Do we require a specific condition when working with indices on solidity

Question

I know that an assert-style exception is generated in the situation if you access an array at a too large or negative index (i.e. x[i] where i >= x.length or i < 0).

But should I check the condition every time?

Following is my code:

pragma solidity ^0.4.0;

contract A{
    byte[10] arr;

    function setElement(uint index, byte value) public {
        require(index >= 0 && index < arr.length); //Should I leave it as is?
        arr[index] = value;
    }

    function getElement(uint index) view public returns (byte) {
        require(index >= 0 && index < arr.length); //Or not?
        return arr[index];
    }
}

Also, how do I refund the remaining gas to the executor?

Answer 1

Your code is written correctly. require is intended to be used to check input parameters whereas assert is to verify the internals of your contract (mostly for testing purposes). If a require condition fails, the remaining gas will be refunded.

You can refer to the section in the solidity documentation here: http://solidity.readthedocs.io/en/develop/control-structures.html?highlight=require#error-handling-assert-require-revert-and-exceptions