Assignment 2 Question 2(c)

Hi all

I don't know if I'm just being dumb, but I'm not understanding question 2(c):

"Write a procedure satisfy(P,L) that receives a binary predicate symbol P and returns the list L of all terms X for which P(X,Y) succeeds for some Y."

Please can someone explain what is actually being asked here?

Thanks
Paulo

First of I think the question might be stated slightly incorrectly. Also, this is my interpretation of what is being asked so don't complain if it turns out to be wrong.

So image you have the following facts:

fact(x1, y1).
fact(x2, y2).
fact(x3, y3).
fact(x4, y4). % fact(xn, yn) etc.

You must write a procedure:

satisfy(fact, L)

that returns:

L = [x1, x2, x3, x4]

Now, the question says "for some Y" and not "for all Y" (meaning it does not care what the value of Y is), which the above does. This means that you need to specify the Y part, but I don't see how you can do this with satisfy(fact, L). I think you will need something like satisfy(fact, Y, L) to specify the Y part.

My assumption might be completely wrong as this is the first time I have used Prolog.

Thanks MotaBoy

I hope you're right. I won't hold it against you if you're not because I have no clue myself

Cheers
Paulo

Here is a slighly better example of what I think "for some Y" means:

for all Y:

fact(green, dog).
fact(blue, cat).
fact(red, dog).
fact(yellow, cat).
fact(green, cat).

satisfy(fact, L).

Result:

L = [green, blue, red, yellow, green]



...and for some Y:

satisfy(fact, cat, L).

Result:

L = [blue, yellow, green]

That does make sense "for some Y", but as you said, it requires an extra variable which the original procedure structure "satisfy(P,L)" does not cater for.
If the question used a real life example, I think it would have been easier to figure out what is actually being asked.

It might be that there is some other way to pass the Y variable to the precedure using just "satisfy(P,L)", but I have come up with a solution yet.

I think I have found the set of predicates to use in the textbook.
Just need to figure out which one would be best suited to answering the question.

I managed to get two versions ( satisfy(P, L) & satisfy(P,Y, L) )working with the stuff from (I think) chapter 6 (built-in predicates?). I'm still not sure about this whole "for some Y" business. Any ideas so far DJ_Paulo?