always false

[jonison]

Dear Everyone:

I am a new starter,and make a programme: here are the code:

matchsort([H|T],YList,XList,[N|M]) :-
nth1(Index,YList,H),
write('******'),nl,write(H),nl,
nth1(Index,XList,N),
write(N),nl,write('------'),nl,
matchsort(T,YList,XList,M).

matchsort([],[],[],[]).

run with

"matchsort([17,12,8,5,4],[4,8,12,5,17],[3,9,6,4,9],List)."

the results are:
******
17
9
------
******
12
6
------
******
8
9
------
******
5
4
------
******
4
3
------
false.

No 'List' value....and with false...

With this false problem, I made some changes, the code are changed as:

matchsort([H|T],YList,XList,[N|M]) :-
nth1(Index,YList,H),
write('******'),nl,write(H),nl,
nth1(Index,XList,N),
write(N),nl,write('------'),nl,
matchsort(T,YList,XList,M).

matchsort([],YList,XList,[]).

run this with

"matchsort([17,12,8,5,4],[4,8,12,5,17],[3,9,6,4,9],List)."
get the results:

******
17
9
------
******
12
6
------
******
8
9
------
******
5
4
------
******
4
3
------
*********[9, 6, 9, 4, 3]
true .

here I get the value of List and true, I donot know why I can get this result,especially for the List value, could you give me explainations about these?

Many Thanks for your patience for New starter like me.

 

[joel76]

matchsort([H|T],YList,XList,[N|M]) :-
    nth1(Index,YList,H),
    write('******'),nl,write(H),nl,
    nth1(Index,XList,N),
    write(N),nl,write('------'),nl,
    matchsort(T,YList,XList,M).
        
matchsort([],[],[],[]).

matchsort([],[],[],[]) never happens because YList and XList are always the same, but matchsort([],YList,XList,[]) is correct, you should write matchsort([],_YList,_XList,[]) with '_' in front of XList and YList because these args are not used.


Now, explanation for the result :

matchsort([H|T],YList,XList,[N|M]) :-
    nth1(Index,YList,H),
    write('******'),nl,write(H),nl,
    nth1(Index,XList,N),
    write(N),nl,write('------'),nl,
    matchsort(T,YList,XList,M).

with matchsort([17,12,8,5,4],[4,8,12,5,17],[3,9,6,4,9],List).

At the first call
matchsort([17 | T], [4,8,12,5,17],[3,9,6,4,9], [9 | M])).
because
nth1(Index, [4,8,12,5,17], ,17), --> Index = 5
write('******'),nl,write(H),nl,
nth1(5 , [3,9,6,4,9],N), ==> N = 9
write('******'),nl,write(H),nl,
matchsort([12,8,5,4]],[4,8,12,5,17],[3,9,6,4,9],,M).

and so on ...


To explain the list [9,6,9,4,3]

 

[jonison]

Many thanks for your reply, yes, this is really helpful.
many many thanks.