Programming Forums - modulo arithmetic question

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LOI Kratong

Sep 26th, 2005 3:44 PM

modulo arithmetic question

I am writing a program that deals with ciphers and codes. When the user enters a set of text and a number after being prompted, they are taken to this function:

for (i = 0; i < len; i++)

    { 

         workingtext[i] = workingtext[i] + ikey; //add the key to the plaintext                                                                                                    

    }                                                                                                   

    return workingtext;

}

where len is the length of workingtext

I want the resulting workingtext to only be a letter a-z, not any of the wierd computer characters (e.g. smiley faces, musical notes and french circumflex accents (just some of my favourites)). I think that it probably just requires a modulo thingy in here somewhere, can some one indicate where?

iignotus

Sep 26th, 2005 5:35 PM

ASCII normal printing characters are only d32 to d126. A-Z is d65-d90, and a-z is d97-d122. http://www.lookuptables.com/

InfoGeek

Sep 27th, 2005 5:44 AM

assuming that the characters entered by the user lie between a-z, you can do

workingtext[i] = 97+(workingtext[i] + ikey)%26;

DaWei

Sep 27th, 2005 7:22 AM

You could also construct an array for your desired character set that would be filled in at runtime in a way sensitive to the locale, then apply your modulus function according to the length of the array. If you're a portability kind of person. I'm (perhaps unfortunately) not.

LOI Kratong

Sep 27th, 2005 1:41 PM

Thanks guys, haven't tried it yet, but from what i can see it should do what i want so thanks...

snow	Sep 27th, 2005 5:07 PM

This is ROT encryption.
More info: http://en.wikipedia.org/wiki/ROT13

LOI Kratong

Sep 28th, 2005 3:07 AM

Ah! I tried it but it didn't work! If you try and use 1 as the key, it outputs with a key of 20!

Anyone know why?

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