Dynamic Arrays for each address

Question

I want to store ids in an array and attach them to an address. For example: There are 0 houses. User1 buys an house so id 0 gets added to the array userHouses[msg.sender]. User2 buys 2 houses so it 1 and 2 gets added to userHouses[msg.sender]. user1 buys another house, id 3 gets added to userHouses[msg.sender] and so on. I could do

mapping(uint => address) houseOwner;

and

houseOwner[0] = msg.sender; // User1
houseOwner[1] = msg.sender; // User2
houseOwner[2] = msg.sender; // User2
houseOwner[3] = msg.sender; // User1

But when there are 1.000.000 houses of 10.000 users it gets too much. So I want instead 10.000 arrays for each address and simply add the houses into that address (like houses[msg.address]). Also it's easier and cheaper to loop only houses of the user instead of looping all houses and check if the house owner is the user.

I know how to make an array (uint[] houses) but I don't know how to make arrays for each address (uint[] houses[msg.sender]?).

Answer 1

If I understand correctly, I believe you want this:

mapping (address => uint[]) houses;

Then you can do things like this:

houses[msg.sender].push(15);

Answer 2

pseudo code update v2

pragma solidity ^0.4.18;
contract Houses {
    struct House {
        address owner;
    }    

    House[] public houses;

    function buyHouse() public payable {
        House memory tempHouse = House(msg.sender);
        houses.push(tempHouse);
    }

    function getTotalHomesSold() public view returns (uint256 count) {
        return houses.length;
    }
}