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Consider the following solidity function:

function example(uint256[] calldata _numbers) external {
    uint256[] memory _evenNumbers;

    // initialize `_evenNumbers` and populate it with even numbers from the `_numbers` array

    someOtherFunction(_evenNumbers);
}

_numbers is a uint256 array containing an unspecified amount of even and odd numbers. someOtherFunction expects an array of even numbers, what's the best way to initialize and populate _evenNumbers with all even numbers from the _numbers array?

The simplest solution would be something like this:

function example(uint256[] calldata _numbers) external {
    uint256[] memory _evenNumbers = new uint256[](_numbers.length);

    // initialize _evenNumbers and populate it with even numbers from the _numbers array
    uint256 _evenCounter;
    for (uint256 i; i < _numbers.length; ++i) {
        if (_numbers[i] % 2 == 0) {
            _evenNumbers[_evenCounter] = _numbers[i];
            ++_evenCounter;
        }
    }

    someOtherFunction(_evenNumbers);
}

This unfortunately doesn't work as _evenNumbers would contain as many trailing zeros as the amount of odd numbers in the original _numbers array, which would be considered valid even numbers.

A way around that would be the following inline assembly code:

assembly {
    mstore(_evenNumbers, _evenCounter)
}

This works and is probably the most gas efficient way to do it but I don't think that using inline assembly can be considered a best practice, but the only other solution I can think of is count the even numbers with a for loop first and then populate _evenNumbers with a second for loop, which is very ugly and inefficient.

1 Answer 1

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I think the most gas-efficient way is moving the check condition into someOtherFunction() and processing the whole array.

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  • My bad, I should've specified that this is just a very simple hypothetical and someOtherFunction is meant to be an external function we have no control over
    – Sam
    Mar 8, 2023 at 15:34
  • I see. In this case, Array.slice(docs.soliditylang.org/en/v0.6.0/types.html#array-slices) should be the most efficient solidity method AFAIK. Mar 9, 2023 at 2:14
  • Unfortunately array slices are only implemented for calldata arrays as of solidity 0.8.x
    – Sam
    Mar 10, 2023 at 11:56

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