usage testing

   Gas usage testing measures the amount of gas consumed by smart contract functions during execution.

2. Step 2: Differentiate from other tests

   It is not about syntax errors, logic correctness, or changing gas prices but about cost measurement.

3. Final Answer:

   To measure how much gas a function consumes when executed -> Option D

4. Quick Check:

   Gas usage testing = measure gas cost [OK]

Hint: Gas testing measures cost, not correctness or syntax [OK]

Common Mistakes:

• Confusing gas usage testing with syntax or logic testing
• Thinking gas usage testing changes gas prices
• Assuming gas usage testing verifies contract correctness

2. Which of the following is the correct way to get gas used from a transaction receipt in Solidity testing frameworks?

easy

A. const gasUsed = receipt.gasUsed;

B. const gasUsed = receipt.gas_price;

C. const gasUsed = receipt.gasLimit;

D. const gasUsed = receipt.gasCost;

Solution

1. Step 1: Identify the correct property for gas used

   Transaction receipts have a property called gasUsed that shows the gas consumed.

2. Step 2: Eliminate incorrect properties

   gas_price is gas price per unit, gasLimit is max gas allowed, and gasCost is not a standard property.

3. Final Answer:

   const gasUsed = receipt.gasUsed; -> Option A

4. Quick Check:

   Gas used = receipt.gasUsed [OK]

Hint: Gas used is in receipt.gasUsed property [OK]

Common Mistakes:

• Using gas_price instead of gasUsed
• Confusing gasLimit with gas used
• Trying to access non-existent gasCost property

3. Consider this JavaScript test snippet for a smart contract function:

const receipt = await contract.methods.myFunction().send({ from: user });
console.log(receipt.gasUsed);

What will this code output?

medium

A. An error because gasUsed is undefined

B. The gas price set for the transaction

C. The amount of gas used by myFunction execution

D. The total gas limit of the block

Solution

1. Step 1: Understand what receipt.gasUsed represents

   The receipt.gasUsed property contains the gas consumed by the transaction.

2. Step 2: Confirm the code logs gas used

   The code logs receipt.gasUsed, so it outputs the gas used by myFunction.

3. Final Answer:

   The amount of gas used by myFunction execution -> Option C

4. Quick Check:

   receipt.gasUsed logs gas used [OK]

Hint: receipt.gasUsed shows gas consumed by function call [OK]

Common Mistakes:

• Thinking gasUsed is gas price or block gas limit
• Expecting gasUsed to be undefined or cause error
• Confusing gasUsed with transaction cost in ether

4. You wrote this test code to measure gas usage:

const receipt = await contract.methods.transfer(to, amount).send({ from: sender });
const gasUsed = receipt.gasPrice;
console.log(gasUsed);

What is the error here?

medium

A. Using receipt.gasPrice instead of receipt.gasUsed to get gas consumed

B. Missing await keyword before contract method call

C. Incorrect method name; should be transferFrom

D. send() does not return a receipt object

Solution

1. Step 1: Check property used for gas consumption

   The code uses receipt.gasPrice, which is gas price per unit, not gas used.

2. Step 2: Identify correct property for gas used

   The correct property to get gas consumed is receipt.gasUsed.

3. Final Answer:

   Using receipt.gasPrice instead of receipt.gasUsed to get gas consumed -> Option A

4. Quick Check:

   Gas used = receipt.gasUsed, not gasPrice [OK]

Hint: Use receipt.gasUsed, not gasPrice, for gas consumption [OK]

Common Mistakes:

• Confusing gasPrice with gasUsed
• Omitting await causing promise issues
• Assuming send() returns something other than receipt

5. You want to compare gas usage of two smart contract functions, funcA and funcB, to find which is cheaper. You run:

const receiptA = await contract.methods.funcA().send({ from: user });
const receiptB = await contract.methods.funcB().send({ from: user });
console.log(receiptA.gasUsed < receiptB.gasUsed ? 'funcA cheaper' : 'funcB cheaper');

What is a potential issue with this approach?

hard

A. You cannot compare gasUsed values directly; must convert to ether

B. Gas usage can vary due to blockchain state; single runs may mislead

C. You must add gasPrice to gasUsed to get total cost

D. send() does not return gasUsed in receipt

Solution

1. Step 1: Understand gas usage variability

   Gas used depends on blockchain state and inputs; single runs may not reflect average cost.

2. Step 2: Identify why single comparison can mislead

   Different states or conditions can cause gas usage to fluctuate, so multiple tests or averaging is better.

3. Final Answer:

   Gas usage can vary due to blockchain state; single runs may mislead -> Option B

4. Quick Check:

   Gas varies by state; test multiple times [OK]

Hint: Gas varies; test multiple times for accurate comparison [OK]

Common Mistakes:

• Assuming gasUsed is fixed for all runs
• Adding gasPrice to gasUsed incorrectly
• Thinking gasUsed must convert to ether for comparison