Why gas efficiency saves money in Blockchain / Solidity - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to calculate the gas cost for a transaction.

Blockchain / Solidity

uint gasUsed = [1]; // gas used by transaction
uint gasPrice = 20 gwei;
uint cost = gasUsed * gasPrice;

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

Amsg.gas

B21000

Cblock.gaslimit

Dgasleft()

Attempts:

3 left

💡 Hint

Common Mistakes

Using gasleft() which returns remaining gas, not used gas.

The typical gas used for a simple transaction is 21000 units.

2fill in blank

medium

Complete the code to calculate the total transaction cost in Ether.

Blockchain / Solidity

uint gasUsed = 21000;
uint gasPrice = 50 gwei;
uint costInWei = gasUsed * [1];
uint costInEther = costInWei / 1 ether;

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

AgasPrice

BgasPrice / 1 ether

CgasUsed

D1 ether

Attempts:

3 left

💡 Hint

Common Mistakes

Dividing gasPrice by 1 ether instead of using it directly.

The cost in wei is gasUsed multiplied by gasPrice.

3fill in blank

hard

Fix the error in the code that calculates gas cost in Ether.

Blockchain / Solidity

uint gasUsed = 30000;
uint gasPrice = 100 gwei;
uint costInWei = gasUsed * gasPrice;
uint costInEther = costInWei [1] 1 ether;

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

A+

B*

C/

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using * instead of / causes wrong conversion.

To convert wei to ether, divide by 1 ether (10^18 wei).

4fill in blank

hard

Fill both blanks to create a dictionary of function gas costs and filter those above 50000 gas.

Blockchain / Solidity

gasCosts = {
    "transfer": 21000,
    "mint": 60000,
    "burn": 45000
};

expensiveFunctions = {k: gasCosts[k] for k in gasCosts if gasCosts[k] [1] [2];

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

A>

B<

C50000

D60000

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of > filters wrong functions.

We want functions with gas cost greater than 50000.

5fill in blank

hard

Fill all three blanks to create a function that returns the gas cost in Ether for a given gasUsed and gasPrice.

Blockchain / Solidity

function calculateCost(uint gasUsed, uint gasPrice) public pure returns (uint) {
    uint costInWei = gasUsed [1] gasPrice;
    uint costInEther = costInWei [2] [3];
    return costInEther;
}

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

A*

B/

C1 ether

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Adding instead of multiplying or dividing causes wrong results.

Multiply gasUsed by gasPrice to get wei, then divide by 1 ether to get Ether.