string - caesar cipher check in ocaml

Question

I want to implement a check function that given two strings s1 and s2 will check if s2 is the caesar cipher of s1 or not. the inter face needs to be looked like string->string->bool.

the problem is that I am not allowed to use any string functions other than String.length, so how can I solve it? i am not permitted any list array, iterations. Only recursions and pattern matching.

Please help me. And also can you tell me how I can write a substring function in ocaml other than the module function with the above restrictions?

Answer 1

My guess is that you are probably allowed to use s.[i] to get the ith character of string s. This is the same as String.get, but the instructor may not think of it in those terms. Without some form of getting the individual characters for the string, I believe that this is impossible. You should probably double check with your instructor to be sure, but I would be surprised if he had meant for you to be unable to separate a string into characters (which is something that you cannot do with pattern-matching alone in Ocaml).

Once you can get individual characters, the way to do it should be pretty clear (you do not need substring to traverse each string recursively).

If you still want to write substring, creating it would be complex since you don't have access to String.create or other similar functions. But you can write your own version of String.create using recursion, one character string literals (like "x"), the ability to set a character in a string to another (like s.[0] <- c), and string concatenation (s1 ^ s2). Again, of course, all of this is assuming that those operators are allowed to be used.