Accessing array elements with inline assembly

Question

Using a loop in inline assembly compared to a regular solidity for loop saves around 20% in gas cost.

I'm struggling however with looping over an array through inline assembly. Is this possible and if so, how?

Many thanks.

pragma solidity ^0.4.21;

contract Loop {
    uint[] array = [1, 2, 3, 4, 5];

    function returnNthElement(uint _index) returns (uint) {
        assembly {
            let index := _index
            let length := mload(sload(array_slot))
            for
               { let i := 0 }
               lt(i, length)
               { i := add(i, 1)}
           {
               //How to return element at index '_index' ? 
           }
        }
    }
}

Answer 1

First, I don't think you need a loop. You can access the location of whatever element by its index in constant time.

As explained in this article, the location of an array's element is calculable the following way : you hash the slot of the array (here, it's 0), and this will give you the "starting location" of the array. You just have to add to it the index to shift to the appropriate location.

You will not that it's not the case all the time, it depends on what type of elements your array contains, because you will have to adapt your shifting (EVM works by 256 bits).

As for the code itself, with the informations you have been given, it should be simple.