Slightly confused to some C# coding

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I have this part of the code in my program

int remainder = sum % 26;

     byte[] code = Encoding.ASCII.GetBytes(check);
     int chk = code[0];
     chk = chk - 65;                  // Subtract 65 from ASCII value for comparison i.e.
                                      // A = 0, B = 1, C = 2 onwards
     /* If we have check letter let say J its ascii value is 74 so chk will have value 9
        and as security number is 1122334455 sum = 165 and reminder = 9
      */
     if (remainder == chk)
     {

My lecturer helped me write that part, but I do not understand what it does. I can't get hold of him and my deadline is in for tomorrow. I have to add up the sum of 10 numbers and then each letter of the alphabet is assigned a number so A =0, B=1 etc etc, what i don't get is how does J = 74 and why do I have to -65? If i take the chk = chk -65 code out of my program then it does not work. Any insight?

Posted 19-Jan-14 9:49am

Member 10506451

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BillWoodruff 19-Jan-14 16:20pm

C. Pallini gives you an excellent explanation below; I'd add that there's no "mystery" here; the values of the ASCII character codes are just arbitrary left-overs ... fossils ... of the evolution of computer programming; they originated in telegraphic codes.

Upper-case ASCII characters are between #65 and #90, lower-case are from #97 to #122.

Sometimes it's a mistake to assume that there's any deep "principle" at work :)

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CPallini · Accepted Answer · 2014-01-19T10:06:00

Solution 1

The mapping you use ('A'<->0, 'B'<->1, 'C'<->2, ...) is handy for computing a checksum on a string but it is not the same usually found on computer, that is the ASCII[^] one ('A'<->65, 'B'<->66, 'C'<->67, ...).

Luckily the two 'encodings' (yours and the ASCII one) differ only for an offset, hence

char_yours = char_ASCII - 65

Posted 19-Jan-14 10:06am

CPallini

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Member 10506451 19-Jan-14 16:38pm

thanks for the help :)

Member 10506451 19-Jan-14 16:49pm

I have another question about what some code does if that is ok? should i just post it here or create a new question thread?