How to access to functions of another Contract and change it's variables

Question

I have to Contracts habibconcert and changer , there is a method for changing the tickets_number in "habib concert" , it works fine when working just with the contract itself but i want to access to to it in another Contract "changer" so I made an object of the "habibconcert" in Changer Class and try to use it in

   function changeTickets(uint new_tickets) returns(uint) {


      firstObject.tickets = new_tickets;

      uint updateTickets =  firstObject.tickets  = firstObject.tickets + new_tickets;
      return ( updateTickets);

        }

but i receive the these errors

          browser/habibconcert.sol:12:11: TypeError: Expression has to be an lvalue.
      firstObject.tickets = new_tickets;
      ^-----------------^


            browser/habibconcert.sol:12:33: TypeError: Type uint256 is not implicitly convertible to expected type function () view external returns (uint256).
      firstObject.tickets = new_tickets;
                            ^---------^ 

                       browser/habibconcert.sol:14:33: TypeError: Expression has to be an lvalue.
      uint updateTickets =  firstObject.tickets  = firstObject.tickets + new_tickets;
                            ^-----------------^

                                       browser/habibconcert.sol:14:56: TypeError: Operator + not compatible with types function () view external returns (uint256) and uint256
      uint updateTickets =  firstObject.tickets  = firstObject.tickets + new_tickets;
                                                   ^-------------------------------^                      

                           browser/habibconcert.sol:14:11: TypeError: Type function () view external returns (uint256) is not implicitly convertible to expected type uint256.
      uint updateTickets =  firstObject.tickets  = firstObject.tickets + new_tickets;
      ^----------------------------------------------------------------------------^

Thanks in advance my two Contracts here below as following

pragma solidity ^0.4.0;


contract changer{


   habibconcert firstObject =    habibconcert(0xdc544654fefd1a458eb24064a6c958b14e579154);

  function changeTickets(uint new_tickets) returns(uint) {


      firstObject.tickets = new_tickets;

      uint updateTickets =  firstObject.tickets  = firstObject.tickets + new_tickets;
      return ( updateTickets);

        }


}





contract habibconcert{

  uint  public tickets = 6;

  address owner_of_tickets;

  uint constant price = 1 ether;

   mapping(address=>uint) public purchasers;

  function habibconcert()payable{

        owner_of_tickets =  msg.sender;
  }


function  displayTickets_numbers()constant returns(uint){

    return tickets;


}

    modifier ownLycreature(){


        require(msg.sender ==  owner_of_tickets) ;

        //   && tickets == 0
        _;

        }



               function changeTickets_number (uint newtickets) ownLycreature{

                     tickets +=newtickets;

               }




function buytickets(uint amount)payable{

      if(msg.value!=(amount*price) || amount>tickets){

          throw;
      }

      purchasers[msg.sender] +=amount;

      tickets -=amount;


    // if(tickets == 0){

    //     //selfdestruct(owner_of_tickets);

    // }


}        


}

Answer 1

You cannot modify directly the attributes of another contract. You need to have setters, and then, you can change the values with these functions.

contract Called {
    uint public myuint;

    function set(uint _var) {
        myuint = _var;
    }

    function get() view returns (uint){
        return myuint;
    }
}

---

interface Called {
    function set(uint);
    function get() view returns (uint);
}

contract Caller {

    Called public called_address;

    function set_address(address _addy) {
        called_address = Called(_addy);
    }

    function set(uint256 _var) {
        called_address.set(_var);
    } 

}