How compute Index of element in a list?

Question

I am starting to play with prolog, and with a Java background it's really difficult for me so here is a silly question:

How will you write an indexOf predicate able to give the index of a given element in a given list ?

My first question is about the predicate arity: I guess it should be 3 such as:

indexOf(List,Element, Index) :- ......

Am I right ? May be this already exists in built-in libraries but I want to learn how to write it. Thanks for your help.

Answer 1

You can do it recursively: Suppose 0-based index (otherwise just change the 0 with 1 on the first clause)

indexOf([Element|_], Element, 0). % We found the element
indexOf([_|Tail], Element, Index):-
  indexOf(Tail, Element, Index1), % Check in the tail of the list
  Index is Index1+1.  % and increment the resulting index

If you want only to find the first appearance, you could add a cut (!) to avoid backtracking.

indexOf([Element|_], Element, 0):- !.
indexOf([_|Tail], Element, Index):-
  indexOf(Tail, Element, Index1),
  !,
  Index is Index1+1.