[Ethereum] Delete or clear a mapping

The best way is simply not to clear them. In many (most?) situations, the array varies in size over time, and emptied slots will eventually be filled again. Instead of shortening the array, keep a separate count of live elements:

uint numElements = 0;
uint[] array;

function insert(uint value) {
    if(numElements == array.length) {
        array.length += 1;
    }
    array[numElements++] = value;
}

function clear() {
    numElements = 0;
}

This means that deleted elements remain in the array, but are ignored, since you can use numElements in place of array.length everywhere.

This also conserves gas on the whole; deleting and reinserting an item to an array costs 5k (deletion) - 10k (refund) + 20k (reinsertion) = 10k net gas, plus another 10k to update the array length twice. Overwriting, in contrast, costs only 5k gas, plus the same 10k to update the numElements variable twice.

In cases where the array is erased only once, and won't grow again, a more sensible alternative may be to spawn a new contract for the life of the event, and have it kill itself at the end, which will refund gas for all live storage elements to the caller.

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;  
  }