Prolog question

Question

Let's assume the result is the third parameter.

a (1, [Hd | Tl], Hd). a (N, [| Tl], Elem): - N> 1, N1 is N - 1, a (N1, Tl, Elem).

I am trying to figure out what this does.

+2

Dan May 03 '10 at 21:05

1 answer

sepp2k · Answer 1 · 2010-05-03T21:43:23+0000

It gives the Nth element of the list. You can read the definition like this:

a( 1, [Hd | Tl], Hd ).

Hd is the first item in the list [Hd | Tl]

, that is, a list starting with Hd followed by a Tl list.

a( N, [ | Tl], Elem ) :- N > 1, N1 is N - 1, a( N1, Tl, Elem).

Elem is the Nth element of the list if and only if it is the N1

th element of its tail, where N1

- N-1

.