[Ethereum] How to access content of mapping in a struct

mappingsoliditystruct

I'm aware of this post:

Accessing a mapping within a struct via Truffle console

However, it seems to me, it does not provide a clear answer. Also, my question is more generic and not just for truffle. Also, there's similar code in here (section "structs") but couldn't find any answer to the below questions.

Case 1:

Assume we have the following pseudo-code in solidity:

mapping (uint => BB) public map_1; 

struct BB{

  mapping (uint => address) map_2;
}

Question : Given a key, how can I access the content of map_2 in Geth or a user interface?

Case 2:

Assume we have the following pseudo-code in solidity:

mapping (uint => BB) public map_1; 

struct AA{
uint val;
}

struct BB{

  mapping (uint => AA) map_2;
}

My question for this case remains the same as the one for case 1.

Best Answer

You can access the case 1s mapping by given the both keys. The key for map_1 and key for map_2. Use pragma solidity ^0.4.20; when compiling

Case 1

contract test {
    struct BB {
        mapping(uint => string) map_2;
    }
    mapping(uint => BB) map_1;

    function test() public {
        map_1[1].map_2[1] = "hello"; //setting temp value
    }

    function maping() public view returns(string) {
        return map_1[1].map_2[1]; //accessing the value
    }
}

Case 2

contract test {
    struct BB {
        mapping(uint => AA) map_2;
    }
    struct AA{
        uint val;
    }
    mapping(uint => BB) map_1;

    function test() public {
        map_1[1].map_2[1].val = 10;
    }

    function maping() public view returns(uint) {
        return map_1[1].map_2[1].val;
    }
}

Related Solutions

[Ethereum] Clearing a custom struct

you could use delete to clear your map.

delete userRegister[addr];

proposition :

pragma solidity ^0.4.0;
contract test {

struct user {
    string name;
    address add;
}

mapping (address => user) public userRegister;

 function add_user (string name, address addr)   {  
     userRegister[addr].name=name;  
      userRegister[addr].add=addr;  
}


function clearStruct (address addr)   {  
   delete userRegister[addr];  

}

function try_it (address addr)  returns (string name) {  

   return userRegister[addr].name ;
}
}

Solidity – Test if a Struct State Variable is Set

Question is possibly a duplicate of this: What is the zero, empty or null value of a struct?

There are a few ways.

You can check a value if you're sure it implicitly indicates a set/not set condition.

if(tx.to > 0) {} // the address is set

You can be explicit if you prefer:

struct Transaction {
    address to;
    uint value;
    bytes data;
    bool isValid; // set to true whenever the struct holds data
}

Then you can:

if(tx.isValid) {
  // do something
}

You'll want to use the explicit method in the case that 0 or empty has meaning your application; meaning you can't use 0 as an indication of valid/inValid.

Not sure it applies in this case, but elimination of doubt about this sort thing is a bonus feature if you buy into the idea of maintaining an index of valid keys to a mapped structure.

Hope it helps.

Update: Some general examples here: Blog: Simple Storage Patterns in Solidity

Best Answer

Related Solutions

[Ethereum] Clearing a custom struct

Solidity – Test if a Struct State Variable is Set

Related Topic