Solidity Mapping – How to Check if List Contains a Value

mappingsolidity

given address[] wallets. What is the correct method to check that the list contains a given address?

Does solidity provide any native list contains function?

If not, is the only way to do a full iteration of the array and check each element?

Are there more memory efficient ways of doing this, maybe a mapping?

Best Answer

Solidity doesn't provide a contains method, you'd have to manually iterate and check.

Using an array for what you're trying to achieve would be a highly inefficient pattern. The best and most cost efficient method is use a mapping data structure. Set the key to be the address and the value to be a boolean. Lists that are too long have the possibility of running out of gas when you're trying to iterate over them.

If you need to iterate through all the keys in the mapping, then you'd need to have an external database to get all the keys. The database can be populated and updated based on events from the smart contract (i.e. an event when the address is added or removed).

Related Solutions

[Ethereum] Dynamic arrays VS mappings when it comes to gas usage

There's some difference in behavior but in your example the gas consumption is about the same. An array has to maintain its length and your mapping uses a counter instead. All data in the array is stored continuously and the mapping is spread in the store but that doesn't affect the costs.

Solidity – Best Method to List All Tokens of a User

This can be achieved by the following:

Declare storage variables at the contract level

mapping(address => uint256[]) public userOwnedTokens;
mapping(uint256 => int256) public tokenIsAtIndex;

mint(tokenId) {
    // Prior minting logic from OpenZeppelin
    userOwnedTokens[msg.sender].push(tokenId);
    uint256 arrayLength = userOwnedTokens[msg.sender].length;
    tokenIsAtIndex[tokenId] = arrayLength;
}

To get all user tokens without loop, all you have to do is, this is possible as the access specifier for userOwnedTokens is public

contractInstance.methods.userOwnedTokens.call(address) // THIS WILL RETURN AN ARRAY OF TOKEN IDs

In case of transfer of tokens, change tokenIsAtIndex[tokenId] to -1

transfer(from, to, tokenId) {
    // Transfer logic
    uint256 tokenIndex = tokenIsAtIndex[tokenId];
    userOwnedTokens[from][tokenIndex] = 000; // TO DENOTE THAT THE TOKEN HAS BEEN TRANSFERRED, YOU CAN USE ANY OTHER NUMBER
 }

Next time when userOwnedTokens is called you can keep a check wherever the tokenId is 000, those tokens have already been transferred and you can decide not to show it to the user on the frontend.

I hope it helps!!!!!

Best Answer

Related Solutions

[Ethereum] Dynamic arrays VS mappings when it comes to gas usage

Solidity – Best Method to List All Tokens of a User

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