Hobbled. Solving Prolog Puzzles.

Ovid on 2005-05-16T06:31:41

Being laid up with a bad foot, I decided to work through more of the Amzi! Prolog tutorial. I really like the tutorial as it's clearly written and show how things operate under the hood. I read the lists chapter to see if I could learn techniques to better explain lists. Unfortunately, they are something that you'll either get or your won't. However, working through each of the exercises is like solving a bunch of mini-logic puzzles. I like that.

Here's my stab at various questions (they can be harder than they look, but as is often the case, a simpler answer is more likely to be correct).

• Remove a given element from a list

  remove(X,[X|Tail],Tail).
  remove(X,[Head|Tail],[Head|Remainder]) :-
      remove(X,Tail,Remainder).

• Find the element after a given element

  next(X,[X|[Y|_]],Y).
  next(X,[_|Tail],Y) :-
      next(X,Tail,Y).

• Split a list into two lists at a given element (this is really simple, but I made it very complicated at first)

  split(X,List, Before, After) :-
      append(Before, [X|After],List).

• Get the last element of a list (isn

  last([X],X).
  last([_|Tail],X) :- last(Tail,X).

• Count the elements in a list

  length([],0).
  length([_|TAIL],N) :-
      length(TAIL,N1),
      N is N1 + 1.

That last one was tricky because the math must be done last to ensure the N1 variable is instantiated since math is not done logically in Prolog.