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String manipulation questions
f#functionalweb

Hello all,

I am working through Robert Pickering's Foundations of F# book. While doing so, I am trying to solve exercises from other programming books and the web to help reinforce my learning. One problem called for reversing a string. Here's the code I came up with:

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#light

open System
let rec reverse str =
    match str with
    | "" -> ""
    | _ -> if String.length str = 1 then
               str
           else     
               Char.ToString (String.get str ((String.length str) - 1))
                   + (reverse (String.sub str 0 ((String.length str) - 1)))
print_endline (reverse "")    
print_endline (reverse "A")
print_endline (reverse "AB")
print_endline (reverse "ABC")
print_endline (reverse "This is my test string")

Console.ReadKey()

It works, but the nice things I can say about my implementation end there. How would you improve this code? Most of the F# code I see is concise, but mine feels syntacticly unwieldy.

I am primarily interested in F# because I feel I am overdue in (deeply) learning a functional language, and F# is probably the only functional language I would be permitted to use at work for the foreseeable future. But am I better off using the imperative features of F# for string manipulation? I am also interested to hear of any good resources for general string manipulation in F#.

Thanks,

Jim

.

Thanks to everyone for their implementations. They are all both insightful and humbling. :)

Jim

By on 12/18/2007 5:00 PM ()

Hi, I see that Julien was faster :-) anyway, my implementation is a bit different, so it might be useful to see them both. In my example, the inner recursive function (reverseAux) is using direct access to the string character at specified index and is generating list of characters (char list) and finally creates a string from this list of characters.

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let reverse (s:string) =
  let rec reverseAux idx acc =
    if (idx = s.Length) then acc
    else reverseAux (idx+1) ((s.[idx])::acc)
  new string(Array.of_list (reverseAux 0 []))

A few things to note - concatation of strings in .NET is a bit inefficient, since it is creating a copy of the whole string (though it will not be problem for short strings), so the sample is creating a list of characters (where append to the beginning should be more efficient).

By on 12/16/2007 4:25 AM ()

I really don't see why you shouldn't do this the obvious way

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let reverse s =
    let n = String.length s
    let cs = Array.zero_create n
    for i = 1 to n do
        cs.[i - 1] <- s.[n - i]
    done
    new string(cs)
By on 12/16/2007 4:34 AM ()

As string implements seq<char>, you can also use

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let reverse (s:string) = new string(s |> Seq.to_array |> Array.rev)
By on 12/18/2007 4:11 PM ()
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  let reverse s =
    let rec reverse s acc =
      match s with
      | "" -> acc
      | s -> reverse (String.sub s 1 (String.length s - 1)) (String.sub s 0 1 :: acc)
    reverse s [] |> (String.concat "")
By on 12/16/2007 4:07 AM ()

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