Receiving Ether in Blockchain / Solidity - Time & Space Complexity

Time Complexity: Receiving Ether

O(n)

Understanding Time Complexity

When a smart contract receives Ether, it runs code to handle the incoming payment.

We want to understand how the time it takes grows as more Ether transfers happen.

Scenario Under Consideration

Analyze the time complexity of the following Solidity code snippet.

contract ReceiveEther {
    event Received(address sender, uint amount);

    receive() external payable {
        emit Received(msg.sender, msg.value);
    }
}
    

This contract receives Ether and emits an event each time it happens.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The receive function runs once per Ether transfer.
• How many times: It runs exactly once for each incoming payment.

How Execution Grows With Input

Each Ether transfer triggers the receive function once, doing a fixed amount of work.

Input Size (n)	Approx. Operations
10	10 calls to receive()
100	100 calls to receive()
1000	1000 calls to receive()

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

Final Time Complexity

Time Complexity: O(n)

This means the total time grows linearly with the number of Ether transfers received.

Common Mistake

[X] Wrong: "The receive function runs once and handles all Ether transfers at once."

[OK] Correct: Each transfer triggers the receive function separately, so the work adds up with each payment.

Interview Connect

Understanding how contract functions run per transaction helps you explain how blockchain handles many users efficiently.

Self-Check

"What if the receive function also updated a growing list of senders? How would the time complexity change?"

Practice

1.

What is the main purpose of the receive function in a Solidity contract?

easy

A. To reject incoming Ether transfers

B. To send Ether from the contract to an external address

C. To initialize contract state variables

D. To accept Ether sent to the contract without any data

Solution

1. Step 1: Understand the role of the receive function

   The receive function is a special function in Solidity that is triggered when the contract receives Ether without any calldata.

2. Step 2: Identify its purpose

   Its purpose is to accept Ether sent directly to the contract without any function call data.

3. Final Answer:

   To accept Ether sent to the contract without any data -> Option D

4. Quick Check:

   receive function = accept Ether without data [OK]

Hint: Remember: receive handles plain Ether transfers only [OK]

Common Mistakes:

• Confusing receive with fallback function
• Thinking receive sends Ether out
• Assuming receive initializes variables

2.

Which of the following is the correct syntax for a receive function in Solidity?

?

easy

A. receive() internal payable {}

B. receive() external payable {}

C. receive() external {}

D. receive() public payable {}

Solution

1. Step 1: Recall receive function requirements

   The receive function must be declared as external and payable to accept Ether without data.

2. Step 2: Match syntax

   receive() external payable {} matches the required syntax: receive() external payable {}.

3. Final Answer:

   receive() external payable {} -> Option B

4. Quick Check:

   receive must be external and payable [OK]

Hint: receive must be external and payable, no public or internal [OK]

Common Mistakes:

• Using public instead of external
• Omitting payable keyword
• Making receive internal or private

3.

Consider this Solidity contract snippet:

contract Test {
    event Received(address sender, uint amount);
    receive() external payable {
        emit Received(msg.sender, msg.value);
    }
}

What happens when someone sends 1 Ether to this contract without calling any function?

medium

A. The contract rejects the Ether and transaction fails

B. The contract accepts Ether but does not emit any event

C. The contract accepts the Ether and emits the Received event

D. The contract calls fallback function instead

Solution

1. Step 1: Analyze the receive function

   The receive function is payable and emits an event with sender and amount when Ether is received without data.

2. Step 2: Understand the behavior on Ether transfer

   Sending 1 Ether without data triggers receive, so Ether is accepted and event emitted.

3. Final Answer:

   The contract accepts the Ether and emits the Received event -> Option C

4. Quick Check:

   receive payable + emit event = accept and notify [OK]

Hint: Sending Ether without data triggers receive, not fallback [OK]

Common Mistakes:

• Thinking fallback is called instead
• Assuming no event is emitted
• Believing Ether is rejected

4.

Identify the error in this Solidity contract snippet that prevents it from receiving Ether:

contract Example {
    receive() public payable {
        // code
    }
}

medium

A. receive function must be external, not public

B. receive function must not be payable

C. receive function must have a return type

D. receive function must be internal

Solution

1. Step 1: Check receive function visibility

   The receive function must be declared as external, not public.

2. Step 2: Confirm payable requirement

   It is correctly marked payable, so no error there.

3. Final Answer:

   receive function must be external, not public -> Option A

4. Quick Check:

   receive must be external, not public [OK]

Hint: receive must be external, not public or internal [OK]

Common Mistakes:

• Using public instead of external
• Removing payable keyword
• Adding return type to receive

5.

You want to create a contract that tracks the total Ether received and rejects any Ether sent with data. Which combination of functions should you implement?

hard

A. Implement receive() external payable to accept Ether and fallback() external to revert on data

B. Implement only fallback() external payable to accept all Ether

C. Implement receive() external without payable and fallback() external payable

D. Implement receive() external payable and fallback() external payable both accepting Ether

Solution

1. Step 1: Accept Ether without data using receive

   The receive function accepts Ether sent without data and can update total received.

2. Step 2: Reject Ether sent with data using fallback

   The fallback function should revert to reject any Ether sent with data.

3. Final Answer:

   Implement receive() external payable to accept Ether and fallback() external to revert on data -> Option A

4. Quick Check:

   receive accepts plain Ether, fallback rejects data [OK]

Hint: Use receive for plain Ether, fallback to reject data [OK]

Common Mistakes:

• Making fallback payable and accepting Ether with data
• Omitting fallback to reject data
• Not marking receive payable