How to return mapping list in Solidity? (Ethereum contract)

Question

I want to make a simple smart contract that has a list, can set item, and can get the list.

Code in solidity:

contract lister {
    mapping(int => string) list;
    int id = 0;
    
    function getList() returns ( /*HERE*/ ) {
        return list;
    }

    function setItemToList(string str) {
        list[id] = str;
        id++;
    }
}

I want to make getList() return the list, but the return type is not compatible. How can I do that?

Solution 1

Bulk access to lists/arrays/etc is painful in Solidity. You rarely see it in contracts. In your case, a possible solution is to provide a function to access one item, using its index, and to let the caller loop from 0 to id.

Solution 2

With mappings, keys are not stored and the values can not be iterated upon, so they are really only good for single-value lookups. In the example you provide, it may be a better choice to use an array.

On the other hand, if you use an array and need to do a search on it (loop through all items), you need to be careful because if there are too many items in your array, it could end up costing a considerable amount of gas to call the function.

Solution 3

You can change the visibility of your variable list, insert public and it will possible to access this by getList.

mapping(int => string) public list;