My guess is that you added another loop similar to the one you are using there, so firstly it proccesses all the and and changes them to b's, then goes back to the start of the string and checks for all the b's and changes them to c's. See where you are going wrong?
Personally i would try a slightly different approach which is much less code.
Im sure you are aweare tha each character in the alphat bet is represented by a number depending on which encoding you are using for example ASCII 'a' is 97. ASCII 'b' is 98. using this logic we can rewrite you code to change all the letters to the next letter in the alphabet in 1 loop.
I would change your for loop to something like this.
for(x=0;x<(zbor.length());x++)
{
zbor[x]=char((int(zbor[x])+1));
}
Now firstly you will notice that the conition of the for loop has been change from
x<100; to
x<(zbor.length());, this means that the for loop will only iterate as many times as there are number of characters in the string.
char((int(zbor[x])+1)). This may look a bit confusing if your new to C++ but its quite simple.firstly it changes the letter at zbor[x] to an integer, then adds one to this to get the next letter in the alphabet then the outside char() converts the number back into its character form before finally storing it back into zbor[x].
Note this is only a quick solution and not a full solution. AS i have left a few things out for you to think about,
1) what happens ro any space's in the string?
2) Whats happens when you reach 'z'?
3) Other punctuation, will not remain the same.
So you made need to add in a few more lines of code to check that it is a letter, and then when you reach z, is the next ccharacter in the alphabet A?.
A small clue for you is that all of the characters in the alphabet lie between the number 65 (A) and 122 (z), assuming its ASCII. So perhaps you should check to see if the value of the letter is within there before changing it.
Hope i didn't talk to much rubbish.
Chris