Solidity 4.24, uint plus

Question

May I know why I can't do additional in uint for solidity 4.24?

function ToUint(bytes memory value) internal pure returns (uint) {
    uint r;
    for (uint i = value.length - 1; i >= 0; i--)
        r = (r << 8) + uint(value[i]);
    return r;
}

I might have over simplified my code and it doesn't exhibit the problem. The original code was above. As soon as I took out (r << 8) +, every thing works.

The << 8 is not an issue. Because it failed even when I have r + uint(value[I])

The length of bytes array is only 6, and all bytes value are < 0x7F

Answer 1

You will have problems with the loop

for (uint i = value.length - 1; i >= 0; i--)

Since i is unsigned once it reaches 0 in the next iteration it will wrap to a very large number becuase i cannot be a negative number.

Answer 2

It does work. You can add uint as per here. I also tried it in Remix.

Answer 3

You were accessing the elements of value wrong, which was causing the reversion.

Here's a working version:

pragma solidity 0.4.24;

contract BytesToUint {

    function toUint(bytes _value) external pure returns (uint) {
        return toUint_(_value);
    }

    function toUint_(bytes memory _value) internal pure returns (uint) {
        uint256 r;

        for(uint256 i = 0; i < _value.length; i++) {
            r += uint256(_value[i]) << (i * 8);
        }
        return r;
    }
}