OCaml中的字符串操作

Question

I want to write these functions in pure OCaml

Length : that calculate the length of a given string

getChar : the same as String.get that return the char positioned at the given index.

I can't use any function that figure in String module or in other modules. Is string are list of char like in other language. Or how to transform a string to a list of char?

Thanks for your help.

Answer 1

The length of a value of type string cannot be obtained without String.length as far as I know. Well, you could write a loop that tries to read all characters until there is an exception, but I think it is rather ugly.

Regarding getChar I don't think it can be done either, unless you allow yourself to use the normal way to read a string: s.[i].

If you can't use anything but OCaml primitives, my guess is that you have to define your own string type, perhaps as a char list as you suggest:

type my_string = char list

Then, just re-define List.length and List.nth.

Answer 2

字符串在ocaml中势在必行，我不认为它在内部使用列表。

Answer 3

As others have pointed out, the statement of the problem is somewhat contradictory. All functions are part of some module. The most widely used functions are part of the Pervasives module, which is opened automatically for you.

Assuming the idea is to use only functions from Pervasives, the only available operations I see are the polymorphic comparison operations, the concatenation operator (^) and some I/O operations. You could also include the indexing operator .[] , which is implemented with the help of the compiler. But you might also exclude it, since it's really just another name for String.get .

This isn't enough to make a useful exercise (as far as I can see). You could find the length of the string using I/O operations but I very much doubt that's what you're looking for.

let string_length s = (* Not what you really want *)
    let oc = open_out "/tmp/myfile" in
    output_string oc s;
    flush oc;
    let res = out_channel_length oc in
    close_out oc;
    res

It might make more sense to work with a list of characters rather than a string. In OCaml these are completely different types. For testing, you could use the following function to change a string to a list of characters. Note that this function doesn't obey your stated limitations:

let list_of_string s =
    let rec ilist n =
        if n = String.length s then []
        else s.[n] :: ilist (n + 1)
    in
    ilist 0

You can do many things with lists using just pattern matching and the functions of Pervasives.