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;
    }
}

Updated function for solidity ^0.8.11

function bytesToUint(bytes memory b) internal pure returns (uint256){
        uint256 number;
        for(uint i=0;i<b.length;i++){
            number = number + uint(uint8(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

A fully working modern solution can be found in this repository, authored by ConsenSys:

function toUint256(bytes memory _bytes, uint256 _start) internal pure returns (uint256) {
    require(_bytes.length >= _start + 32, "toUint256_outOfBounds");
    uint256 tempUint;

    assembly {
        tempUint := mload(add(add(_bytes, 0x20), _start))
    }

    return tempUint;
}

The accepted answer is not working in Solidity ^0.5 anymore; it produces a type conversion error.