C++ char hex value to byte - c++

Question

I have a hex value in a char pointer (for example 'F3'), and I would like to convert it to byte, because I want it to put in an array. I know that there are many solutions, but they are not what I wanted.
Thanks in advance!
EDIT:
Okay, maybe I have not written everything.
What I have now:
char aChar[5];
itoa (j, aChar, 16);
j is now 3, and I just want it in byte. Atoi, scanf doesn't help, those are the different solutions.

Answer 1

Since you've tagged this C++ and not C, I'm not going to use any C functions (except assert() to demonstrate the behaviour, edge conditions, et cetera). Here's a sample file. Let's call it hex2byte.cpp:
#include <sstream>
#include <cassert>
unsigned char hex2byte(const char* hex)
{
unsigned short byte = 0;
std::istringstream iss(hex);
iss >> std::hex >> byte;
return byte % 0x100;
}
int main()
{
const char* hex = "F3";
assert(hex2byte(hex) == 243);
assert(hex2byte("") == 0);
assert(hex2byte("00") == 0);
assert(hex2byte("A") == 10);
assert(hex2byte("0A") == 10);
assert(hex2byte("FF") == 255);
assert(hex2byte("EEFF") == 255);
assert(hex2byte("GG") == 00);
assert(hex2byte("a") == 10);
assert(hex2byte("0a") == 10);
assert(hex2byte("f3") == 243);
assert(hex2byte("ff") == 255);
assert(hex2byte("eeff") == 255);
assert(hex2byte("gg") == 00);
}
Make it:
% make hex2byte
g++ -Wall -Wextra -Wshadow -pedantic -Weffc++ -Werror hex2byte.cpp -o hex2byte
Run it:
% ./hex2byte
No assertions. Add error handling to taste (such as checking for when hex == NULL, et cetera).

Answer 2

A byte is usually simply an unsigned char
myArray[n] = (unsigned char)*p;
Or do you mean that you have a string representation of an hex value?

Answer 3

Given a char * with "F3":
char *hexstr = "F3";
Then you can do this:
unsigned char byteval =
(((hexstr[0] >= 'A' && hexstr[0] <= 'Z') ? (10 + hexstr[0] - 'A') :
(hexstr[0] >= 'a' && hexstr[0] <= 'z') ? (10 + hexstr[0] - 'a') :
(hexstr[0] >= '0' && hexstr[0] <= '9') ? (hexstr[0] - '0') : 0) << 4) |
((hexstr[1] >= 'A' && hexstr[1] <= 'Z') ? (10 + hexstr[1] - 'A') :
(hexstr[1] >= 'a' && hexstr[1] <= 'z') ? (10 + hexstr[1] - 'a') :
(hexstr[1] >= '0' && hexstr[1] <= '9') ? (hexstr[1] - '0') : 0);
I'm sorry for its ugliness; I'm sure it can be improved.
You can turn this into a function:
inline unsigned char hextobyte(const char *s) {
return
(((s[0] >= 'A' && s[0] <= 'Z') ? (10 + s[0] - 'A') :
(s[0] >= 'a' && s[0] <= 'z') ? (10 + s[0] - 'a') :
(s[0] >= '0' && s[0] <= '9') ? (s[0] - '0') : 0) << 4) |
((s[1] >= 'A' && s[1] <= 'Z') ? (10 + s[1] - 'A') :
(s[1] >= 'a' && s[1] <= 'z') ? (10 + s[1] - 'a') :
(s[1] >= '0' && s[1] <= '9') ? (s[1] - '0') : 0);
}

Answer 4

I can think of at least five ways:
using sscanf with %x
using strtol with the correct base
using istringstream (though you'll have to down cast from unsigned short to unsigned char)
boost::spirit
hand rolled loop to parse character-by-character
None of the ways you listed would work. But your question still isn't really clear, you have a char which somehow you converted to hex using itoa and now you want to convert to a byte!?! what's wrong with a cast? e.g. unsigned char byte = static_cast<unsigned char>(charvalue);