Padding the front of a list with N items

[WarwickB]

Hi,
I am trying to refine an algorithm that otherwise depends on too many calls to list reverse predicate.

I am trying to define a predicate that takes a list and an integer as arguments, and prepends integer number of items at the front of the list. In this case the items are zeros and the list is a list of integers.

make_pad(Zs, N):- N1 is N-1,
make_pad([0|Zs], N1).
make_pad([_], 0).

Is what I have, but it seems to be nesting the list ad infinitum till the list runs out of stack.

As a less important aside, I am using WinXP and SWI Prolog and I can't seem to get the spy function to work.

Any ideas on either problem would be greatly appreciated.

Best
Warwick

 

[WarwickB]

make_pad(Zs, N):- N > 0,
N1 is N-1,
make_pad([0|Zs], N1).


missing clause (N>0), it works now.
Silly Me.

Nevertheless I am getting an awful lot of insufficiently instantiated arguments I would just LOVE to know at which point in the code its occuring.

Any assistance with debug trace, spy still very much needed.

Thanks
Warwick