Reentrancy guard pattern in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a boolean variable named 'locked' initialized to false.

Blockchain / Solidity

bool [1] = false;

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

AisLocked

Block

Cstatus

Dlocked

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different variable name like 'lock' or 'status' which is not standard.

The reentrancy guard uses a boolean variable named locked to track if the function is already running.

2fill in blank

medium

Complete the modifier to check if the contract is not locked before running the function.

Blockchain / Solidity

modifier noReentrant() {
    require(![1], "Reentrant call");
    _;
}

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

Alocked

BisLocked

Clock

Dstatus

Attempts:

3 left

💡 Hint

Common Mistakes

Checking the wrong variable or missing the negation operator.

The modifier checks that locked is false before allowing the function to run.

3fill in blank

hard

Fix the error in the modifier to set the locked state correctly before and after the function runs.

Blockchain / Solidity

modifier noReentrant() {
    require(!locked, "Reentrant call");
    locked = [1];
    _;
    locked = false;
}

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

Atrue

Bstatus

Clocked

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Setting locked to false before the function runs, which disables the guard.

Set locked = true before running the function to prevent reentrancy.

4fill in blank

hard

Fill both blanks to complete the safe withdraw function using the noReentrant modifier and resetting the locked state.

Blockchain / Solidity

function withdraw(uint amount) public noReentrant {
    require(balances[msg.sender] >= amount, "Insufficient balance");
    balances[msg.sender] -= amount;
    (bool success, ) = msg.sender.call{value: amount}([1]);
    require(success, "Transfer failed");
    locked = [2];
}

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

Agas(2300)

Bfalse

Ctrue

Dgas()

Attempts:

3 left

💡 Hint

Common Mistakes

Using gas(2300) which limits gas and can cause failures.

Not resetting locked to false.

Use gas() to forward all gas in the call and set locked = false after the transfer to unlock.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps addresses to their balances only if balance is greater than zero.

Blockchain / Solidity

mapping(address => uint) public balances;

function positiveBalances(address[] memory users) public view returns (mapping(address => uint) memory) {
    return {user: balances[user] for user in users if balances[user] [1] 0};
}

modifier noReentrant() {
    require(!locked, "Reentrant call");
    locked = [2];
    _;
    locked = [3];
}

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

A>

Btrue

Cfalse

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using < or wrong boolean values in the modifier.

The comprehension filters balances greater than zero using >. The modifier sets locked = true before and locked = false after the function.