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From: Bill.Viggers@comp.vuw.ac.nz (Bill Viggers)
Newsgroups: comp.lang.prolog
Subject: Re: simple question
Message-ID: <1991Mar20.225013.21372@comp.vuw.ac.nz>
Date: 20 Mar 91 22:50:13 GMT
References: <WAYNE.91Mar19112927@offa.cs.odu.edu>
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	For starters its easiest if you give Prolog functions, names that
	describe what they do.  Personally I also dislike
	have two functions with the same name and different arity.

	Thus your program could be re-written as:

writeln(S):-
	write(S),
	nl.

makelist:-
	writeln('Enter the verticies'),
	getlist(List),
	write(List).

getlist(L):-
	read(Item),
	addtolist(Item,List).

addtolist(end,_):-!.

addtolist(Item,List):- 
	List=[Item|List],
	writeln('Another one...'),
	getlist(List).


Now it is clearer what is happening.
	The first time though an item is got, and the list is set equal to it.
	The second time though, another item is got, and then the line:

	List=[Item|List],

	is executed, and it will fail as List is not equal to the new item 
	followed by the previous item.

	An example:
		| ?-makelist.
		Enter the verticies
		|: 3.
		Another one...
		|: 4.
		
		no

	We can se getlist is called, it reads 3, and calls addtolist
	addtolist sets List equal to [3], then sends that list on to another
	call to getlist.  This time getlist reads a 4, and sends that, along
	with the current list to addtolist.  Addtolist trys to set
	[3] = [4|3]. 
	Which it fails to do.

	A solution then could be:


writeln(String):-
	write(String),
	nl.

makelist:-
	writeln('Enter the verticies'),
	getlist(List),
	write(List).

getlist(Thelist):-
	read(Item),
	addtolist(Item,_,Thelist).

addtolist(end,_,end):-!.

addtolist(Item,List,Thelist):- 
	Thelist=[Item|List],
	writeln('Another one...'),
	getlist(List).


	This returns a list, with the last item being an 'end'
	eg:


	| ?- makelist.
	Enter the verticies
	|: bill.
	Another one...
	|: viggers.
	Another one...
	|: end.
	[bill,viggers|end]
	yes


	I hope this helps.
 
B.