"ocaml_beginners"::[] tail recursive

Hello everybody,
  how can I make this sum (fun x -> x < 2) [1;2;3];; a
tail recursive funtion?:
   
  let sum f alist = let rec sum f alist num = 
match alist with []->num
|hd::tl->sum (if (f hd)=true then (num++) else num) tl
num;;
   
  Ty

 		
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On Sun, 10 Sep 2006 18:54:19 +0200, Tyson Fugel
<tfugelyahoo.com> wrote:

> Hello everybody,
>   how can I make this sum (fun x -> x < 2)
[1;2;3];; a tail recursive  
> funtion?:
>  let sum f alist = let rec sum f alist num =
> match alist with []->num
> |hd::tl->sum (if (f hd)=true then (num++) else num)
tl num;;
>  Ty

not the most efficient but short answer

let sum f alist = List.length (List.filter f alist)


or

let sum f alist =
    let rec aux num = function
      | [] -> num
      | h :: t when f h -> aux (succ num) t
      | h :: t -> aux (num) t
    in
    aux 0 alist

val sum : ('a -> bool) -> 'a list -> int =
<fun>

#  sum (fun x -> x < 2) [1;2;3];;
- : int = 1

I think the trick is that you should pass the new value of
num to the  
called recursive function.

peter


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Thanks Peter,
  I've changed my code like that, but it doesn't count
correctly, I suppose that I'd put a function instead of
last myfun.
  let sum f alist =let rec mysum myfun alst num=
     match alst with
       [] -> num
     | x:s
-> mysum myfun xs (num+1)
 in mysum f alist 0;;
Yerkin
Peter Halacsy <peterhalacsy.com> wrote:
  On Sun, 10 Sep 2006 18:54:19 +0200, Tyson Fugel wrote:

> Hello everybody,
> how can I make this sum (fun x -> x < 2)
[1;2;3];; a tail recursive 
> funtion?:
> let sum f alist = let rec sum f alist num =
> match alist with []->num
> |hd::tl->sum (if (f hd)=true then (num++) else num)
tl num;;
> Ty

not the most efficient but short answer

let sum f alist = List.length (List.filter f alist)


or

let sum f alist =
let rec aux num = function
| [] -> num
| h :: t when f h -> aux (succ num) t
| h :: t -> aux (num) t
in
aux 0 alist

val sum : ('a -> bool) -> 'a list -> int = 

# sum (fun x -> x < 2) [1;2;3];;
- : int = 1

I think the trick is that you should pass the new value of
num to the 
called recursive function.

peter


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On Mon, 11 Sep 2006 00:24:37 +0200, Tyson Fugel
<tfugelyahoo.com> wrote:

> Thanks Peter,
>   I've changed my code like that, but it doesn't
count correctly, I  
> suppose that I'd put a function instead of last myfun.
>   let sum f alist =let rec mysum myfun alst num=
>      match alst with
>        [] -> num
>      | x:s
-> mysum myfun xs (num+1)
>  in mysum f alist 0;;
> Yerkin


The function f is missing from  mysum. This code calculates
the length of  
alist, isn't it?

As far as I understand you want to count the number of
elements of a list  
that satisfies the predicate f.

>   let sum f alist =let rec mysum myfun alst num=
>      match alst with
>        [] -> num
>      | x:s
-> mysum myfun xs (num+1)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
         (* this matches every element of alist *)
         (* change to *)
         (* | x:s
-> if f x then myfun xs (num+1) else myfun xs (num) *)
>  in mysum f alist 0;;

peter


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2006/9/10, Tyson Fugel <tfugelyahoo.com>:
> Hello everybody,
>   how can I make this sum (fun x -> x < 2)
[1;2;3];; a tail recursive funtion?:
>
>   let sum f alist = let rec sum f alist num =
> match alist with []->num
> |hd::tl->sum (if (f hd)=true then (num++) else num)
tl num;;

Mmm, it is already a tail recursive function : nothing is
done after
the recursive call to sum. But this is not ocaml code (num++
???). You
want:

 let sum f alist =
   let rec sum_rec f alist num =
     match alist with
     | [] -> num
     | hd::tl ->
	 sum_rec f tl (if (f hd)=true then (num+1) else num) in
   sum_rec f alist 0;;


