[Ethereum] Returning 2D array with dynamic sizes from a function

arrayssolidity

This 2D array works when setting the array lenghts with constant values. But i'd prefer to use a dynamic array size:

function funWith2DArray(address[] array1,  address[] array2) public view returns (uint[50][50]) {
    uint[50][50] memory result;
    // ... 
    return result; 
}

This 1D array works, when dynamically setting the array length:

function funWith1DArray(address[] array1,  address[] array2) public view returns (uint[]) {
    uint[] memory result = new uint[](array1.length * 2);
    // ... 
    return result; 
}

How can I do a 2D array with dynamic lengths?:

function funWith2DArray_2(address[] array1,  address[] array2) public view returns (uint[][]) {
    // Not valid syntax
    // uint[][] memory result = new uint[][](array1.length * 2)(array1.length * 2);
    // Not correct either, error: Identifier must be declared a constant
    // uint[array1.length * 2][array1.length * 2] memory result;
    return result; 
}

Note: This function must have the view modifier so I can call it (reading data only) without paying gas.

My current workaround is just cramming all the response data into one array and just knowing where to look to find it (which is okay)

Best Answer

It's not possible yet. See Solidity 0.4.21 FAQ here (search for "dynamic array") -> http://solidity.readthedocs.io/en/develop/frequently-asked-questions.html

It is also not possible to return string[] since technically it's also a 2-dim array.

Related Solutions

[Ethereum] How to handle dynamic size string array in solidity

Solidity cannot return string arrays in current version. One workaround is to serialize the string array and then de-serialize it on client side.

An example :

https://hackernoon.com/serializing-string-arrays-in-solidity-db4b6037e520

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;
  }
}

Related Topic

Solidity Dynamic Uint Array – Does push() Return Reference or New Memory Variable?