CODEWITHSUNDEEP
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Greg
Greg

Reputation: 35

How to assign variables in a recursion?

Suppose you have a bunch of boxes inside other boxes, i.e.

inBox(a,b).
inBox(b,c).
inBox(c,d).

inBox(x,y) meansbox x is in box y. Now I want to write a method to determine if one box is inside of another box, e.g. insideBox(a,d) = true since a is inside b, which is inside c, which is inside d. So it appears to be a naturally recursive problem and my attempt at solving it fails to initialize my variable to "move one box up":

insideBox(X, Y) :- inBox(X, Y), inBox(X, Z), insideBox(Z,Y).

The logic is to first check the base case of whether or not box X is currently inside box Y, if not then call inBox(X, Z) to get a value for which box X is in and then recall the recursive method. I have tried looking at the trace but can't reason what is going on:

| ?- insideBox(a,d).
  1    1  Call: insideBox(a,d) ? c
  2    2  Call: inBox(a,d) ? c
  2    2  Fail: inBox(a,d) ? c
  1    1  Fail: insideBox(a,d) ? c

I've tried other stuff like Z is inBox(X, _) for setting the variable Z but this hasn't worked either and I have no clue how Z is ending up with the value of a.

Upvotes: 1

Views: 103

Answers (1)

Enigmativity
Enigmativity

Reputation: 117037

Try this:

?- insideBox(a,d).

inBox(a,b).
inBox(b,c).
inBox(c,d).

insideBox(X,Z) :- inBox(X,Z).
insideBox(X,Z) :- inBox(X,Y), insideBox(Y,Z).

Just a side-note: be careful with the terms you use. You don't "assign" variables, instead you "unify" them. Also you're not writing "methods", you're writing "predicates".

Upvotes: 2

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