Trying to write a C++ function to un-hex encode a 8 bit char and turn it into an integer

Question

I have a C++ function that converts a unsigned long into a 8 bit hex char string. I need to come up with a reverse function that takes a 8 bit hex char string and converts it into an unsigned integer representing it's bytes.

Original UINT -> char[8] method:

std::string ResultsToHex( unsigned int EncodeResults)
{

        std::cout << "UINT Input ==>";
        std::cout << EncodeResults;
        std:cout<<"\n";

        char _HexCodes[] = "0123456789ABCDEF";
        unsigned int HexAccum = EncodeResults;
        char tBuf[9];
        tBuf[8] = '\0';
        int Counter = 8;
        unsigned int Mask = 0x0000000F;
        char intermed;

        // Start with the Least significant digit and work backwards

        while( Counter-- > 0 )
        {

            // Get the hex digit
            unsigned int tmp = HexAccum & Mask;

            intermed = _HexCodes[tmp];

            tBuf[Counter] = intermed;

            // Next digit
            HexAccum = HexAccum >> 4;
        }

        std::cout << "Hex char Output ==>";
        std::cout << tBuf;
        std::cout << "\n";

        return std::string(tBuf);
    }

And here is the function I am trying to write that would take a char[8] as input and convert into a UINT:

 unsigned int ResultsUnhex( char tBuf[9])
{
        unsigned int result = 0;
        std::cout << "hex char Input ==>";
        std::cout << tBuf;
        std:cout<<"\n";

        //CODE TO CONVERT 8 char (bit) hex char into unsigned int goes here......
        //
        // while() {}
        //
        ///


        std::cout << "UINT Output ==>";
        std::cout << result;
        std::cout << "\n";

        return result;
    }

I am new to bit shifts, any help would be greatly appreciated :).

Some effort on your side would be greatly appreciated from our side. The idea here is that you give us a SMALL piece of the puzzle that you can't solve, not all 500 pieces on the table in a jumbled mess, saying "I don't see where these pieces go together". It is really just about doing the same thing as you have done for "make a number into hex-string" the other way around. — Mats Petersson, Jul 02 '13 at 22:02
possible duplicate of [convert hex string to signed integer](http://stackoverflow.com/questions/1070497/c-convert-hex-string-to-signed-integer?lq=1) — quetzalcoatl, Jul 02 '13 at 22:10
The answer might wait for you in the question what the **reverse** bitshift operation is for that one you see in the sample function. — πάντα ῥεῖ, Jul 02 '13 at 22:30
"8 bit hex char string" makes no sense -- hex chars encode 4 bits each. Perhaps you mean "8 hex char string" (32 bits total)? Or perhaps a 2 char hex string, for 8 bits total? — Chris Dodd, Jul 02 '13 at 22:38
[overly complicated](http://coliru.stacked-crooked.com/view?id=f45e2659c570dd3797c4ff39af82aeaf-f674c1a6d04c632b71a62362c0ccfc51) — Mooing Duck, Jul 02 '13 at 23:09

score 2 · Answer 1 · answered Jul 02 '13 at 22:45

You need to scan the string and convert each hexadecimal character back to it's corresponding 4 bit binary value. You can do this with a simple set of if statements that checks the character to see if it's valid and if so convert it back.

After a character has been converted, shift your result variable left by 4 bits then stuff the converted value into the lower 4 bits of the result.

#include <stdexcept>

unsigned int ConvertHexString(char *str)
{
    unsigned int result = 0;
    while(*str)
    {
        char ch = *str++;

        // Check for 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9
        if(ch >= '0' && ch <= '9')
        {
            ch -= '0';
        }
        // Check for a, b, c, d, e, f
        else if(ch >= 'a' && ch <= 'f')
        {
            ch = 10 + (ch - 'a');
        }
        // Check for A, B, C, D, E, F
        else if(ch >= 'A' && ch <= 'F')
        {
            ch = 10 + (ch - 'A');
        }
        // Opps! Invalid character
        else
        {
            throw std::invalid_argument("Unknown character in hex string");
        }


        // Mmmm! Stuffing! Don't forget to check for overflow!
        result = (result << 4) | ch;
    }

    return result;
}

There are several ways to do this but I figured a simple example to get you started would be more helpful.

Obligatory example using the conversion function.

#include <iostream>
int main()
{
    std::cout
        << std::hex
        << ConvertHexString("1234567")
        << ConvertHexString("90ABCDEF")
        << std::endl;
}

Outputs...

123456790abcdef

I'd say he's supposed to use bit shift operations to solve this problem to get a good grade for the solution. Ahhh! Sorry didn't spot it immediately ;o) ... — πάντα ῥεῖ, Jul 02 '13 at 22:54
@g-makulik There's one bit shift in there...it'll get the OP _started_. — Captain Obvlious, Jul 02 '13 at 22:56

score 0 · Answer 2 · edited May 23 '17 at 12:30

0

Why do you do it manually? There's plenty of tools that do that for you. For example, stringstream.

This converts an int to a hex-string:

std::stringstream stream;
stream << std::hex << your_int;
std::string result( stream.str() );

[code copied from Integer to hex string in C++ - please see that thread, there's plenty of examples. Upvote there instead of here.]

Add to that "0x" and some field-width manipulators and you may reduce the ResultsToHex to three..five lines.

The stringstream also works the other way. Using >> operator you can just as easily read a hexstring into an integer variable. [Converting Hexadecimal to Decimal, Hex character to int in C++, convert hex buffer to unsigned int, ...]

edited May 23 '17 at 12:30

Community

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answered Jul 02 '13 at 22:07

quetzalcoatl

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Presumably the homework lesson is about "hex numbers and bit-shifts", not "how do you do this in the fewest lines using already written code". – Mats Petersson Jul 02 '13 at 22:11
1

@MatsPetersson: `I need to come up with a reverse function that takes a 8 bit hex char string and converts it into an unsigned integer representing it's bytes`. Not a word about bitshift. For me, it looks like trying to solve the problem by concentrating on "what is integer" "what is a byte" "what is Hex" instead of just doing it by normal/standard means. But, I agree, that also might be a homework. We'll probably hear about it soon :) – quetzalcoatl Jul 02 '13 at 22:15
3

Indeed not. But then most homework students don't post "I'm at chapter 3 of my C++ book, which is about integers, bits and bytes". The fact that the rest of the posted code uses none of the C++ strings, vectors, etc, indicates this. But I guess your answer will be at least as useful as an obfuscated one that I might have written had I been in a devious mood. ;) – Mats Petersson Jul 02 '13 at 22:18

Trying to write a C++ function to un-hex encode a 8 bit char and turn it into an integer

2 Answers2