Solidity – How to Delete a Mapping

mappingsolidity

Fixed question:

I have a mapping:

mapping (bytes32 => mapping (bytes32 => bytes32[])) items;

With some elements in it:

items["fruits"]["apples"].push("first apple");
items["fruits"]["apples"].push("second apple");
items["fruits"]["apples"].push("third apple");
items["fruits"]["plums"].push("first plum");
items["vegetables"]["carrots"].push("first carrot");

I want to delete all fruits. The code below does not work:

delete items["fruits"]

The error I get is:

Error: Unary operator delete cannot be applied to type mapping(bytes32 => bytes32)

Original question:

I have a mapping:

mapping (bytes32 => mapping (bytes32 => bytes32)) items;

With some elements in it:

items["fruits"]["apples"] = "first apple";
items["fruits"]["apples"] = "second apple";
items["fruits"]["apples"] = "third apple";
items["fruits"]["plums"] = "first plum";
items["vegetables"]["carrot"] = "first carrot";

I want to delete all fruits. The code below does not work:

delete items["fruits"]

The error I get is:

Error: Unary operator delete cannot be applied to type mapping(bytes32 => bytes32)

Best Answer

Unfortunately you can't delete a mapping. From the solidity docs at: http://solidity.readthedocs.io/en/develop/types.html

delete has no effect on whole mappings (as the keys of mappings may be arbitrary and are generally unknown). So if you delete a struct, it will reset all members that are not mappings and also recurse into the members unless they are mappings. However, individual keys and what they map to can be deleted.

So you could do something like delete items[i][j] but you would need to know i and j in advance.

One common pattern is to keep an additional array (e.g. bytes32[]) into which you store the keys to your mapping, which you could then iterate over. Or in your case you could keep a mapping from "fruits" to an array of fruits, and then iterate over this when you want to delete all fruits. You would need to maintain these secondary data structures as you add fruits to your main items mapping.

Hope this helps!

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;  
  }

Solidity – How to Create a Simple Fixed-Size Array

The fixed arrays doesn't have push member, you need to use the index to set the value:

pragma solidity ^0.4.24;

contract Garbage {
    address[3] public addresses;
    uint idx;

    function addAddress(address _address) public {
        require(idx < addresses.length);

        addresses[idx] = _address;
        idx++;
    }
}

Best Answer

Related Solutions

Solidity Memory Management – Does ‘Delete’ Free Mapping Space

Solidity – How to Create a Simple Fixed-Size Array

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