[Caml-list] List.sort

[Caml-list] List.sort

[Johann Spies]

I apologise for asking a newbie question on this list, but the
beginners-list seems to have a problem.  After 24 hours my message
did not appear on the list and there was actually no activity the last
few days.

I am trying to figure out how to use List.sort.

According to the manual the Persvasives.compare function is suitable
to use with List.sort.  I can't figure out how to use it:
----------------------------
# k;;
- : int list = [3; 2; 1]
# List.sort (compare 1 2) k;;
This expression has type int but is here used with type 'a -> 'a ->
int
# List.sort (+) k;;
- : int list = [1; 2; 3]
# a;;
- : char list = ['d'; 'b'; 'g']
# List.sort (+) a;;
This expression has type char list but is here used with type int list
# List.sort (compare 'a' 'b') a;;
This expression has type int but is here used with type 'a -> 'a ->
int
# compare 'a' 'b';;
- : int = -1
#
---------------------
What is confusing to me is that the manual says:

"Sort a list in increasing order according to a comparison
function. The comparison function must return 0 if it arguments
compare as equal, a positive integer if the first is greater, and a
negative integer if the first is smaller. For example, the compare
function is a suitable comparison function. " 

I could not find a single example on how to use this function in the
manual.  Can somebody show me how to use it please?

Johann
-- 
Johann Spies          Telefoon: 021-808 4036
Informasietegnologie, Universiteit van Stellenbosch

     "My son, if sinners entice thee, consent thou not." 
                               Proverbs 1:10
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Re: [Caml-list] List.sort

[Stefano Zacchiroli]

On Wed, Jun 19, 2002 at 10:54:54AM +0200, Johann Spies wrote:
> I am trying to figure out how to use List.sort.

Look at the List.sort type:

  ('a -> 'a -> int) -> 'a list -> 'a list

the first "argument" required by List.sort have type "function that
takes two arguments of the same type and return an int".
The type of compare is:

  'a -> 'a -> int

you then have to pass the function compare itself without applying it
(as I saw in one of your example).
This is a sample usage:

  List.sort compare [3;2;1]

Indeed "List.sort compare" have type

  'a list -> 'a list

Cheers.

-- 
Stefano Zacchiroli - undergraduate student of CS @ Univ. Bologna, Italy
zack@cs.unibo.it | ICQ# 33538863 | http://www.cs.unibo.it/~zacchiro
"I know you believe you understood what you think I said, but I am not
sure you realize that what you heard is not what I meant!" -- G.Romney
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Re: [Caml-list] List.sort

[Henri Dubois-Ferriere]

A simple example: 

# let l = [4; 3; 2; 1];;
val l : int list = [4; 3; 2; 1]
# List.sort compare l;;
- : int list = [1; 2; 3; 4]


# List.sort;;
- : ('a -> 'a -> int) -> 'a list -> 'a list = <fun>

you can interpret the signature as follows:

1st argument, of type ('a -> 'a -> int): 'compare' function
a function that takes two 
arguments of identical type, and returns an int.

2nd argument, of type 'a list : list of the *same* type as the arguments 
of the function passed as the compare function.
   

> According to the manual the Persvasives.compare function is suitable
> to use with List.sort.  I can't figure out how to use it:
> ----------------------------
> # k;;
> - : int list = [3; 2; 1]
> # List.sort (compare 1 2) k;;
> This expression has type int but is here used with type 'a -> 'a ->
> int

indeed, (compare 1 2) evaluates to an int :

# compare 1 2;;
- : int = -1


> # List.sort (+) k;;
> - : int list = [1; 2; 3]
> # a;;

this is accepted because (+) takes the right arguments (two ints). Your 
list is sorted, but that is just by coincidence ;)

> - : char list = ['d'; 'b'; 'g']
> # List.sort (+) a;;
> This expression has type char list but is here used with type int list

As noted above, the second argument passed to List.sort takes a list whose 
elements must be of same type as the compare function. Here your compare 
function (+) takes int's, and you are giving a list of char's ...

> # List.sort (compare 'a' 'b') a;;
> This expression has type int but is here used with type 'a -> 'a ->
> int
> # compare 'a' 'b';;
> - : int = -1
> #
> ---------------------

same argument as for the first case: (compare 'a' 'b') is a char, not a 
function.

Henri

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Re: [Caml-list] List.sort

[sebastien FURIC]

Henri Dubois-Ferriere a écrit :

 <snip>

> > # compare 'a' 'b';;
> > - : int = -1
> > #
> > ---------------------
> 
> same argument as for the first case: (compare 'a' 'b') is a char, not a
                                                            ^^^^^^
> function.

 You meant an int, of course!

 Cheers,

 Sebastien.
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Re: [Caml-list] List.sort

[Remi VANICAT]

Johann Spies <jspies@sun.ac.za> writes:

> I apologise for asking a newbie question on this list, but the
> beginners-list seems to have a problem.  After 24 hours my message
> did not appear on the list and there was actually no activity the last
> few days.

well, you should have wait longer, I already answer you there.



-- 
Rémi Vanicat
vanicat@labri.u-bordeaux.fr
http://dept-info.labri.u-bordeaux.fr/~vanicat
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Re: [Caml-list] List.sort

[Alessandro Baretta]

Johann Spies wrote:
> 
> According to the manual the Persvasives.compare function is suitable
> to use with List.sort.  I can't figure out how to use it:
> ----------------------------
> # k;;
> - : int list = [3; 2; 1]
> # List.sort (compare 1 2) k;;
> This expression has type int but is here used with type 'a -> 'a ->
> int  ...

         Objective Caml version 3.04+13 (2002-06-05)

# let k = [ 3; 2 ; 1];;
val k : int list = [3; 2; 1]
# List.sort compare k;;
- : int list = [1; 2; 3]


Does this solve your problem? You have to realize that 
functions (like Pervasives.compare) are first class citizens 
in O'Caml. That is, you can pass a function like List.sort 
*another function* as a actual parameter.

This is a fairly elementary question. You should try to 
avoid posting questions like this on this list; however, I 
am more than willing to lend a hand, insofar as possible, so 
you can write to me personally, but please mention "Caml" in 
the subject.

Alex


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Re: [Caml-list] List.sort

[Johann Spies]

On Wed, Jun 19, 2002 at 07:27:10PM +0200, Alessandro Baretta wrote:
> This is a fairly elementary question. You should try to 
> avoid posting questions like this on this list; however, I 
> am more than willing to lend a hand, insofar as possible, so 
> you can write to me personally, but please mention "Caml" in 
> the subject.

Thanks for all the answers. 

Johann

-- 
Johann Spies          Telefoon: 021-808 4036
Informasietegnologie, Universiteit van Stellenbosch

     "There is therefore now no condemnation to them which 
      are in Christ Jesus, who walk not after the flesh, but
      after the Spirit."              Romans 8:1 
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