CODEWITHSUNDEEP
prolog
NLSteveO
NLSteveO

Reputation: 48

Having trouble with a prolog recursion program

So the problem given is as follows

Write a program printfigure() to print out the corresponding part of the following figure for input capital character given by a user. Test your program.

For example, when an input is C the print out will be:

Input a capital character: C The corresponding figure is as the follows: A BB CCC

Anyways, I feel like I have mostly figured this out but the code keeps printing past the given letter. I think it's a problem with my recursion and the base case being skipped. Here is my code so far:

 alphabet(['A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z']).

printfigure :- write('Input a capitol character: '),get0(L),
    write('The corresponding figure is as follows: '),nl,
    alphabet(A),member(L,A,0).

member(X,[X|_],S) :- X =:= X, C1 is S+1, abc(X,C1).
member(X,[H|T],S) :- C1 is S+1, abc(H,C1), member(X,T,C1).

abc(_,0) :- nl.
abc(Q,N) :- write(Q), N1 is N-1, abc(Q,N1).

Any help is great, i'm just hoping to understand what is going wrong.

Upvotes: 1

Views: 121

Answers (1)

CapelliC
CapelliC

Reputation: 60004

A way to debug (in any language) it's to separate functionally independent parts.

printfigure :- write('Input a capitol character: '),get0(L),
    write('The corresponding figure is as follows: '),nl,
    printfigure(L).
printfigure(L) :-
    alphabet(A),member(L,A,0).

and test with the input that (I presume) you think will be fed to your code by get0/1

1 ?- printfigure('C').
A
BB
ERROR: =:=/2: Arithmetic: `'C'/0' is not a function
   Exception: (9) so:member('C', ['C', 'D', 'E', 'F', 'G', 'H', 'I'|...], 2) ? aabort

you can see that

  • when you're matching elements from alphabet/1, actually you're using them inappropriately.
  • but you're not matching them: since get0/1 returns a character code (a number), you should change your alphabet to alphabet([0'A,0'B,0'C...]). or use a different input predicate, like read/1, or convert the character code to an atom (with atom_codes(A, [C]))

then correct the other error...

Upvotes: 1

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