[Ethereum] Can’t get length of array? (not querying for property)

arrayssolidity

mapping(uint256 => address) public userList;

function getUserCount() public constant returns(uint userCount) {
 return userList.length;
}

function newUser(address _address) internal returns(bool success) {
    userList.push(_address);
    return true;
  }

function getUser(uint256 _userId) public constant returns(address){
      return(userList[_userId]);
  }

throws error "member length can not be found" – can somebody explain to me why this doesn't work? i've looked at the storage patterns list and saw that they would add an additional array to have a user count – i was really trying to avoid that so that on high demand two users would not get the same id, by setting their id to length – was just planning to push the new users without setting an id manually

Best Answer

In the code above userList is not an array, it is a mapping. Mapping's do not have a length property because we can't know what the size of a mapping is.

If you convert mapping(uint256 => address) public userList; to address[] public userList; then it should work as expected.

Also, regarding your last point. It's fine for you to use userList.length to determine a user's ID, even if two users are added in the same block/at the same time, as the transactions are processed sequentially, so they will still get different ID's.

Related Solutions

Solidity – Handling Unimplemented Functions in Contracts

The problem is that totalSupply() implementation is missing. Seems like autogenerated getters do not count. So you need to explicitly create the function.

contract StandardToken is Token {
    uint256 _totalSupply;

    function totalSupply() constant returns (uint256 totalSupply) {
        totalSupply = _totalSupply;
    }
}

Solidity – How to Resolve Remix Compilation Error: Invalid Array Length, Expected Integer Literal or Constant Expression

I changed your smart contract implementation with this smart contract.

I put notes just so you'd know your errors.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

// NOTE: Import IERC20 contract
import "@openzeppelin/contracts/token/ERC20/IERC20.sol";

// NOTE: You must to implement IERC20 for override ERC20 methods!
contract NewToken is IERC20 {

  string public constant name = "NewToken";
  string public constant symbol = "NTKN";
  uint8 public constant decimals = 18;
  uint256 totalSupply_ = 10000000000000000000000000; 

  mapping(address => uint256) public balances;
  mapping(address => mapping (address => uint256)) allowed;

  // NOTE: I removed the events because it already defined into IERC20
  
  constructor() { 
    balances[msg.sender] = totalSupply_;
  }

  function totalSupply() public override view returns (uint256) {
    return totalSupply_;
  }

  // NOTE: You must to change signature function adding address parameter
  function balanceOf(address _address) public override view returns(uint256) {
    return balances[_address];
  }

  function transfer(address to, uint256 value) public override returns (bool) {
      require(value <= balances[msg.sender]);
      balances[msg.sender] = balances[msg.sender]-value;
      balances[to] = balances[to]+value;
      // NOTE: The event is called  Transfer not 'transfer'
      emit Transfer(msg.sender, to, value);
      return true;
  }

  function approve(address spender, uint256 value) public override returns (bool) {
      allowed[msg.sender][spender] = value;
      emit Approval(msg.sender, spender, value);
      return true;
  }

  function allowance(address owner, address spender) public override view returns (uint) {
      return allowed[owner][spender];
  }

  function transferFrom(address from, address to, uint256 value) public override returns (bool) {
      require(value <= balances[from]);
      require(value <= allowed[from][msg.sender]);

      balances[from] = balances[from]-value;
      allowed[from][msg.sender] = allowed[from][msg.sender]+value;
      balances[to] = balances[to]+value;
      emit Transfer(from, to, value);
      return true;
  }
}

Best Answer

Related Solutions

Solidity – Handling Unimplemented Functions in Contracts

Solidity – How to Resolve Remix Compilation Error: Invalid Array Length, Expected Integer Literal or Constant Expression

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