Is there any alternative for for loop?

Question

I have used in burn() function of NFT Contract.

 function burn(address to, uint256 tokenId) external {
    require(_exists(tokenId), "Contract: operator query for nonexistent token");
    
    for (uint256 i = 0; i < _tokenIds.length; i ++) {
        if (_tokenIds[i] == tokenId) {
            _tokenIds[i] = _tokenIds[_tokenIds.length - 1];
            _tokenIds.pop();
        }
    }
    
    // Clear metadata (if any)
    delete _Info[tokenId];
    delete _tokenOwnersMap[tokenId];
    delete _tokenApprovalsMap[tokenId];

    emit Burn(to, tokenId);
}

According to some resources on internet, loops and use of delete cause more gas consumption, is that correct? Any other alternative for delete?

Answer 1

Not entirely correct. In fact, delete reduces gas costs. Read about gas refunds for example here: https://ethereum.stackexchange.com/a/859/31933

The problematic part is looping. That has two main issues:

1. If you loop based on length of an array (or any value which is not predetermined) the loop may run for too long if there are lots of entries and the transaction runs out of gas

2. Structures which require a loop are often expensive to use due to the loop

Depending on the requirements it is sometimes possible to remove a loop (and its array) and use mappings instead. But that's only true if your data structure can be represented with a mapping and if you don't really need to iterate over it (you can't iterate over a mapping).

Answer 2

To avoid for loop, I use a mapping to map from id to index. Index is the index of nft in the array which you store the nfts.

  mapping(uint=>uint) private _idToNftIndex;

I write a function:

uint256[] private _nftsArray;

function _removeTokenFromAlTokenEnumeration(uint tokenId) private{
    uint lastTokenIndex=_allNfts.length-1;
    uint tokenIndexToRemove=_idToNftIndex[tokenId];
    uint lastTokenId=_nftsArray[lastTokenIndex];
    // after this we will have two lastTokenid in the array
    _nftsArray[tokenIndexToRemove]=lastTokenId;
    _idToNftIndex[lastTokenId]=tokenIndexToRemove;
    delete _idToNftIndex[tokenId];
    _nftsArray.pop();
  }

let's say

_idToNftIndex={1:0,2:1,3:2}
// storing tokenId's here
_nftsArray=[1,2,3]

I want to delete the 1st index in _nftsArray which is tokenId 2. After this line of code _nftsArray[tokenIndexToRemove]=lastTokenId; my array would be

 _nftsArray=[1,3,3]

then I remove the last item with _nftsArray.pop(). So my array would be

 _nftsArray=[1,3]

Answer 3

you can use mapping to avoid for loop.

e.g.

mapping (uint=>uint) _tokenIds;
function burn(address to,uint256 tokenId) external{
    _tokenIds["tokenId"]
   //....other functions
}