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From: ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe)
Newsgroups: comp.lang.prolog
Subject: Re: Simple Prolog Question
Message-ID: <3542@goanna.cs.rmit.oz.au>
Date: 10 Aug 90 08:18:42 GMT
References: <2157@cernvax.UUCP>
Organization: Comp Sci, RMIT, Melbourne, Australia
Lines: 50

In article <2157@cernvax.UUCP>, julian@cernvax.UUCP (julian bunn) writes:
> I am trying to write a program that will remove leading blank
> characters from a list.

You're clearly not using the QP library, because it's all there...

> remove_blanks([H|T],T) :- H=32, remove_blanks(T,_).
> remove_blanks(S,S).

(a) Let's look at that first clause, and let's consider the case
    where you called remove_blanks("  sargon", Ans).

    This unifies with the head of the first clause, giving
	H = (0'  ) = 32
	T = " sargon"
	Ans = T = " sargon"
    the body becomes
	32 = 32, remove_blanks(" sargon", _dont_dare_tell_me_this_answer_)
    the first goal of which succeeds, and then your recursive call says
	'discard leading blanks from " sargon" AND THEN THROW AWAY THE
	ANSWER'

    So you want your first clause to be

	remove_blanks([0' |T], Answer) :- remove_blanks(T, Answer).

(b) The second clause will _also_ succeed in this case, with
	S = "  sargon"
	Answer = S = "  sargon".
    You presumably don't want this to happen.

A pure way of doing it is

	remove_leading_blanks([],    []).
	remove_leading_blanks([H|T], [H|T]) :- H =\= " ".
	remove_leading_blanks([H|T], R    ) :- H =:= " ",
		remove_leading_blanks(T, R).

If you don't mind a red cut, you can do

	remove_leading_blanks([32|T], R) :- !,
		remove_leading_blanks(T, R).
	remove_leading_blanks(T, R).

(I changed the name to remove_leading_blanks/2 because it doesn't remove
                              ^^^^^^^
_all_ blanks as the name suggests; trailing blanks and consecutive
internal blanks are left alone.)
-- 
Taphonomy begins at death.