Thread: Changing leftmost digit of hexadecimal number.

I am trying to write a function that changes the first (leftmost) hexadecimal digit in a 32-bit unsigned integer using bitwise operators. The function needs to have two parameters: the first is the integer to be manipulated, the second the replacement digit. I can change the rightmost digit, but the leftmost digit ... This is what I have so far:

Code:

#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>

uint32_t changeLeftmost (uint32_t num1, int digit);

int main (void) {

    uint32_t num1 =0x5F8FDF91 ;
    int digit;
    int changedHex;

    printf ("Here is the unchanged hex digit: %#06X\n", num1);
    printf ("Enter the digit that will replace the leftmost digit:\n");
    scanf ("%d", &digit);
   
    changedHex = changeLeftmost (num1, digit);
    printf("Changed leftmost digit:\n");
    printf ("%#06X\n", changedHex);
   
   
    system ("pause");
    return 0;
}

uint32_t changeLeftmost (uint32_t num1, int digit){

/*
Your function should accept two parameters of uint32_t.
Inside the function create three variables of uint32, temp, result, and clear
Clear the last hex digit using an and statement and put into temp
Bit shift the replacement digit into the right place
Combine the two numbers together with an or statement
Return result to main.
*/
    uint32_t temp;
    uint32_t result;
    uint32_t clear;

    //This is where I am trying to shift and replace the digit.

    clear = num1 & 0xFFFFFFF0; //This makes the rightmost digit able to be changed. That I can do.
    temp = clear;
    temp = digit << 7;
    result = temp | num1;

    return result;
}

Using masks? I'm not sure how to use those. Is there a way for me to change the leftmost digit, without that? I want to input 0xB and change the 5 in 0x5F8FDF91 to B. I'm just trying to make sense of these instructions specifically:
"Your function should accept two parameters of uint32_t.

Inside the function create three variables of uint32, temp, result, and clear
Clear the last hex digit using an and statement and put into temp
Bit shift the replacement digit into the right place
Combine the two numbers together with an or statement
Return result to main."
Thank you for all your help.

Well, I am still learning, so it's not all the clear that to me that I must use masks and my book isn't a lot of help. My number is 0x5F8FDF91, if the user enters B, then the 5 in my hex number changes to B, not the 0x part. Thanks.

Yeah, that's what I understood from the directions I was given, which tells me to "Clear the last hex digit using an and statement and put into temp", and from my understanding that's what

Code:

 
clear = num1 & 0xFFFFFFF0; 
temp = clear;

does. Since the rightmost is the last hex digit. Thanks.