Solidity – Using Arrays as Key or Value in Mapping

contract-developmentmappingsolidity

Is it possible to do a mapping with an Array as key/value ? Where an array would exist for each value ?

I'm actually looking to store structs in a mapping but can't seem to have it work.

Struct Foo{
uint x
}

mapping(uint => Foo[])foo; and even simply
mapping([] => uint)foo / (uint => [])foo; don't seems to work.

something so i can do :

foo.1.push(FooInstance)

foo.1[0] = FooInstance

Solidity Doc reads

Mapping types are declared as mapping _KeyType => _ValueType, where _KeyType can be almost any type except for a mapping and _ValueType can actually be any type, including mappings.

So aren't array considered a data type in solidity?

Best Answer

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;
    }
}

Related Solutions

Solidity Memory Management – Does ‘Delete’ Free Mapping Space

Based on my Debug observation:

I have tested that:

Firstly I insert data into my map data-structure as I explained in my question.
Secondly, I have returned the address of first mapped index, which is 0, this could be done for other indexes as well (0:10).
Without a delete operation I can retrieve the data perfectly fine from the address of mapped Foo[]. But if I did the delete operation, Solidity gives an error.

This debug result: leads me to conclude that memory is freed or address does not contain any valid information.

Test code:

set_txn_hash     = my_contract.transact().map_insert();
contract_address = unmigrated_chain.wait.for_receipt(set_txn_hash)

for x in range(0, 9):
   for y in range(0, 9):
         output           = my_contract.call().get_(x, y);
         print(output);  //returns correct value.

set_txn_hash     = my_contract.transact().map_remove_map();                                             
contract_address = unmigrated_chain.wait.for_receipt(set_txn_hash)                                      

set_txn_hash     = my_contract.transact().get_map_address(0);
contract_address = unmigrated_chain.wait.for_receipt(set_txn_hash)

output           = my_contract.call().get_map();
print(output);

output           = my_contract.call().try_();
print(output);  //Solidity gives error if delete map_remove_map() is called

Additional functions inside my Contract:

  function map_insert(){
    for(uint i = 0; i < 10; i++)
      for(uint j = 0; j < 10; j++)
        add(i, i*10 + j);
  }

  function map_remove_map(){
    for(uint i = 0; i < 10; i++)
      delete_(i);
  }

  function map_remove_map_item(){
    for(uint i = 0; i < 10; i++)
      for(uint j = 0; j < 10; j++)
        delete_items(i, j);
  }

  function get_map_address(uint id) {
    uint addr;
    Foo[] a = foo[id];

    assembly{
    addr := a
        }
    map_address = addr;
  }

  function get_map() constant returns (uint){
    return map_address;
  }

  function try_() constant returns (uint ){
    uint addr = map_address;
    Foo[] a;
    assembly{
    a := addr
        }
    return a[1].x;  
  }

[Ethereum] Is it possible to have enum as a mapping key type

That feature is not currently implemented. A simple workaround is to do

pragma solidity ^0.4.4;

 contract TestContract {
    enum TestEnum { ONE, TWO, THREE }
    mapping (bytes32 => uint) testMapping;

    function getValueOne() constant returns(uint) {
        return testMapping[sha3(TestEnum.ONE)];
    }

}

Just use the hash of the enum for the key instead of the actual enum item, and it will work fine.

Best Answer

Related Solutions

Solidity Memory Management – Does ‘Delete’ Free Mapping Space

[Ethereum] Is it possible to have enum as a mapping key type

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