Payable functions in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Solidity payable function call?

Consider the following Solidity contract snippet. What will be the value of contractBalance after calling deposit with 1 ether?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Wallet {
    uint public contractBalance;

    function deposit() public payable {
        contractBalance += msg.value;
    }
}

A contractBalance will be 0 because msg.value is not added

B contractBalance will be 1000000000000000000 (1 ether in wei)

C contractBalance will be 1 because ether is counted as 1

D The code will not compile due to missing payable keyword

💡 Hint

Remember that msg.value holds the amount of wei sent with the transaction.

The deposit function is marked payable, so it can receive ether. The msg.value contains the amount sent in wei. Adding it to contractBalance updates the balance correctly.

❓ Predict Output

intermediate

What error occurs when calling a non-payable function with ether?

Given this Solidity contract, what happens if you call withdraw and send 1 ether along with the call?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Bank {
    function withdraw() public {
        // withdraw logic
    }
}

A Transaction reverts with error: 'non-payable function cannot accept ether'

B Transaction succeeds and ether is accepted

C Transaction reverts with 'out of gas' error

D Transaction succeeds but ether is ignored

💡 Hint

Check if the function is marked payable or not.

Functions not marked payable reject any ether sent with the call. The transaction reverts with an error indicating the function cannot accept ether.

🔧 Debug

advanced

Why does this payable fallback function not receive ether?

Examine the contract below. Why does sending ether to this contract fail to increase its balance?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Test {
    fallback() external {
        // fallback without payable
    }
}

A Fallback function is not marked payable, so it rejects ether

B Fallback function must be named receive to accept ether

C Fallback functions cannot be external

D Contract balance cannot increase without a deposit function

💡 Hint

Check the fallback function signature and payable keyword.

Fallback functions must be marked payable to accept ether. Without payable, any ether sent causes the transaction to revert.

🧠 Conceptual

advanced

Which statement about payable functions is true?

Select the correct statement about payable functions in Solidity.

A All functions can receive ether regardless of payable keyword

B Payable keyword is optional if the contract has a receive function

C Payable functions automatically reject ether sent

D Only functions marked payable can receive ether during a call

💡 Hint

Think about how Solidity controls ether transfers to functions.

In Solidity, only functions marked payable can receive ether. Other functions reject ether and cause the transaction to revert.

❓ Predict Output

expert

What is the contract balance after these calls?

Given the contract below, what is the value of address(this).balance after these calls? 1. Call deposit with 2 ether 2. Call withdraw with 1 ether Assume withdraw sends ether back to the caller.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Vault {
    function deposit() public payable {
        // Accept ether
    }

    function withdraw(uint amount) public {
        payable(msg.sender).transfer(amount);
    }
}

A 0 ether

B 2 ether (2000000000000000000 wei)

C 1 ether (1000000000000000000 wei)

D 3 ether

💡 Hint

Consider how much ether is sent and withdrawn from the contract balance.

The contract receives 2 ether in deposit. Then it sends 1 ether back in withdraw. So the contract balance is 1 ether.

Practice

1. What is the main purpose of a payable function in a blockchain smart contract?

easy

A. To allow the function to receive cryptocurrency along with the call

B. To make the function run faster

C. To restrict the function to only the contract owner

D. To prevent the function from modifying state variables

Solution

1. Step 1: Understand the role of payable

   The payable keyword marks a function that can accept cryptocurrency (like Ether) when called.

2. Step 2: Compare other options

   Options A, C, and D describe unrelated behaviors. Only To allow the function to receive cryptocurrency along with the call correctly describes the purpose of payable.

3. Final Answer:

   To allow the function to receive cryptocurrency along with the call -> Option A

4. Quick Check:

   Payable = Receive money [OK]

Hint: Payable means function can accept money sent [OK]

Common Mistakes:

• Thinking payable makes function faster
• Confusing payable with access control
• Believing payable prevents state changes

2. Which of the following is the correct syntax to declare a payable function in Solidity?

easy

A. payable function deposit() public {}

B. function deposit() payable public {}

C. function payable deposit() public {}

D. function deposit() public payable {}

Solution

1. Step 1: Recall Solidity function declaration order

   In Solidity, the visibility modifier (e.g., public) comes before payable keyword.

