How to convert bytes to uint in Solidity

Question

Hey is there a way to convert bytes to uint in Solidity?

Answer 1

You can use this function to convert bytes to unit:

pragma solidity ^0.4.23;

contract mycontract {

    function bytesToUint(bytes b) public returns (uint256){
        uint256 number;
        for(uint i=0;i<b.length;i++){
            number = number + uint(b[i])*(2**(8*(b.length-(i+1))));
        }
        return number;
    }
}

The amount of gas used depends on the length of your bytes variable, but this is cheap. Converting a bytes variable of length 20 uses about 420 gas.

Hope this helps

Answer 2

Slicing an uint out of bytes using inline assembly

function sliceUint(bytes bs, uint start)
    internal pure
    returns (uint)
{
    require(bs.length >= start + 32, "slicing out of range");
    uint x;
    assembly {
        x := mload(add(bs, add(0x20, start)))
    }
    return x;
}

Answer 3

The fully working modern solution from Consensys dude can be found here:

https://github.com/GNSPS/solidity-bytes-utils/blob/master/contracts/BytesLib.sol

The accepted answer's code is not compiling in Solidity >0.5; with implicit type conversion error.