C/C++: Converting hexadecimal value in char to integer

Question

I have hexadecimal values stored as characters:

char A = \'0\';
char B = \'6\';
char C = \'E\';

... I need them coverted to integers. I kno

Answer 1

int v = (A > '9')? (A &~ 0x20) - 'A' + 10: (A - '0');

is correct for ASCII. For other character sets, a similar approach would work, but you would then want toupper instead of &~ 0x20.

Answer 2

You could try strtol. But strtol needs a 0 terminated char *, so:

long x = strtol((char[]){A, 0}, NULL, 16);

Answer 3

#include <stdio.h>
#include <stdlib.h>


int xtoi(char c)
{
    int v = -1;
    char w=toupper(c);
    if(w >= 'A' && w <= 'F'){
        v = w - 'A' + 0x0A;
    }else if (w >= '0' && w <= '9'){
        v = w - '0';
    }

    return v;
}

int main(int argc,char **argv)
{
    printf("v:%p\n",argc == 2 ? xtoi(argv[1][0]):-1);
    return 0;
}

Answer 4

You can explicity state you want to use base 16 with strtol

char C = 'E';
int num = strtol (&C ,NULL,16); // 14

Beware that C is not a null terminated array, simply solved if you can change your characters to the following:

char C[] = "E";
int num = strtol(C, NULL, 16);

Answer 5

In C:

const char chrs[] = "0123456789ABCDEF";

int value = -1; // sentinel
const char *loc = strchr(chrs, ch);
if (loc)
    value = loc - chrs;

or, using C++:

const std::string chrs("0123456789ABCDEF");
int value = chrs.find(ch);

Answer 6

The obvious solution is to make a one character string, and use the standard conversion techniques on it:

std::istringstream tmp( std::string( 1, A ) );
tmp >> anInt;

It's a bit heavy, however (to put it lightly), and probably not very efficient in terms of runtime (a litote if I ever heard one).

One simple and robust solution is a table lookup:

static char const digits[] = "0123456789ABCDEF";
int value = std::find(digits, digits + 16, A) - digits;
if (value >= 16) {
    //  A wasn't a hex digit...
}

Alternatively (and much faster) is to use the character as an index into a table:

static signed char const values[] =
{
    -1, -1, -1, -1, -1, -1, -1, -1,     //  0x00-0x07
    //  ...
     0,  1,  2,  3,  4,  5,  6,  7,     //  0x30-0x37
     8,  9, -1, -1, -1, -1, -1, -1,     //  0x38-0x3F
    -1, 10, 11, 12, 13, 14, 15, -1,     //  0x40-0x47
    //  ...
    -1, 10, 11, 12, 13, 14, 15, -1,     //  0x60-0x67
    //  ...
};


int value = values[static_cast<unsigned char>( A )];

Note that this is very dependent on the encoding; you might want to build it up at runtime from a list of digits, to avoid any encoding issues. Something like:

struct ValueTable
{
    signed char values[256];
    ValueTable()
    {
        std::fill( begin(values), end(values), -1 );
        static char const digits[] = "0123456789ABCDEF";
        for ( int i = 0; i < 16; ++ i ) {
            values[ digits[i] = i;
            values[ tolower( digits[i] ) ] = i;
        }
    }
};
static ValueTable const map;

//   ...
int value = map.values[ static_cast<unsigned char>( A ) ];