[Ethereum] Getting key from Solidity mapping by value

Mappings can only use elementary types (address, uint, bytes, string) as keys, and more specifically any type for which sha3() is defined. This means structs and arrays currently can't be used as keys.

mapping(uint => Foo[]) foo; should work, and does for me. Your code has some syntax errors, which might be the problem (struct shouldn't be capitalized, uint x needs a semicolon)

[] is not a type, you need some elementary type in front of it, like uint[].

Elements in arrays are accessed with bracket notation, not dot notation.

This contract should work:

contract Bar{

    struct Foo{
        uint x;
    }
    mapping(uint => Foo[]) foo;

    function add(uint id, uint _x) public {
        foo[id].push(Foo(_x));
    }

    function get(uint id, uint index) public returns(uint){
        return foo[id][index].x;
    }
}

I tried both, and measured gas for both transactions. I got 107864 with a struct, and 107856 with two mappings.... I'm pretty sure that the two bytes32 take all the gas, and the data structure are negligible in comparison.

So I'm using the cleaner one in my opinion, the struct.