Solidity Mapping – How to Delete an Element from a Mapping

contract-developmentmappingsoliditystruct

I have a mapping like this:

struct data {  
   string name;  
   string nickname;  
}

mapping(address => data) public user;

What is the correct way to delete one element from the variable user? Do I only have to call delete(user[address])

do I also have to delete user[address].name and user[address].nickname?

Best Answer

Yes, delete user[someAddress]; will work with structs that do not contain a mapping.

For this question, because name and nickname are not mappings, they will be deleted (set to zero) automatically: there is no need to do something like "delete user[someAddress].name".

http://solidity.readthedocs.io/en/develop/types.html#delete

if you delete a struct, it will reset all members that are not mappings and also recurse into the members unless they are mappings

Caveat:

delete has no effect on whole mappings (as the keys of mappings may be arbitrary and are generally unknown)

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] Efficient approach to delete element from array in Solidity

Source: https://github.com/su-squares/ethereum-contract/blob/master/contracts/SuNFT.sol

Here you go:

Algorithm:

uint[] assets;
mapping(uint=>uint) indexOfAsset;

function removeAssetFromArray(uint _assetToDelete) {
  uint index = indexOfAsset[_assetToDelete];
  if (!index) return;

  if (assets.length > 1) {
    assets[index] = assets[assets.length-1];
  }
  assets.length--; // Implicitly recovers gas from last element storage
}

Caveat: this approach assumes an ordered set, not an array. In other words, it assumes the array has no duplicate items.

Best Answer

Related Solutions

Solidity Memory Management – Does ‘Delete’ Free Mapping Space

[Ethereum] Efficient approach to delete element from array in Solidity

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