Vigenere formula assigning new values to plaintext

Question

First some sample code:

 1 // preliminary headers and such :-)
   ...

30 string key = argv[1];  // BAZ
   // validate key
   ...

35 string plain = GetString();  // test
   ...
38 string cipher = plain;
   // loop through the plaintext converting it and placing it into the cipher string
   ...
61    printf("plain: %i, plain[i]"); // prints out 116 (t) assuming i=0
62    cipher[i] = (plain[i] - 'a' + key[v] - 'a') % 26 + 'a';
63    printf("plain: %i, plain[i]"); // prints out 85 (U)
   ...
82 // the final printout of "test" using "BAZ" as the keyword is "U_lU"

Why is the value of plain[i] changing? Does setting cipher = plain work both ways? I thought assigning a variable to another variable was a one way thing, kind of like making a copy. To my knowledge line 62 should't do anything to the variable plain, just alter the contents of cipher[i]. BTW - I've copied the formula to it's own .c file and hard coded the key and individual characters into the formula and it seems to work just fine. The problem is plain[i] changes before the actual formula is run.

The reason I set the value of cipher to plain in line 38 is that I have to initialize the string. If I use NULL (as clang suggests) or just empty "", its length can't be altered. I tried using cipher[strlen(plain)] since a string is an array of chars, but it throws 8 errors back at me.

Also, oddly enough, the upper case version of the formula works just fine. I didn't include it here, because it seems irrelevent.

Thanks in advance.

Answer 1

when you do string cipher = plain you aren't creating a copy, but just another representation.

You don't really need cipher at all. You can just printf your new value using somthing like

printf("%c", (plain[i] - 'a' + key[v] - 'a') % 26 + 'a');

Don't forget that you need to handle upper and lowercase differently. the above only works for lowercase.