Accessing public variables of smart contracts

Question

I've got the following contract:

pragma solidity ^0.4.11;

contract Zusatzaufgabe {

    struct Student {
        string name;
        string vorname;
        uint matnr;
        uint punkte;
        bool[] solved;
    }

    address[] public addressen;

    mapping(address => Student) public studenten;

    modifier hurdle(uint _n) {
        require(_n == msg.value/2);
        _;
    }

    modifier onlyTillTest() {
        require(block.timestamp < 1499702400);
        _;
    }

    function Zusatzaufgabe(){

    }

    function register(string _name, string _vorname, uint _matnr) onlyTillTest(){
        studenten[msg.sender] = Student(_name,_vorname,_matnr,1,new bool[](3));
        addressen.push(msg.sender);
    }

    function onepoint() payable hurdle(10000) onlyTillTest(){
        if(studenten[msg.sender].solved[0] == false){
            studenten[msg.sender].solved[0] = true;
            studenten[msg.sender].punkte += 1;
        }
    }

    function twopoints(uint _value) onlyTillTest(){
        if(studenten[msg.sender].solved[1] == false
        && _value == studenten[addressen[0]].matnr
        ){
            studenten[msg.sender].solved[1] = true;
            studenten[msg.sender].punkte += 2;
        }
    }

    function sixpoints(bytes32 _h, uint8 _v, bytes32 _r, bytes32 _s) onlyTillTest(){
        if(
            studenten[msg.sender].solved[2] == false
            && ecrecover(_h, _v, _r, _s) == msg.sender
            && _h == sha3(this)
        ){
            studenten[msg.sender].solved[2] = true;
            studenten[msg.sender].punkte += 6;
        }
    }
}

To sucessfully use the function twopoints, I need to get the value of studenten[addressen[0]].matnr.

How can I access this by console with web3?

MyContract.studenten[addressen[0]].matnr doesn't work, even though it is public.

ReferenceError: addressen is not defined

How do I access this value?

Answer 1

This unit test works for me with the above contract using node 8.1.2 and truffle 3.4.3.

contract('Zusatzaufgabe', function(accounts) {
  it("Retrieve", async function() {
    const zusatzaufgabe = await Zusatzaufgabe.deployed();

    // Populate some entries
    await zusatzaufgabe.register("hola", "hola", 123, { from: accounts[1] });
    await zusatzaufgabe.register("toma", "joni", 321, { from: accounts[2] });

    // This returns Zusatzaufgabe.addressen[0]
    const addressen = await zusatzaufgabe.addressen.call(0);
    assert.equal(addressen, accounts[1]);

    // This returns Zusatzaufgabe.studenten[Zusatzaufgabe.addressen[0]]
    const student = await zusatzaufgabe.studenten.call(addressen);
    assert.equal(student[2].toNumber(), 123);
  });
});

The last call will return a tuple with the fields from struct Student. It will return [name, vorname, matnr, punkte]. The field solved is not returned, see https://solidity.readthedocs.io/en/develop/contracts.html#getter-functions for the documentation about getters automatically generated. Also int fieds (matnr and punkte) are returned as bignums.

Answer 2

i need to get the value of 'studenten[addressen[0]].matnr'

how can i access this by console with web3?

It could be done with Contract function like this:

function getMatnr(address addr) constant returns(uint) {
    return studenten[addr].matnr;
}

And in web3:

contract.getMatnr.call(addr, function(err, result) {

});

ReferenceError: addressen is not defined

how do i access this value?

address[] public addressen; for me, it is enough for publicity of addressen array

Check addressen array after you call register function. I think, it is still empty.

contract.addressen.call(function(err, res) {

});

Try to send tx with register function without onlyTillTest modifier.

UPDATE

If you had getMatnr, what you need to do in web3 is:

MyContract.addressen.call(function(err, addresses) {
   if (err) return console.log(err);
   var addr = addresses[0];

   MyContract.getMatnr.call(addr, function(err, matnr) {
       if (err) return console.log(err);
       console.log(matnr);
   });
});