Prolog list question

Hello,

I have created a predicate that takes a list and a character, and
returns the elements beginning with that character:

begin_with([H|T],X) :- sub_string(H,0,1,_,X), !, write(H), nl,
begin_with(T,X).
begin_with([H|T],X) :- begin_with(T,X).

Because of the 'write(H)' clause, Prolog just outputs the elements.
How can I save the returned elements in a list like for example:

begin_with([my,name,is,martin],m,List).

and Prolog returns: List = [my,martin]
true.

Thanks for your help!

On Mar 29, 1:55 pm, alley <edwardmus...@gmail.com> wrote:
> Hello,
>
> I have created a predicate that takes a list and a character, and
> returns the elements beginning with that character:
>
> begin_with([H|T],X) :- sub_string(H,0,1,_,X), !, write(H), nl,
> begin_with(T,X).
> begin_with([H|T],X) :- begin_with(T,X).
>
> Because of the 'write(H)' clause, Prolog just outputs the elements.
> How can I save the returned elements in a list like for example:
>
> begin_with([my,name,is,martin],m,List).
>
> and Prolog returns: List = [my,martin]
>                              true.
>
> Thanks for your help!

Think about solving this with a recursive approach,
i.e. tackling a "base" case and reducing the solution
of any longer list to a shorter one:

What result should an empty list produce?

If a list is not empty, what should we do with the
first element that, together with a call to
begin_with/2 on the tail of that list, produces
the desired result?

regards, chip

On Mar 29, 10:18=A0pm, Chip Eastham <hardm...@gmail.com> wrote:
> On Mar 29, 1:55 pm, alley <edwardmus...@gmail.com> wrote:
>
>
>
>
> 
> 
> 
> 
> 
> 
> 
>
> Think about solving this with a recursive approach,
> i.e. tackling a "base" case and reducing the solution
> of any longer list to a shorter one:
>
> What result should an empty list produce?
>
> If a list is not empty, what should we do with the
> first element that, together with a call to
> begin_with/2 on the tail of that list, produces
> the desired result?
>
> regards, chip- Hide quoted text -
>
> - Show quoted text -

I added a base case: begin_with([],X). I've tried several options, but
can't get a correct implementation to save the output in a list...

On Mar 30, 12:00 pm, alley <edwardmus...@gmail.com> wrote:
> On Mar 29, 10:18 pm, Chip Eastham <hardm...@gmail.com> wrote:
>
>
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> 
> 
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> 
> 
> 
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>
> I added a base case: begin_with([],X). I've tried several options, but
> can't get a correct implementation to save the output in a list...

Remember that the version of begin_with which "saves"
output in a list needs three arguments.  If we start
with an empty list, the output would also be an empty
list, right?

Prolog has a feature called "anonymous variable" given
by underscore.  It matches anything, and references to
underscore need not match the same thing.  So I'd say:

begin_with([],_,[]).  /* base case */

Now how about writing a case (or two) which deal with
a nonempty list, treating the head of the list (first
element) and its tail?

regards, chip

On Mar 30, 8:01=A0pm, Chip Eastham <hardm...@gmail.com> wrote:
> On Mar 30, 12:00 pm, alley <edwardmus...@gmail.com> wrote:
>
>
>
>
> 
> 
> 
> 
> 
> 
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> Remember that the version of begin_with which "saves"
> output in a list needs three arguments. =A0If we start
> with an empty list, the output would also be an empty
> list, right?
>
> Prolog has a feature called "anonymous variable" given
> by underscore. =A0It matches anything, and references to
> underscore need not match the same thing. =A0So I'd say:
>
> begin_with([],_,[]). =A0/* base case */
>
> Now how about writing a case (or two) which deal with
> a nonempty list, treating the head of the list (first
> element) and its tail?
>
> regards, chip- Hide quoted text -
>
> - Show quoted text -

First of all thanks for your help.

OK, so if the list is empty, the output should be an empty list:
begin_with([],X,[]).

If the list has a head and a tail, if the head matches the right part,
it should be the head of the new list: begin_with([H|T],X,[H]) :-
sub_string(H,0,1,_,X), !, begin_with(T,X,Y).

But then, when the recursion occurs on the tail, its head overrides
the new list's head. I must be missing something here....

In message
<5c2ce075-01c2-4eac-abc5-4234808514b0@y24g2000hsd.googlegroups.com>,
alley <edwardmuscat@gmail.com> writes
>On Mar 30, 8:01_pm, Chip Eastham <hardm...@gmail.com> wrote: 
>
>First of all thanks for your help.
>
>OK, so if the list is empty, the output should be an empty list:
>begin_with([],X,[]).
>
>If the list has a head and a tail, if the head matches the right part,
>it should be the head of the new list: begin_with([H|T],X,[H]) :-
>sub_string(H,0,1,_,X), !, begin_with(T,X,Y).

You don't need the cut.  And more important, you need to do something
with Y.

>But then, when the recursion occurs on the tail, its head overrides
>the new list's head. I must be missing something here....

It doesn't "override" (or overwrite?) it.  The recursion involves a
whole new invocation of the predicate, with a whole new H, etc.

You also need to decide what happens if  sub_string(H,0,1,_,X)  fails.

Nick
--
Nick Wedd    nick@maproom.co.uk

On Mar 31, 12:23=A0am, Nick Wedd <n...@maproom.co.uk> wrote:
> In message
> <5c2ce075-01c2-4eac-abc5-423480851...@y24g2000hsd.googlegroups.com>,
> alley <edwardmus...@gmail.com> writes
>
>
>
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> 
 
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s. 
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t 
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> You don't need the cut. =A0And more important, you need to do something
> with Y.
> 
>
> It doesn't "override" (or overwrite?) it. =A0The recursion involves a
> whole new invocation of the predicate, with a whole new H, etc.
>
> You also need to decide what happens if =A0sub_string(H,0,1,_,X) =A0fails.=[/color
]

>
> Nick
> --
> Nick Wedd =A0 =A0n...@maproom.co.uk- Hide quoted text -
>
> - Show quoted text -

Thanks guys for your help.