keep getting the Singleton variable error

Question

i am a beginner and use swi-Prolog. Can someone tell me why this piece of code doesn't work?

inp:- write('Enter the string'),nl,read(X),write(X).
abc:- subtract(X,['at','in','to','of'],L),write(L).

I keep getting the Singleton Variable[X] error. Thanks.

Answer 1

X in inp predicate and X in abc predicate - are totally different variables, not connected in any way.

You probably want something like this:

inp(X) :- write('Enter the string'),nl,read(X),write(X).
abc(X) :- subtract(X,['at','in','to','of'],L),write(L).

And then use it like inp(X), abc(X).