How to edit nibbles inside a variable? - c++

Question

Suppose I have the following code.
int main(int argc, char *argv[])
{
size_t value = stoul(argv[1], 0, 16);
size_t nibble = stoul(argv[2]);
size_t replacement = stoul(argv[3]) % 16;
cout << hex << value << '\n';
}
I want to write supplementary to this code such that I can type ./a.out value nibble replacement in my terminal, and it will replace whatever digit is in the specified nibble by the replacement. So for example, I would want to type ./a.out 22334 3 11 so that the output I get is 2b334. Nibble here indicates the nibble offset. How would I make my program access the specified nibble?

Answer 1

value &= ~( 0xF << ( nibble * 4 ) );
value |= replacement << ( nibble * 4 );
First we create a mask.
1 1 1 1 0xF
1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0xF << ( nibble * 4 )
1 1 1 1 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 ~( 0xF << ( nibble * 4 ) )
We use this to clear bits from the original value.
0 0 1 0 0 0 1 0 0 0 1 1 0 0 1 1 0 1 0 0 value
& 1 1 1 1 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 mask
-----------------------------------------------
0 0 1 0 0 0 0 0 0 0 1 1 0 0 1 1 0 1 0 0 value & mask
Then we prepare the value to insert.
1 0 1 1 replacement
1 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 replacement << ( nibble * 4 )
Then merge it in.
0 0 1 0 0 0 0 0 0 0 1 1 0 0 1 1 0 1 0 0 value & mask
1 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 replacement << ( nibble * 4 )
| -----------------------------------------------
0 0 1 0 1 0 1 1 0 0 1 1 0 0 1 1 0 1 0 0

Answer 2

You can use the output stream class to work with a string. This makes it much easier to replace any position.
#include <iostream>
#include <sstream>
int main (int argc, char *argv[])
{
std::ostringstream outstream_value;
outstream_value << std::hex << std::stoul(argv[1], 0, 16);
std::string hexstr = outstream_value.str();
const std::size_t length = hexstr.length();
std::ostringstream outstream_replace;
outstream_replace << std::hex << std::stoul(argv[3]) % 16;
const char replace_char = outstream_replace.str()[0];
const std::size_t nibble = std::stoul(argv[2]);
if ( length >= nibble + 1 )
hexstr[length - nibble - 1] = replace_char;
std::cout << hexstr;
return 0;
}