Gas usage testing in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a gas variable initialized to zero.

Blockchain / Solidity

uint256 gasUsed = [1];

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Agasleft()

B1

C0

D100

Attempts:

3 left

💡 Hint

Common Mistakes

Using gasleft() directly instead of zero

Initializing with 1 or 100

The gasUsed variable should start at zero to measure gas consumption properly.

2fill in blank

medium

Complete the code to record the starting gas before a function call.

Blockchain / Solidity

uint256 startGas = [1]();

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Agasleft

BgasUsed

Cblock.gaslimit

Dgas

Attempts:

3 left

💡 Hint

Common Mistakes

Using gasUsed variable

Using block.gaslimit which is total gas limit

gasleft() returns the remaining gas at the current point in execution.

3fill in blank

hard

Fix the error in calculating gas used after function execution.

Blockchain / Solidity

gasUsed = startGas - [1]();

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AgasUsed

Bblock.gaslimit

Cgasleft

Dgasleft()

Attempts:

3 left

💡 Hint

Common Mistakes

Using gasleft without parentheses

Using block.gaslimit incorrectly

gasleft() must be called as a function to get current gas left.

4fill in blank

hard

Fill both blanks to create a function that returns gas used by a call.

Blockchain / Solidity

function measureGas() public returns (uint256) {
    uint256 start = [1]();
    // some operation
    uint256 end = [2]();
    return start - end;
}

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Agasleft

Bblock.timestamp

Cgasleft()

Dblock.number

Attempts:

3 left

💡 Hint

Common Mistakes

Using block.timestamp or block.number instead of gasleft()

Both start and end gas measurements use gasleft() function to get current gas left.

5fill in blank

hard

Fill all three blanks to create a gas measurement wrapper function.

Blockchain / Solidity

function gasWrapper() public returns (uint256) {
    uint256 [1] = [2]();
    performAction();
    uint256 gasUsed = [1] - [3]();
    return gasUsed;
}

Drag options to blanks, or click blank then click option'

AstartGas

Bgasleft

Cgasleft()

DendGas

Attempts:

3 left

💡 Hint

Common Mistakes

Using gasleft without parentheses

Using wrong variable names

We declare startGas variable, assign gasleft() at start, then subtract gasleft() after action.

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