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On Sunday 10 September 2006 17:54, Tyson Fugel wrote:
> Hello everybody,
>   how can I make this sum (fun x -> x < 2)
[1;2;3];; a tail recursive
> funtion?:
>
>   let sum f alist = let rec sum f alist num =
> match alist with []->num
>
> |hd::tl->sum (if (f hd)=true then (num++) else num)
tl num;;

# let sum f list =
    List.fold_left ( + ) 0 (List.filter f list);;
val sum : (int -> bool) -> int list -> int =
<fun>

or the deforested:

# let sum f list =
    List.fold_left (fun n e -> if f e then n+1 else n) 0
list;;
val sum : ('a -> bool) -> 'a list -> int =
<fun>

-- 
Dr Jon D Harrop, Flying Frog Consultancy Ltd.
Objective CAML for Scientists
http://www.ffconsultancy.com/products/ocaml_for_scient
ists


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On Sunday 10 September 2006 17:54, Tyson Fugel wrote:
> Hello everybody,
>   how can I make this sum (fun x -> x < 2)
[1;2;3];; a tail recursive
> funtion?:
>
>   let sum f alist = let rec sum f alist num =
> match alist with []->num
>
> |hd::tl->sum (if (f hd)=true then (num++) else num)
tl num;;

# let sum f list =
    List.fold_left ( + ) 0 (List.filter f list);;
val sum : (int -> bool) -> int list -> int =
<fun>

or the deforested:

# let sum f list =
    List.fold_left (fun n e -> if f e then n+1 else n) 0
list;;
val sum : ('a -> bool) -> 'a list -> int =
<fun>

-- 
Dr Jon D Harrop, Flying Frog Consultancy Ltd.
Objective CAML for Scientists
http://www.ffconsultancy.com/products/ocaml_for_scient
ists


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Hi,
  What is the difference between lp1 and lp2? Can I say that
both functions are forward recursion, not a tail recursion,
because they return "Stack overflow during evaluation
(looping recursion?)" then I call lp1() or lp2()? A
tail recursive function shouldn't return "Stack
overflow", am I right? 
  let rec lp1 () = lp1(); ()
let rec lp2 () = lp2();;
  val lp1 : unit -> unit = <fun>
val lp2 : unit -> 'a = <fun>


 		
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Actually, lp2 doesn't overflow the stack. The fact the last
thing it
does is call itself makes it tail-recursive.

lp1 is not tail recursive because the last thing it does is
return
unit (after calling itself).

On 9/19/06, Tyson Fugel <tfugelyahoo.com> wrote:
> Hi,
>  What is the difference between lp1 and lp2? Can I say
that both functions are forward recursion, not a tail
recursion, because they return "Stack overflow during
evaluation (looping recursion?)" then I call lp1() or
lp2()? A tail recursive function shouldn't return
"Stack overflow", am I right?
>  let rec lp1 () = lp1(); ()
> let rec lp2 () = lp2();;
>  val lp1 : unit -> unit = <fun>
> val lp2 : unit -> 'a = <fun>
>
>
>
> ---------------------------------
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>
>
>
>
>
>
>
>
>
>
>


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but if you trace it lp2() finally shows a stack overflow

Jonathan Roewen <jonathan.roewengmail.com> wrote:       
  Actually, lp2 doesn't overflow the stack. The fact the
last thing it
does is call itself makes it tail-recursive.

lp1 is not tail recursive because the last thing it does is
return
unit (after calling itself).

On 9/19/06, Tyson Fugel <tfugelyahoo.com> wrote:
> Hi,
> What is the difference between lp1 and lp2? Can I say
that both functions are forward recursion, not a tail
recursion, because they return "Stack overflow during
evaluation (looping recursion?)" then I call lp1() or
lp2()? A tail recursive function shouldn't return
"Stack overflow", am I right?
> let rec lp1 () = lp1(); ()
> let rec lp2 () = lp2();;
> val lp1 : unit -> unit = <fun>
> val lp2 : unit -> 'a = <fun>
>
>
>
> ---------------------------------
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>
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> Archives up to August 22, 2005 are also downloadable at
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>
>
>
>
>
>
>
>
>
>
>


         

 		
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