List comprehensions

List comprehensions

[Jon Harrop]

I didn't know OCaml had list comprehensions built-in:

$ ocaml camlp4oof.cma
        Objective Caml version 3.10.0

        Camlp4 Parsing version 3.10.0

# [x + 2*x + x/2 | x <- [1; 2; 3; 4]];;
- : int list = [3; 7; 10; 14]
#

Cool! :-)

-- 
Dr Jon D Harrop, Flying Frog Consultancy Ltd.
OCaml for Scientists
http://www.ffconsultancy.com/products/ocaml_for_scientists/?e

Re: [Caml-list] List comprehensions

[Joel Reymont]

On Aug 18, 2007, at 3:28 PM, Jon Harrop wrote:

> # [x + 2*x + x/2 | x <- [1; 2; 3; 4]];;
> - : int list = [3; 7; 10; 14]

How do you do something like this?

[x + 2*x + y/2 | x <- [1; 2; 3; 4], [y <- [3; 4; 5]];;

That is a list comprehension with more than one variable where all  
variables come from lists.

	Thanks, Joel

--
http://wagerlabs.com

Re: [Caml-list] List comprehensions

[Jon Harrop]

On Saturday 18 August 2007 15:44:04 Joel wrote:
> On Aug 18, 2007, at 3:28 PM, Jon Harrop wrote:
> > # [x + 2*x + x/2 | x <- [1; 2; 3; 4]];;
> > - : int list = [3; 7; 10; 14]
>
> How do you do something like this?
>
> [x + 2*x + y/2 | x <- [1; 2; 3; 4], [y <- [3; 4; 5]];;
>
> That is a list comprehension with more than one variable where all
> variables come from lists.

Ooh, look:

# let rec sort = function
    | [] -> []
    | x::xs -> sort [y | y <- xs; y<x] @ [x] @ sort [y | y <- xs; y>=x];;
val sort : 'a list -> 'a list = <fun>
# sort [3;1;6;2;7;4;9;8];;
- : int list = [1; 2; 3; 4; 6; 7; 8; 9]

Hmm, maybe we could form some sort of rudimentary lathe...

-- 
Dr Jon D Harrop, Flying Frog Consultancy Ltd.
OCaml for Scientists
http://www.ffconsultancy.com/products/ocaml_for_scientists/?e

Re: [Caml-list] List comprehensions

[Nicolas Pouillard]

Excerpts from Joel Reymont's message of Sat Aug 18 16:44:04 +0200 2007:
> 
> On Aug 18, 2007, at 3:28 PM, Jon Harrop wrote:
> 
> > # [x + 2*x + x/2 | x <- [1; 2; 3; 4]];;
> > - : int list = [3; 7; 10; 14]
> 
> How do you do something like this?
> 
> [x + 2*x + y/2 | x <- [1; 2; 3; 4], [y <- [3; 4; 5]];;
> 
> That is a list comprehension with more than one variable where all  
> variables come from lists.
> 

Use `;' to separate generators. And you have an extra `['.

[x + 2*x + y/2 | x <- [1; 2; 3; 4]; y <- [3; 4; 5]];;

-- 
Nicolas Pouillard aka Ertai

Re: [Caml-list] List comprehensions

[skaller]

On Sat, 2007-08-18 at 15:28 +0100, Jon Harrop wrote:
> I didn't know OCaml had list comprehensions built-in:
> 
> $ ocaml camlp4oof.cma
>         Objective Caml version 3.10.0
> 
>         Camlp4 Parsing version 3.10.0
> 
> # [x + 2*x + x/2 | x <- [1; 2; 3; 4]];;
> - : int list = [3; 7; 10; 14]
> #
> 
> Cool! :-)

Since you're fishing, I'm biting -- what bait do you
need to catch a camlp4oof?


-- 
John Skaller <skaller at users dot sf dot net>
Felix, successor to C++: http://felix.sf.net

Re: [Caml-list] List comprehensions

[Nathaniel Gray]

On 8/18/07, skaller <skaller@users.sourceforge.net> wrote:
> On Sat, 2007-08-18 at 15:28 +0100, Jon Harrop wrote:
> > I didn't know OCaml had list comprehensions built-in:
> >
> > $ ocaml camlp4oof.cma
> >         Objective Caml version 3.10.0
> >
> >         Camlp4 Parsing version 3.10.0
> >
> > # [x + 2*x + x/2 | x <- [1; 2; 3; 4]];;
> > - : int list = [3; 7; 10; 14]
> > #
> >
> > Cool! :-)
>
> Since you're fishing, I'm biting -- what bait do you
> need to catch a camlp4oof?

I guess just 0.3.10.

[n8gray@golux]% locate camlp4oof
/usr/local/godi/bin/camlp4oof
/usr/local/godi/bin/camlp4oof.opt
/usr/local/godi/lib/ocaml/std-lib/camlp4/camlp4oof.cma

[n8gray@golux]% rlwrap ocaml camlp4oof.cma
        Objective Caml version 3.10.0

        Camlp4 Parsing version 3.10.0

# [x + 2*x + x/2 | x <- [1; 2; 3; 4]];;
- : int list = [3; 7; 10; 14]
# [x + 2*x + x/2 - y | x,y <- [1,2; 2,3; 3,4; 4,5]];;
- : int list = [1; 4; 6; 9]

-- 
>>>-- Nathaniel Gray -- Caltech Computer Science ------>
>>>-- Mojave Project -- http://mojave.cs.caltech.edu -->

Re: [Caml-list] List comprehensions

[Adrien]

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I just tried this to avoid a big block of ugly code but it doesn't work with
3.09:

# [x + 2*x + x/2 | x <- [1; 2; 3; 4]];;
Characters 15-16:
  [x + 2*x + x/2 | x <- [1; 2; 3; 4]];;
                       ^
Syntax error


So it's a very recent addition, no surprise you didn't know had this. ;)



---

Adrien Nader

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