How to convert bytes data into a uint in Solidity?

Question

I'm looking to convert a bytes into an int.

For example, this works in Solidity 0.4.23, but fails to explicit type conversion error in later versions.

function bytesToUint(bytes memory b) public view 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;
}

Does anybody know how to do this in later versions of solidity?

Answer 1

Bytes is a tightly packed array of bytes1.

Since Solidity 0.5.0 this change was introduced :

Type Checker: Disallow conversions between bytesX and uintY of different size.

Given that the current doc states that :

uint and int are aliases for uint256 and int256, respectively.

Your syntax tries to cast a bytes1 to uint256, this is forbidden since 0.5.0.

You can achieve your goal by just being more precise on the types, see this example :

pragma solidity ^0.8;

contract MyContract {
    
    function bytesToUint(bytes memory b) public pure returns (uint256){
        uint256 number;
        for(uint i=0;i<b.length;i++){
            number = number + uint8(b[i]);
        }
        return number;
    }

}

The only important change is uint8(b[i]) instead of uint(b[i[). Casting bytes1 to a type of equal length.

So for an explicit conversion from bytes (containing tightly packed bytes1) you need :

uint(uint8(b[i]))

But anyway, implicit conversion from uint8 to uint is perfectly fine as in the previous exemple.

Hope that helps !

Answer 2

I was having issues with hroussille's answer. This answer worked for me

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;
}