Ascii to Hex

This is a discussion on Ascii to Hex within the C Programming forums, part of the General Programming Boards category; Is there a function available in C to easily lookup the hex value for an ascii char? I.e. If I ...

  1. #1
    Matt Conway bobthebullet990's Avatar
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    Ascii to Hex

    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!
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  2. #2
    and the hat of int overfl Salem's Avatar
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    sprintf( str, "%%%02x", currentChar );
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    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

  4. #4
    Woof, woof! zacs7's Avatar
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    Don't look them up...
    Convert decimal (ie the chars position in the ascii table) to hex and ta dar.

  5. #5
    Matt Conway bobthebullet990's Avatar
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    Quote Originally Posted by zacs7 View Post
    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.
    Many junglists take pride in their belongin to what may be referred to as a globalised drum & bass subculture, as a subculture though, it is not nearly as distinct at gothic or punk!

  6. #6
    Technical Lead QuantumPete's Avatar
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    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
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    Quote Originally Posted by bobthebullet990 View Post
    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

  8. #8
    Matt Conway bobthebullet990's Avatar
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    Nice one! thanks guys! ...and just one more quick question...

    If i can do printf("&#37;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!
    Many junglists take pride in their belongin to what may be referred to as a globalised drum & bass subculture, as a subculture though, it is not nearly as distinct at gothic or punk!

  9. #9
    Technical Lead QuantumPete's Avatar
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    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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