Char string to hex char array in C++ [duplicate]

Question

I have a CLinux device, that i need to transform some data. The input is something like

char mytest[] = "020441010C0000003131203131203131313103";

and i need to have

mytest2[] = {0x02,0x04,0x41,0x01,0x0C,0x00,0x00,0x00,0x31,0x31,0x20,0x31,0x31,0x20,0x31,0x31,0x31,0x31,0x03}

This is the code that i've tried, but it doesn't compile cause the sstream is not on the device OS.. Can some give me another solution to transform the data?

#include <iostream>
#include <sstream>


int main() {
std::string myStr = "1122AA010A";

std::stringstream ss;
int n;
for(int i = 0; i<myStr.length(); ) {
    std::istringstream(myStr.substr(i,2))>>std::hex>>n;
    ss<<(char)n;
    i += 2;
    }

std::string result = ss.str();

std::cout<<"\n"<<result<<"\n";
return 0;
}

Seva Alekseyev · Accepted Answer · 2019-03-19 21:04:44Z

1

Parsing hex is pretty easy. Convert each digit to its hex value, and combine them together into a single char.

int FromHexDigit(char c) //Returns a number from 0 to 15
{
    if(c >= '0' && c <= '9')
        return c-'0';
    else if(c >= 'A' && c <= 'F')
        return c-'A' + 10;
    else if(c >= 'a' && c <= 'f')
        return c-'a' + 10;
    else
        return -1; //Not a hex digit :(
}

Then the main loop becomes:

std::string result;
for(int i = 0; i<myStr.length(); i += 2) {
    char d1 = myStr[i], d2 = myStr[i+1]; //Those are the two consecutive hex digits
    int n = FromHexDigit(d1)*16 + FromHexDigit(d2); //Convert to a number
    result += (char)n; //Put into the result string
}

Note the i+=2 portion in the for loop header. We're processing the string in two byte chunks. There's no handling of the situation that the string has an odd number of characters.

edited Mar 19, 2019 at 21:04

answered Mar 19, 2019 at 17:35

Seva Alekseyev

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3 Comments

SE-LM Over a year ago

What do you mean with convert each digit to its hex value ? There's no way to work like with substrings, working with 2 digits at at time? ( cause they're already in hex )

bruno Over a year ago

@SE-LM you can do a substring of size 2 then make a sstring with it then extract in hexa, but this is much more expensive

bruno Over a year ago

is strange to put the result in a string, a vector<unsigned> or equivalent seems more natural no ?

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