Solidty - Return an array of UINTS from a struct

Question

I have a mapping that is storing bytes32=>uint. I want to write a function that loops through this mapping and only returns a list of the uints.

I know you cannot have a dynamic array in the function as it has to be in memory. But I need to store them in an array to be able to return the array in the function.

I just want all the ints to be returned in an array so I can access it on my frontend via JS.

This is what I have so far (that doesn't work):

function getAllVotes () returns (uint[]) {
    bytes32[] returnVotes;

    for(uint i = 0; i < candidateList.length; i++) {
      returnVotes[i] = bytes32(votesReceived[candidateList[i]]);
    }

    return returnVotes;

  }

Rob Hitchens · Accepted Answer · 2017-09-02T20:49:46.547

Your unbounded for loop is an anti-pattern because it will cease to operate at scale when the gas cost exceeds the block gas limit. Ouch.

Break it up into multiple fixed-cost operations. Iteration is the caller's responsibility.

I also noticed you're declaring a storage variable inside a function. Not conventional. Below will give you "free" getters voteList(uint row) and voteStructs[bytes32 key]. The client can 1) get the list length, 2) get the keys one at a time, and 3) get the complete structure for any key.

contract Vote {

  struct VoteStruct {
    bool wasCast;
    // more stuff about votes
  }

  bytes32[] public voteList;
  mapping(bytes32 => VoteStruct) public voteStructs;

  function getVoteCount() public constant returns(uint voteCount) {
    return voteList.length;
  }   
}

In case it helps, there some patterns that combine mappings with key lists over here: Are there well-solved and simple storage patterns for Solidity?

Your snippet refers to a candidatesList and does some computation that you might want the smart contract to tally as the votes come in so you aren't tempted to iterate later.

Hope it helps.

Solidty - Return an array of UINTS from a struct

1 Answers1