How to get a specific hex char from a uint256?

Question

lets say I have the following uint256 value: 0x70997970c51812dc3a010c7d01b50e0d17dc79c8000403

This uint256 is packed with various data that I would like to be able to parse.

How would I extract the value 4 (third number from the right) or the value 3 (last number)?

Answer 1

you can use right shift >> in order to move to the rigth the item that you want

for example if you want the index 1 (from rigth to left) you need to move one times to the rigth

and because every hexadecimal value needs 4 bits to be represented you need to multiply by 4

uint move = index * 4;

and then you can use the and operator in order to clear the rest of the numbers

  function getParsed(uint256 value, uint index) external pure returns (uint cleared){
    uint move = index * 4;
    return (value >> move) & 0x000000000000000000000000000000000000000000000000000000000000000f ;        
}

so if you send the parameters:

0x00000000000000000070997970c51812dc3a010c7d01b50e0d17dc79c8000403 and 0 you will get the 3

0x00000000000000000070997970c51812dc3a010c7d01b50e0d17dc79c8000403 and 1 you will get the 0

0x00000000000000000070997970c51812dc3a010c7d01b50e0d17dc79c8000403 and 1 you will get the 4

---

if you want to use yul:

   function getParsed(bytes32 value, uint index) external pure returns (uint8 cleared){
     assembly {          
        let move := mul(index,4)
        let shifted := shr(move, value)    
        cleared := and(0x000000000000000000000000000000000000000000000000000000000000000f,shifted)       
    }
}

this line here:

 let move := mul(index,4)

use mul in order to get the multiplication in the our example index = 1 so the result will be 4 (1*4)

and then you need to use shr in order to shift to the rigth

let shifted := shr(move, value) 

and finally we use and in order to clear our value