Converting hexadecimal number to its 2's complement

Question

I'm supposed to write a program in C that switches hex number to its 2's complement. e.g) 0000008f to ffffff71

so far I have figured out how to print out 2'c complement of binary, but hex seems like a different story to me. Any suggestions about which direction I'm suppose to looking at?

"so far I have figured out how to print out 2'c complement of binary," --> post that code. — chux - Reinstate Monica, Apr 19 '20 at 11:37
A "hexadecimal number" is a string. So convert the value to an integer, take the 2's complement, and convert back to a string. — Weather Vane, Apr 19 '20 at 11:45
Is there a specified bound on the number of (hex) digits in your input number? — John Bollinger, Apr 19 '20 at 12:34

score 2 · Accepted Answer · answered Apr 19 '20 at 12:33

To compute the two’s complement of a n-digit hexadecimal numeral, either:

complement each digit (exchange 0 for F, 1 for E, and so on) and then add one to the whole numeral, or
subtract the numeral from (In hexadecimal) one followed by n zeroes.

If the number is not a whole number of hexadecimal digits, some adjustments to the above must be made for the first digit:

For the first method, complement the first digit using the appropriate number of bits. For example, with 22 bits, the first hexadecimal digit uses only two bits, so exchange 0 for 3, 1 for 2, 2 for 1, and 3 for 0.
For the second method, subtract from the appropriate power of two. For example, for 22 bits, subtract from (hexadecimal) 400000.

Converting hexadecimal number to its 2's complement

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