Solidity In-Memory Array – How to Fill Dynamic In-Memory Arrays

arraysmemoryremixsoliditystorage

For some reason solidity doesn't allow to push values into memory array

Member "push" is not available in bytes32[] memory outside of
storage.

Here is sample contract code:

pragma solidity ^0.4.21;

contract Foo {
    function getRange(uint n) public pure returns(uint[]) {
        uint[] memory result;
        for (uint i = 0; i < n; i++)
            if (someCondition(i))
               result.push(i);
        return result;
    }
}

I could allocate maximum possible array of size n and then shrink it, but it may hurt performance (n could be in order of magnitude of 100000 while final length of result is 0..100). For example, someCondition could be isPrime. In this case we have large N (which makes impossible to preallocate an array) and small list of resulting prime numbers.

How could it be done?

I ended up with preallicating array of size n and then shrinking it once I know it's final length:

function getRange(uint n) public pure returns(uint[]) {
    uint tookCount = 0;
    uint[] memory result = new uint[](n);
    for (uint i = 0; i < n; i++)
        if (someCondition(i)) {
            result.push(i);
            tookCount++;
        }

    uint[] memory trimmedResult = new uint[](tookCount);
    for (uint j = 0; j < trimmedResult.length; j++) {
        trimmedResult[j] = result[j];
    }
    return trimmedResult;
}

Best Answer

Dynamic arrays are only available in storage, not in memory. In your case, the size of the result array is known upfront (n). So, you can just declare an array with length of n. Then you can fill it up using i, which goes from 0 to n - 1

pragma solidity ^0.4.21;

contract Foo {
    function getRange(uint n) public pure returns(uint[]) {
        uint[] memory result = new uint[](n);
        for (uint i = 0; i < n; i++)
            result[i] = i;
        return result;
    }
}

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 – Handling Unimplemented Functions in Contracts

The problem is that totalSupply() implementation is missing. Seems like autogenerated getters do not count. So you need to explicitly create the function.

contract StandardToken is Token {
    uint256 _totalSupply;

    function totalSupply() constant returns (uint256 totalSupply) {
        totalSupply = _totalSupply;
    }
}

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