Does anArray.length-- delete the last element?

[fulldecent]

Normally I send questions to StackOverflow.

But here I see an assertion that calling .length-- on an array will delete the last element and recover gas.

https://github.com/OpenZeppelin/openzeppelin-solidity/blob/master/contracts/token/ERC721/ERC721Token.sol#L151

The people on that team are generally smart so I'm going to assume it is true.

If it is true then can the documentation please be update to explain this feature?

[chriseth]

Yes, it will delete the last element and then reduce the size by one. What happens is explained in the code snippet in https://solidity.readthedocs.io/en/v0.4.24/types.html#members but we should probably clarify what "resize" means in the description of the length member.

[fulldecent]

Ok, I learned two interesting things today.

• The complexity of array.length = x is O(1) if x > array.length and it is O(n) if x < array.length.
• Calling .length-- on an empty array will set the length to 2^256-1.

Yesterday I thought .length = x was always O(1). That is why I was surprised and thought .length-- was different than .length = .length-1. Now I realize I was wrong in a different way.

I am working now to update the documentation. PR coming.

Here are the black box test cases:

pragma solidity ^0.4.23;

contract Array {
    uint[] public numbers;
    
    function length() external view returns (uint) {
        return numbers.length;
    }
    
    function numbers() external view returns (uint[]) {
        return numbers;
    }
    
    function inc() external {
        numbers.length++;
    }
    
    function dec() external {
        numbers.length--;
    }
    
    function plusOne() external {
        numbers.length = numbers.length + 1;
    }

    function plusALot() external {
        numbers.length = numbers.length + 2**20;
    }
    
    function minusOne() external {
        numbers.length = numbers.length - 1;
    }

    function minusALot() external {
        numbers.length = numbers.length - 2**20;
    }
    
    function pushOne() external {
        numbers.push(1);
    }
    
    function setSizeToTen() external {
        numbers.length = 10;
    }
    
    /*
    function pop() external {
        numbers.pop();
    }
    */
}

[fulldecent]

Documentation fix added at be74b6d