Solidity Mapping – How to Return Multiple Values from a Mapping

You need to break the interface down into smaller chunks. In practice, you'll want to extend the internal storage pattern a little, I think.

Iterable mapping comes to mind. You keep lists of keys to mappings in arrays. Something like this:

  struct VoterStruct {
    address voterAddress;
    uint256 tokensBought;
    mapping (bytes32 => uint256) tokensUsed;
    bytes32[] tokenIndex; // a list of mapping keys that exist for the voter
  }

  mapping (address => VoterStruct) public voterStructs;

Then you can expose functions that allow iterating over lists while the contract returns one set of values at a time. Very sketchy to inspire some ideas:

// client can discover how may tokens are in the list for one voter
function getVoterTokenCount(address voter) returns(uint tokenCount) {
    return voterStructs[voter].tokenIndex.length;}

// client can discover the tokenId for a voter in a given row
function getVoterTokenAtIndex(address voter, uint index) returns(bytes32 tokenId) {
    return voterStructs[voter].tokenIndex[index];}

// client can discover token used by voter and tokenId
function getVoterTokenUsed(address voter, bytes32 tokenId) returns(uint tokensUsed) {
    return voterStructs[voter].tokensUsed[tokenId];}

// append keys to the list as you go
function insertVoterToken(address voter, bytes32 tokenId) returns(bool success) {
    voterStructs[voter].tokenIndex.push(tokenId);
    return true;}

Hope I didn't flub the syntax. It might be possible to sketch out the contract a little more. Would need a general description of what it's going to do. When I use this pattern, I always add some rules to ensure there are no duplicate keys, and so on.

Anyway, possibly the hints will inspire some ideas.

Hope it helps.

You can't loop through the keys or count the keys in a mapping (but patterns exist for accomplishing such functionality using a few more variables).

You're on the right path, tracking it yourself. For example:

pragma solidity ^0.4.8;

contract Vote {

  uint public totalVotes;

  mapping (address => uint) public voterCount;

  function vote(uint votes) returns(bool success) {
    totalVotes += votes;
    voterCount[msg.sender] += votes; // cumulative
    return true;
  }
}

You wouldn't want to iterate over the voters in the contract in any case because that would put an upper bound on the number of voters before the system stops working - due to rising transaction cost hitting the block gas limit.

I can show you how to make the voter list iterable but it's not obvious that it's really needed to count the votes as we go. Keeping it simple for first answer.

Hope it helps.