prolog dynamic predicates difference between /1 and /2

Question

I am a beginner in the Prolog language and I am in a situation where dynamic/1 is being used. In my code there is a line, for example that goes like this:

:- dynamic actual_position/1, at/2, holding/1, talked/1, examined/1.

I want to understand why actual_position has a /1 as compared to at which has a /2.

Thank you!

Answer 1

The number after the slash (/) is the arity of the predicate: it is the number of parameters it takes. So member/2 means a member predicate or functor with two parameters.

The arity is important since, just like in Java for example, one can overload predicate names: one can define multiple predicates with the same name but a different arity. For example append/2 [swi-doc] concatenates a list of lists to a single list, whereas append/3 [swi-doc] appends two lists together in a single list.