Ascii to Hex

**bobthebullet990** · 08-31-2007

Is there a function available in C to easily lookup the hex value for an ascii char? I.e. If I wanted to encode so called 'special' chars to their hex values as with URLs;

so: "hello world" becomes "hello%20world"

or would I have to create my own switch-case statement for checking current char and print it to a string such as:

Code:

...
/* loop through string, until at end of string */
/* code to read current char in string */
...
switch currentChar{
  case ' ' : sprintf(str, "%20", null); break;
  case '%' : sprintf(str, "%25", null); break;
  ...
  default: return 1; /* ERROR! - should never be! */
}

or maybe even a few iteration statements to find the special chars that require encoding!

**Salem** · 08-31-2007

Like
sprintf( str, "%%%02x", currentChar );

**matsp** · 08-31-2007

If you need a % in a sprintf, then you need to use %%, as % is a formatting prefix.

But if you wan't an ASCII to Hex conversion, something like this would do:

Code:

void asctohex(char a, char *s) 
{
     char c;
     c = (a >> 4) & 0x0f;
     if (c <= 9) c+= '0'; else c += 'a' - 10;
     *s++ = c;
     c = a & 0x0f;
     if (c <= 9) c+= '0'; else c += 'a' - 10;
     *s++ = c;
     *s = 0;
}

Alternatively, if you fancy using sprintf, use something like:

Code:

    switch(currentChar) {
       case ' ':
       case '%':
       ...
          sprintf(str, "%%%2x", currentChar);
          break;
       default:
          break;
    }

--
Mats

**zacs7** · 08-31-2007

Don't look them up...
Convert decimal (ie the chars position in the ascii table) to hex and ta dar.

**bobthebullet990** · 08-31-2007

Originally Posted by zacs7

Don't look them up...
Convert decimal (ie the chars position in the ascii table) to hex and ta dar.

How do I find the chars position in the ASCII table? i.e. how to find that char 'a' is denery value 97 and hex value 61

thanks.

**QuantumPete** · 08-31-2007

The following will convert any string into a string of hexvalues:

Code:

char * conv_to_hex (char * input, int max_len) {
    int ii = 0;
    char * out_string = (char *) malloc0 (sizeof (char) * ((max_len*2)+1));
    
    for (ii = 0; ii < max_len; ii++) {
        sprintf ( (out_string+(ii*2)), "&#37;02x", (unsigned char) (*(input+ii)));
    }

    out_string[(max_len*2)] = '\0';

    return out_string;
}

QuantumPete

**matsp** · 08-31-2007

Originally Posted by bobthebullet990

How do I find the chars position in the ASCII table? i.e. how to find that char 'a' is denery value 97 and hex value 61

thanks.

if you do printf("%d", 'a'); then you'll get 97, replace %d with %x and you'll get 61 - chars are already their ASCII value (unless you are running on really old IBM hardware that uses the EBCDIC character set of course - but I doubt those are very common these days, and you probably would have said if that's what your system was).

--
Mats

**bobthebullet990** · 08-31-2007

Nice one! thanks guys! ...and just one more quick question...

If i can do printf("%d", 'a'); to print the denery and printf("%x", 'a'); to print the hex, is it then possible to do:

sprintf(str, "%d", 'a'); and sprintf(str, "%x",'a'); and the same effect will occur?

Thanks!

**QuantumPete** · 08-31-2007

yeah, but you'll need a string to print *into*

Code:

sprintf("&#37;d", 'a'); /* Won't compile */
sprintf (out_string, "%d", 'a'); /* Will compile */

QuantumPete

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