Thread: hex to dec question

  1. #1
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    hex to dec question

    Code:
    int htoi(char s[])
    
    {
        int i = 0; /* Iterate over s */
        int n = 0; /* Built up number */
    
     
        /* Remove "0x" or "0X" */
        if ( s[0] == '0' && s[1] == 'x' || s[1] == 'X' )
            i = 2;
     
        while ( s[i] != '\0' )
        {
            int t;
     
            if ( s[i] >= 'A' && s[i] <= 'F' )
    
                t = s[i] - 'A' + 10;
    
            else if ( s[i] >= 'a' && s[i] <= 'f' )
                t = s[i] - 'a' + 10;
            else if ( s[i] >= '0' && s[i] <= '9' )
                t = s[i] - '0';
            else
                return n;
     
            n = 16 * n + t;
            ++i;
        }
     
        return n;
    
    }
    Hi, I'm working through the problems in the K&R C book. Anyway I've read around and I can't understand this particular line?

    Code:
    t = s[i] - 'A' + 10;
    I don't understand why the 10 is being added. Could anyone please explain?

  2. #2
    C++ Witch laserlight's Avatar
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    Why is 'A' is subtracted from s[i]? Could you please explain?
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
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  3. #3
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    EDIT: Ok so I think I understand it. A = 10 in hex, B = 11 ...F = 16. The reason the numeric value of A is being substracted and then 10 added to it is so that the number can be repesented by it's hex value. For example:

    If s[i] = 'B' which has a deciman value of 66. Then

    t = s[i] - 'A' + 10;


    will equal to t = 66 - 65 + 10 which is 11.
    11 is the hex value of B.
    Last edited by rbz; 08-23-2012 at 12:02 PM.

  4. #4
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    Quote Originally Posted by rbz View Post
    EDIT: Ok so I think I understand it.
    That's exactly it. As you can see it's only done if s[i] is between 'A' and 'F' or 'a' and 'f'. For all other cases '0' is subtracted since 0-9 is covered in the decimal set.

  5. #5
    Algorithm Dissector iMalc's Avatar
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    I know this is not very common, but I always write it like this:
    Code:
    BinVal = AsciiVal - 'A' + 0xA;
    This to me makes it clear that you're moving a value from being based at 'A' to being based at 0xA, and feels somewhat like you're removing a magic number too.
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