Withdrawal patterns in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a withdrawal function that sends funds to a user.

Blockchain / Solidity

function withdraw(uint amount) public {
    require(balances[msg.sender] >= amount, "Insufficient funds");
    payable(msg.sender).[1](amount);
}

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

AsendFunds

Bapprove

Cdeposit

Dtransfer

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'deposit' instead of 'transfer' to send funds.

Using 'approve' which is for token allowances, not sending Ether.

The transfer function is used to send Ether to the user safely.

2fill in blank

medium

Complete the code to update the user's balance after withdrawal.

Blockchain / Solidity

function withdraw(uint amount) public {
    require(balances[msg.sender] >= amount, "Insufficient funds");
    balances[msg.sender] [1] amount;
    payable(msg.sender).transfer(amount);
}

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A-=

B*=

C+=

D/=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+=' which would increase the balance incorrectly.

Using '*=' or '/=' which are not appropriate here.

We subtract the withdrawn amount from the user's balance using -=.

3fill in blank

hard

Fix the error in the withdrawal pattern to prevent reentrancy attacks.

Blockchain / Solidity

function withdraw(uint amount) public {
    require(balances[msg.sender] >= amount, "Insufficient funds");
    balances[msg.sender] [1] amount;
    payable(msg.sender).transfer(amount);
}

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

A-=

B+=

C=

D*=

Attempts:

3 left

💡 Hint

Common Mistakes

Updating balance after transfer, which can cause reentrancy.

Using '=' which overwrites balance incorrectly.

To prevent reentrancy, update the balance before transferring funds. The operator -= subtracts the amount.

4fill in blank

hard

Fill both blanks to implement the Checks-Effects-Interactions pattern correctly.

Blockchain / Solidity

function withdraw(uint amount) public {
    require(balances[msg.sender] >= amount, "Insufficient funds");
    balances[msg.sender] [1] amount;
    payable(msg.sender).[2](amount);
}

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A-=

Bsend

Ctransfer

D+=

Attempts:

3 left

💡 Hint

Common Mistakes

Sending Ether before updating balance.

Using 'send' which returns a boolean and requires extra checks.

First subtract the amount from balance using -=, then transfer funds with transfer.

5fill in blank

hard

Fill all three blanks to create a safe withdrawal function using the withdrawal pattern.

Blockchain / Solidity

function withdraw(uint amount) public {
    uint balance = balances[msg.sender];
    require(balance [1] amount, "Insufficient funds");
    balances[msg.sender] [2] amount;
    (bool success, ) = payable(msg.sender).[3]{value: amount}("");
    require(success, "Transfer failed.");
}

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

A>=

B-=

Ccall

D+=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+=' to update balance which increases it.

Using 'transfer' instead of 'call' which is safer in some cases.

Not checking the success of the call.

Check if balance is greater or equal to amount with >=, subtract amount from balance with -=, and use call to send Ether safely.