Gas and transaction fees in Blockchain / Solidity - Time & Space Complexity

Time Complexity: Gas and transaction fees

O(n)

Understanding Time Complexity

When working with blockchain, gas and transaction fees affect how much work a transaction requires.

We want to understand how the cost grows as transactions do more work.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

function executeTransaction(actions) {
  let totalGas = 0;
  for (let i = 0; i < actions.length; i++) {
    totalGas += actions[i].gasCost;
    // perform action
  }
  return totalGas;
}

This code calculates the total gas needed by adding up gas costs for each action in a transaction.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through each action in the transaction.
• How many times: Once for every action in the list.

How Execution Grows With Input

As the number of actions grows, the total gas calculation takes longer because it adds each action's cost.

Input Size (n)	Approx. Operations
10	10 additions
100	100 additions
1000	1000 additions

Pattern observation: The work grows directly with the number of actions.

Final Time Complexity

Time Complexity: O(n)

This means the time to calculate gas grows in a straight line as more actions are added.

Common Mistake

[X] Wrong: "Gas calculation time stays the same no matter how many actions there are."

[OK] Correct: Each action adds work, so more actions mean more time to add up gas costs.

Interview Connect

Understanding how gas calculation scales helps you explain blockchain transaction costs clearly and shows you know how work grows with input size.

Self-Check

"What if the actions array was sorted by gas cost before summing? Would the time complexity change?"

Practice

1. What does gas represent in a blockchain transaction?

easy

A. The amount of computational work required to process the transaction

B. The total amount of cryptocurrency sent in the transaction

C. The number of transactions in a block

D. The time it takes to confirm a transaction

Solution

1. Step 1: Understand the role of gas in blockchain

   Gas is a unit that measures how much computational effort is needed to execute operations like transactions or smart contracts.

2. Step 2: Differentiate gas from other transaction elements

   Gas is not the amount of cryptocurrency sent, nor the number of transactions or confirmation time.

3. Final Answer:

   The amount of computational work required to process the transaction -> Option A

4. Quick Check:

   Gas = computational work [OK]

Hint: Gas measures work, not money or time [OK]

Common Mistakes:

• Confusing gas with transaction value
• Thinking gas is transaction speed
• Mixing gas with block size

2. Which of the following is the correct formula to calculate transaction fees?

easy

A. Transaction fees = gas price ÷ gas used

B. Transaction fees = gas price - gas used

C. Transaction fees = gas used + gas price

D. Transaction fees = gas used x gas price

Solution

1. Step 1: Recall the transaction fee formula

   Transaction fees are calculated by multiplying the gas used by the gas price.

2. Step 2: Verify the formula correctness

   Dividing, adding, or subtracting gas used and gas price does not represent the fee calculation.

3. Final Answer:

   Transaction fees = gas used x gas price -> Option D

4. Quick Check:

   Fees = gas used x gas price [OK]

Hint: Multiply gas used by gas price for fees [OK]

Common Mistakes:

• Using division instead of multiplication
• Adding gas used and gas price
• Subtracting gas values

3. Given a transaction that used 21000 gas units and a gas price of 50 gwei, what is the total transaction fee in gwei?

medium

A. 21000 gwei

B. 1,050,000 gwei

C. 420 gwei

D. 50 gwei

Solution

1. Step 1: Identify given values

   Gas used = 21000 units, Gas price = 50 gwei per unit.

2. Step 2: Calculate total fee

   Total fee = gas used x gas price = 21000 x 50 = 1,050,000 gwei.

3. Final Answer:

   1,050,000 gwei -> Option B

4. Quick Check:

   21000 x 50 = 1,050,000 [OK]

Hint: Multiply gas used by gas price directly [OK]

Common Mistakes:

• Multiplying incorrectly
• Confusing gas units with gwei
• Using gas price as total fee

4. Identify the error in this transaction fee calculation code snippet:

gas_used = 30000
gas_price = 20
transaction_fee = gas_used + gas_price
print(transaction_fee)

medium

A. Using addition instead of multiplication for fee calculation

B. Incorrect variable names

C. Missing print statement

D. Gas price should be divided by gas used

Solution

1. Step 1: Review the code logic

   The code adds gas_used and gas_price instead of multiplying them.

2. Step 2: Understand correct fee calculation

   Transaction fee should be gas_used x gas_price, not addition.

3. Final Answer:

   Using addition instead of multiplication for fee calculation -> Option A

4. Quick Check:

   Fee = gas_used x gas_price, not + [OK]

Hint: Fees multiply gas used by gas price, not add [OK]

Common Mistakes:

• Adding gas values instead of multiplying
• Confusing variable names
• Ignoring correct formula

5. A user wants to speed up their transaction by increasing the gas price from 40 gwei to 60 gwei. If the gas used remains 21000, how much more will the user pay in transaction fees compared to before?

hard

A. 600,000 gwei more

B. 1,260,000 gwei more

C. 420,000 gwei more

D. 840,000 gwei more

Solution

1. Step 1: Calculate original fee

   Original fee = 21000 x 40 = 840,000 gwei.

2. Step 2: Calculate new fee

   New fee = 21000 x 60 = 1,260,000 gwei.

3. Step 3: Find difference

   Difference = 1,260,000 - 840,000 = 420,000 gwei more.

4. Final Answer:

   420,000 gwei more -> Option C

5. Quick Check:

   Increase = (60-40)x21000 = 420,000 [OK]

Hint: Subtract old fee from new fee to find extra cost [OK]

Common Mistakes:

• Calculating total fee instead of difference
• Mixing gas price and gas used
• Incorrect multiplication