Structs giving error while creating nft on ropsten

Question

I am trying to create 10 random nfts with different supplies using solidity version 6 and remix ide. However I keep getting the following errors:

contracts/bullrun_cards.sol:29:16: TypeError: Type struct MyNFT.Cards memory is not implicitly convertible to expected type struct MyNFT.Cards storage ref[] storage ref. card = Cards({supply:_supply, description:_desc,card_id:newNftTokenId}); ^--------------------------------------------------------------^

contracts/bullrun_cards.sol:30:35: TypeError: Type struct MyNFT.Cards storage ref[] storage ref is not implicitly convertible to expected type struct MyNFT.Cards storage ref. allCards[newNftTokenId-1]=card;

contracts/bullrun_cards.sol:31:15: TypeError: Integer constant expected. Cards[newNftTokenId-1].push(card); ^-------------^

My code is as follows:

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.12;

import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/solc-0.6/contracts/token/ERC721/ERC721.sol";
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/solc-0.6/contracts/utils/Counters.sol";
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/solc-0.6/contracts/access/Ownable.sol";

contract MyNFT is ERC721, Ownable {
    
    using Counters for Counters.Counter;
    Counters.Counter private _tokenIds; //Counter is a struct in the Counters library
    uint constant total_supply =7777;
    
    constructor() ERC721("MyNFT", "MNFT") public {}
    
    struct Cards{
        uint supply;
        string description;
        uint card_id;
    }
    
    Cards[] public card;
    mapping(uint =>Cards)public allCards;
    
    function createCard(uint _supply, string memory _desc) public returns(uint){
        assert(_supply<total_supply);
        _tokenIds.increment();
        uint256 newNftTokenId = _tokenIds.current();
        card = Cards({supply:_supply, description:_desc,card_id:newNftTokenId});
        allCards[newNftTokenId-1]=card;
        Cards[newNftTokenId-1].push(card);
        
    }
    
    function mintToken( uint _s, string memory _d,  string memory _tokenURI) public onlyOwner
    {
        uint newid=createCard(_s, _d);
        address _address = msg.sender;
        _safeMint(_address, newid);
        _setTokenURI(newid, _tokenURI);
        
    }
}

please kindly help me in resolving this issue

Answer 1

First, I don't see why you link a token ID with a supply value. When you transfer a token, you use the tokenId then what will happen with the supply? Will it go entirely to the new owner?

Then you declare a public (not a good idea, keep it private) Cards[] public card which you overwrite when creating a new card (card = Cards({supply:_supply, description:_desc,card_id:newNftTokenId});). Maybe use another variable name, like newCard or _card, or whatever.

Another thing is when you want to add the new card to the cards list, don't do it like Cards[newNftTokenId-1].push(card);, you need to push the new card to the card property: card.push(_newCard)