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

Challenge - 5 Problems

❓ Predict Output

intermediate

Gas cost difference between two functions

Consider two smart contract functions that perform the same task but with different gas usage. What is the output showing the gas cost difference?

Blockchain / Solidity

functionA() uses 50000 gas units
functionB() uses 30000 gas units
Gas price is 20 gwei
Calculate the cost difference in gwei.

A 1000000 gwei

B 200000 gwei

C 600000 gwei

D 400000 gwei

💡 Hint

Multiply the gas units difference by the gas price.

The difference in gas units is 20000. Multiplying by 20 gwei gives 400000 gwei saved.

🧠 Conceptual

intermediate

Why does gas efficiency save money?

Which of the following best explains why improving gas efficiency in smart contracts saves money?

A Because gas price decreases when contracts are efficient

B Because using less gas reduces the total fee paid for executing the contract

C Because less gas used means fewer transactions per block

D Because efficient contracts run faster on the blockchain nodes

💡 Hint

Think about how gas relates to transaction fees.

Gas efficiency reduces the amount of gas consumed, which directly lowers the transaction fee paid.

🔧 Debug

advanced

Identify the gas inefficiency in this Solidity code

Which line in the code below causes unnecessary gas usage? contract Example { uint[] public data; function addData(uint x) public { for(uint i = 0; i < data.length; i++) { if(data[i] == x) { return; } } data.push(x); } }

A The data.push(x) call

B The return statement inside the loop

C The for loop iterating over data array

D The public visibility of data array

💡 Hint

Loops over dynamic arrays cost gas proportional to their length.

Iterating over the entire data array every time addData is called uses more gas as the array grows.

📝 Syntax

advanced

Which code snippet correctly reduces gas usage by avoiding storage writes?

Choose the code snippet that saves gas by minimizing storage writes in Solidity.

A

uint count;
function increment() public {
  count++;
}

B

uint count;
function increment() public {
  uint temp = count;
  temp++;
  count = temp;
}

C

uint count;
function increment() public {
  count = count + 1;
}

D

uint count;
function increment() public {
  uint temp = count + 1;
  count = temp;
}

💡 Hint

Look for the simplest way to update a variable.

Using count++ directly is the simplest and most gas-efficient way to increment a storage variable.

🚀 Application

expert

Calculate total gas cost for multiple transactions

A smart contract function costs 45000 gas per call. You plan to call it 100 times. The gas price is 25 gwei. What is the total cost in gwei?

A 112500000 gwei

B 1125000 gwei

C 4500000 gwei

D 225000000 gwei

💡 Hint

Multiply gas per call by number of calls, then by gas price.

Total gas = 45000 * 100 = 4,500,000 gas units. Total cost = 4,500,000 * 25 = 112,500,000 gwei.

Practice

1. Why does gas efficiency matter when writing blockchain smart contracts?

easy

A. Because it makes the code run faster on a regular computer

B. Because it increases the size of the blockchain

C. Because it reduces the transaction cost paid in cryptocurrency

D. Because it allows unlimited transactions without fees

Solution

1. Step 1: Understand what gas represents in blockchain

   Gas is a fee paid to execute operations on the blockchain, measured in cryptocurrency.

2. Step 2: Connect gas efficiency to cost savings

   Using less gas means paying less cryptocurrency for the same operation.

3. Final Answer:

   Because it reduces the transaction cost paid in cryptocurrency -> Option C

4. Quick Check:

   Gas efficiency = lower transaction cost [OK]

Hint: Gas means fee; less gas means less money spent [OK]

Common Mistakes:

• Confusing gas with code speed on a computer
• Thinking gas increases blockchain size
• Believing gas allows free transactions

2. Which of the following Solidity code snippets is more gas efficient?

easy

A. Using for loops with large fixed ranges

B. Using string for storing numbers

C. Using multiple require statements instead of one

D. Using uint8 instead of uint256 for small numbers

Solution

1. Step 1: Identify data types and their gas costs

   Smaller data types like uint8 use less storage and gas than larger types like uint256.

2. Step 2: Compare options for gas efficiency

   Using uint8 for small numbers saves gas compared to storing numbers as strings or using inefficient loops.

3. Final Answer:

   Using uint8 instead of uint256 for small numbers -> Option D

4. Quick Check:

   Smaller data types = less gas [OK]

Hint: Use smallest suitable data type to save gas [OK]

Common Mistakes:

• Using strings to store numbers wastes gas
• Multiple require statements add unnecessary gas
• Large loops increase gas cost

3. Consider this Solidity function:

function add(uint256 a, uint256 b) public pure returns (uint256) {
    uint256 c = a + b;
    return c;
}

If you replace uint256 with uint8 for variables a, b, and c, what is the main effect on gas usage?

medium

A. Gas usage will decrease because smaller types use less storage

B. Gas usage will increase because smaller types are slower

C. Gas usage will stay the same because addition cost is fixed

D. Gas usage will cause an error due to overflow

Solution

1. Step 1: Understand gas cost of data types

   Smaller data types like uint8 use less gas for storage and operations than uint256.

2. Step 2: Consider the effect on gas usage

   Replacing uint256 with uint8 reduces gas because less storage and computation is needed.

3. Final Answer:

   Gas usage will decrease because smaller types use less storage -> Option A

4. Quick Check:

   Smaller type = less gas [OK]

Hint: Smaller integer types reduce gas cost [OK]

Common Mistakes:

• Assuming smaller types slow down code
• Ignoring gas savings from smaller storage
• Confusing overflow errors with gas usage

4. This Solidity function is intended to save gas by using uint8 but causes an error:

function multiply(uint8 a, uint8 b) public pure returns (uint8) {
    uint8 result = a * b;
    require(result >= a, "Overflow");
    return result;
}

What is the main problem causing the error?

medium

A. Using uint8 causes syntax errors

B. The require condition does not correctly detect overflow

C. Multiplication is not allowed on uint8

D. The function should return uint256 instead

Solution

1. Step 1: Analyze the overflow check logic

   The condition result >= a is not a reliable way to detect overflow in multiplication.

2. Step 2: Understand why this causes errors

   If overflow occurs, the check might pass incorrectly or fail, causing unexpected errors.

3. Final Answer:

   The require condition does not correctly detect overflow -> Option B

4. Quick Check:

   Incorrect overflow check causes errors [OK]

Hint: Use safe math libraries for overflow checks [OK]

Common Mistakes:

• Assuming uint8 causes syntax errors
• Believing multiplication is disallowed on uint8
• Ignoring overflow risks in small types

5. You want to optimize a smart contract function that stores user balances. Which approach saves the most gas and money?

hard

A. Use mapping(address => uint8) if balances never exceed 255

B. Store balances as strings to allow flexible formatting

C. Store balances as uint256 and update them frequently

D. Use arrays to store balances and loop through them on each update

Solution

1. Step 1: Identify the best data structure and type for gas efficiency

   Using a mapping with uint8 saves gas by using less storage and avoiding loops.

2. Step 2: Compare other options for gas cost

   Using uint256 wastes storage if values are small; strings and arrays increase gas due to size and looping.

3. Final Answer:

   Use mapping(address => uint8) if balances never exceed 255 -> Option A

4. Quick Check:

   Smaller types + mappings = gas savings [OK]

Hint: Use smallest type and mappings to save gas [OK]

Common Mistakes:

• Using large types unnecessarily wastes gas
• Storing numbers as strings is inefficient
• Looping over arrays increases gas cost