[Ethereum] How to store strings in an array in solidity

soliditystring

I am trying to build a simple application on ethereum where strings can be added/deleted from an array and the contents of this array can be shown in a frontend UI.

The basic structure of the smart contract is:

pragma solidity ^0.4.18;

contract ExampleApp {
  <string array over here>

  function add(string x) public {

  }

  function delete(string x) public{

  }
}

I have 2 questions: How can I initialise a string array and secondly how to delete a specific item from the string array? Do I have to iterate through the array to find the string or is there some simpler way?

Best Answer

Initializing a dynamic length string array is done just like any other type:

string[] someStrings;

Deleting a specific item from an array of any type by the value itself is not possible without looping thru all elements until you find the target element. You could use something like a Set instead, though that's not a native type in Solidity.

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] Array of Strings in Solidity

Life is easier if you can assume 32 bytes. If you are able to restrict the multihash formats you accept or assume sha2-256, which is common now, you could just drop some meta-data so that they fit in 32 bytes. To convert to a bytes32, base-58-decode the data, then strip what should be 0x1220 off the front. The 12 indicates the format (sha2-256), and the 20 is the length of the data, ie 32 bytes.

If you want to handle all the legal multihash formats properly, you could concatenate them together as a single byte array, then manage a separate array holding the start offset of each entry.

Best Answer

Related Solutions

Solidity Memory Management – Does ‘Delete’ Free Mapping Space

[Ethereum] Array of Strings in Solidity

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