Change bit of hex number with leading zeros in C++,(C)

Question

I have this number in hex string:

002A05.

I need to set 7-th bit of this number to 1, so after conversion I will get

022A05
         


        

Answer 1

It sounds to me like you have a string, not a hex constant, that you want to manipulate. You can do it pretty easily by bit twiddling the ascii value of the hex character. If you have char representing a hex character like char h = '6';, char h = 'C';, or char h = '';, you can set the 3rd from the left (2nd from the right) bit in the number that the character represents using:

 h = h > '7' ? h <= '9' ? h + 9 : ((h + 1) | 2) - 1 : h | 2;

So you can do this to the second character (4 + 3 bits) in your string. This works for any hex string with 2 or more characters. Here is your example:

char hex_string[] = "002A05";

// Get the second character from the string
char h = hex_string[1];

// Calculate the new character
h = h > '7' ? h <= '9' ? h + 9 : ((h + 1) | 2) - 1 : h | 2;

// Set the second character in the string to the result
hex_string[1] = h;

printf("%s", hex_string); // 022A05