Allowing multiple polling for user

Question

Is there a way to allow user to vote for multiple proposals. For example: User A can vote for proposals :P1,P2,P3,etc. Once A has voted for P1 he should not be allowed to vote for P1 but is eligible to vote for P2 and P3. I am able to write a contract that shows which proposals the candidate has voted for. Can any one help!!

Answer 1

You can keep users' eligibility to vote for a particular proposal in a structure like

mapping(uint => mapping(address => bool)) hasVoted;
//      ^proposal IDs   ^users     ^already voted or not
mapping(uint => uint) votesCount;

and then check if they are eligible to vote, here's an example in Solidity:

modifier canVote(address user, uint proposal){
    require(!hasVoted[proposal][user]);
    _;
}

function vote(uint proposal)
public 
canVote(msg.sender, proposal) //throw if user has already voted
{
    hasVoted[proposal][msg.sender] = true; //make note of the fact he's voting now
    votesCount[proposal]++;
    //other voting logic you need
}

Take care to use a suitable type for your proposal IDs, like uint8 or whatever is enough.