initialise array of struct inside a new struct

Question

Following up on the question of 2d array of different types: 2d array of different types?

How would one go about initializing a new array of structs (Vote) when creating a new struct (Project)

pragma solidity ^0.4.25;

contract Votinggame{

       struct Vote {
          address voter;
          string comment;
      }

        struct Project{
            int id;
            string name;
             int votes;
            Vote[] vote;
        }

        Project[] public projects;
        int idcounter=-1;

     function addNewProject(string name) public {

             Project memory newProject = Project({
                 id:idcounter+1,
                 name:name,
                 votes:0

                 // How to initialise new vote array of struct Vote inside Project struct ??
                 // vote: new Vote[](0) -> this is not working "UnimplementedFeatureError: Copying of type struct HotOrNotCrypto.Vote memory[] memory to storage not yet supported."

             });
           idcounter=newProject.id;
           projects.push(newProject); 
        }

      function vote(uint index , string comment) public {
        Project storage project = projects[index];
        project.votes++;     
        Vote memory v;
        v.voter = msg.sender;
        v.comment = comment;
        project.voters.push(v);
      }
    }
 function getProjects() public view returns (uint){
         return (projects.length);
       }
}

Also to extend on this how would one go about displaying vote Array of Vote struct in React. At the moment I'm displaying projects Array of a projects struct using .fill().map()

  static async getInitialProps(){

    const getProjects = await Contract.methods.getProjects().call();
    const projectsList = await Promise.all(
    Array(parseInt(getProjects))
    .fill()
    .map((element, index)=> {
      return Contract.methods.projects(index).call()
    }));
  const sorted_projects = _.orderBy(projectsList, ['votes'], ['desc']);


   return { sorted_projects};

  }

and then render new table row for each struct element like so:

  renderRow(){
    return (

      this.props.sorted_projects.map((sorted_projects,key)=>{
      return <RequestRow
        id= {key+1}
        key ={key}
        sorted_projects ={sorted_projects}
      />;

  }));

I guess this will be sort of similar with the difference that I would have to provide which project index I want votes array to display

Answer 1

Your best bet is to use the following way:

    function addNewProject(string name) public {
        uint lastIndex = projects.length++;
        Project storage project = projects[lastIndex];
        project.id = ++idcounter;
        project.name = name;
    }

All undefined storage is already defined as zero-ish. A zero-ish array is an empty array. So:

• project.votes is already 0, no need to set it.
• project.vote is already initialised to an empty array.

---

Plus, you need not do project.votes++; because that is the same information as project.voters.length, so save yourself some gas.

You may want to make Project[] private projects; and write the getter yourself, so that you can return project.voters.length among others.

Oh, and better naming: function getProjectCount()

Answer 2

As the error message asserts, this isn't something Solidity supports yet. One of the common patterns instead is to store a separate array of Votes, and maintain an index into those (using uint256[]) in the Project struct:

pragma solidity ^0.4.24;

contract Votinggame{
  struct Vote {
    address voter;
    string comment;
  }

  struct Project{
    uint256 id;
    string name;
    uint256 votes;
    uint256[] voteArr;
  }

  Project[] public projects;
  Vote[] public votes;

  uint256 projectCounter = 0;
  uint256 voteCounter = 0;

  function addNewProject(string name) public {
    projects.push(Project({
      id: projectCounter,
      name: name,
      votes: 0,
      voteArr: new uint256[] (0)
    }));

    projectCounter += 1;
  }

  function vote(uint index , string comment) public {
    Vote memory v;
    v.voter = msg.sender;
    v.comment = comment;
    votes.push(v);

    projects[index].votes += 1;
    projects[index].voteArr.push(voteCounter);

    voteCounter += 1;
  }
}

To access this, then, you:

1. Pull the Project from projects using the project index
2. Inspect the project.votes value
3. Iterate from 0 to votes - 1 getting the vote's index
4. Pull the Vote from votes using the vote index