ERC-721 NFT standard in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the ERC-721 interface.

Blockchain / Solidity

interface IERC721 is IERC165 {
    function [1](address owner) external view returns (uint256);
}

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Abalance_of

Bbalanceof

CbalanceOf

Dbalance

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect function names like 'balance' or 'balance_of'.

Misspelling the function name with wrong capitalization.

The function name in the ERC-721 standard is balanceOf, which returns the number of NFTs owned by an address.

2fill in blank

medium

Complete the code to emit the Transfer event when an NFT is transferred.

Blockchain / Solidity

event Transfer(address indexed from, address indexed to, uint256 indexed [1]);

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AtokenId

Bid

CnftId

DassetId

Attempts:

3 left

💡 Hint

Common Mistakes

Using generic or incorrect parameter names like 'id' or 'nftId'.

Omitting the 'indexed' keyword which is required for event filtering.

The ERC-721 standard defines the Transfer event with the third parameter named tokenId, representing the NFT's unique identifier.

3fill in blank

hard

Fix the error in the function signature for safeTransferFrom with data parameter.

Blockchain / Solidity

function safeTransferFrom(address from, address to, uint256 tokenId, bytes calldata [1]) external;

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Apayload

BextraData

Cinfo

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect parameter names like 'extraData' or 'payload'.

Omitting the 'calldata' keyword for the bytes parameter.

The ERC-721 standard defines the last parameter of safeTransferFrom as bytes calldata data, which allows passing additional data with the transfer.

4fill in blank

hard

Fill both blanks to complete the mapping declaration for token owners and balances.

Blockchain / Solidity

mapping(uint256 => address) private [1];
mapping(address => uint256) private [2];

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A_owners

B_balances

Cowners

Dbalances

Attempts:

3 left

💡 Hint

Common Mistakes

Using variable names without underscores.

Swapping the two mappings' variable names.

In the ERC-721 implementation, _owners maps token IDs to owner addresses, and _balances maps owner addresses to their token count.

5fill in blank

hard

Fill all three blanks to complete the function that checks if an address is approved or owner of a token.

Blockchain / Solidity

function _isApprovedOrOwner(address spender, uint256 tokenId) internal view returns (bool) {
    address owner = [1](tokenId);
    return (spender == owner || spender == [2](tokenId) || isApprovedForAll(owner, [3]));
}

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AownerOf

BgetApproved

Cowner

Dspender

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect function names like 'owner' instead of 'ownerOf'.

Confusing the parameters in the isApprovedForAll call.

Using wrong variable names for the spender.

The function uses ownerOf to get the token owner, getApproved to get the approved address for the token, and compares spender to check permissions.