2. Step 2: Check each option

   function deposit() public payable {} uses correct order: function deposit() public payable {}. Options A, B, and D have incorrect keyword order or syntax.

3. Final Answer:

   function deposit() public payable {} -> Option D

4. Quick Check:

   Correct order: visibility then payable [OK]

Hint: Visibility modifier before payable keyword [OK]

Common Mistakes:

• Placing payable before function keyword
• Switching order of public and payable
• Using payable as a function name

3. What will be the output of the following Solidity function when called with 2 Ether?

contract Test {
    uint public balance;
    function deposit() public payable {
        balance = msg.value;
    }
}

medium

A. balance will be 2000000000000000000 (2 Ether in wei)

B. balance will be 2

C. Compilation error

D. balance will be 0

Solution

1. Step 1: Understand msg.value in Solidity

   msg.value holds the amount of wei (smallest Ether unit) sent with the transaction. 2 Ether equals 2 * 10^18 wei = 2000000000000000000 wei.

2. Step 2: Analyze the assignment to balance

   The function sets balance = msg.value, so balance stores the wei amount sent, which is 2000000000000000000.

3. Final Answer:

   balance will be 2000000000000000000 (2 Ether in wei) -> Option A

4. Quick Check:

   msg.value stores wei amount sent [OK]

Hint: msg.value is in wei, not Ether [OK]

Common Mistakes:

• Assuming msg.value is in Ether
• Expecting balance to be zero
• Thinking function won't compile

4. Identify the error in this Solidity function that is intended to receive Ether:

function receiveFunds() public {
    require(msg.value > 0, "Send some Ether");
}

medium

A. require condition is incorrect

B. Function must be marked payable to receive Ether

C. Function visibility should be private

D. msg.value cannot be used inside functions

Solution

1. Step 1: Check function declaration for payable

   To receive Ether, a function must be declared with the payable keyword. This function lacks it.

2. Step 2: Validate other options

   The require condition is valid, visibility can be public, and msg.value is accessible in payable functions.

3. Final Answer:

   Function must be marked payable to receive Ether -> Option B

4. Quick Check:

   Missing payable causes Ether rejection [OK]

Hint: Add payable to accept Ether in function [OK]

Common Mistakes:

• Forgetting payable keyword
• Misusing require condition
• Changing visibility unnecessarily

5. You want to create a smart contract function that accepts Ether and updates the sender's balance only if at least 1 Ether is sent. Which of the following implementations correctly achieves this?

hard

A. function deposit() payable public {
if (msg.value > 1) {
balances[msg.sender] = msg.value;
}
}

B. function deposit() public {
require(msg.value >= 1 ether, "Minimum 1 Ether required");
balances[msg.sender] += msg.value;
}

C. function deposit() public payable {
require(msg.value >= 1 ether, "Minimum 1 Ether required");
balances[msg.sender] += msg.value;
}

D. function deposit() public payable {
if (msg.value >= 1) {
balances[msg.sender] += msg.value;
}
}

Solution

1. Step 1: Check for payable keyword and require condition

   function deposit() public payable { require(msg.value >= 1 ether, "Minimum 1 Ether required"); balances[msg.sender] += msg.value; } correctly marks the function as payable and uses require(msg.value >= 1 ether) to enforce minimum Ether sent.

2. Step 2: Validate balance update and syntax

   function deposit() public payable { require(msg.value >= 1 ether, "Minimum 1 Ether required"); balances[msg.sender] += msg.value; } updates balances[msg.sender] by adding msg.value. Other options either miss payable, use wrong comparison, or incorrect balance update.

3. Final Answer:

   function deposit() public payable { require(msg.value >= 1 ether, "Minimum 1 Ether required"); balances[msg.sender] += msg.value; } -> Option C

4. Quick Check:

   Payable + require with 1 ether check [OK]

Hint: Use require with 1 ether and payable function [OK]

Common Mistakes:

• Omitting payable keyword
• Using incorrect Ether comparison
• Not updating balance correctly