How to call a function in foreach with local variables as arguments? - prolog

Question

I have a knapsack problem to solve in B-Prolog. I have to write something like:
knapsack(X, Indexes, Quantity, Weights, Values, Capacity)
X - array with number of ith item that is put in the bag
Indexes - [0, ..., NumOfItems-1]
Quantity - array with number of available ith items
Weights - array with weight of each item on ith position (only 1 item)
Value - array with value of each item (only 1 item)
Capacity - limit for bag capacity
The task must be solved using a predicate:
get([X|_], 0, X).
get([_|X], I, Y) :- I>0, I1 is I-1, get(X, I1, Y).
which returns the element on ith index, considering they can start at 0. Then Y has the value of that element.
Also, we must maximize the item values. I tried to make it work not only for fixed number of items.
Since I'm just a beginer for Prolog, I had an idea, but it doesn't wort of course.
knapsack(X, Indexes, Quantity, Weights, Values, Capacity) :-
/*
Find the last index of items,
Calculate NumOfItems as last index+1,
Make an array X with NumOfItems elements,
Define domen for each item (ith element in X) as 0..NumOfThatItem
(meaning I can take zero or more up to the max number of that element that is avaliable)
*/
last(Indexes, Elem),
NumOfItems is Elem+1,
length(X, NumOfItems),
foreach(I in 1..NumOfItems, get(Quantity, I, K), X[I]::0..K),
/*
Set up the constraint that sum of item weights must not be bigger than bag capacity
*/
sum([X[I]*T : I in 1..NumOfItems], get(Weights, I, T)) #=< Capacity,
/*
Maximize the values of items in the bag, and find all possible combinations for it
*/
labeling([maximize( sum([X[I]*V : I in 1..NumOfItems, get(Values, I, V)]))], X),
/*
This is the part of a task, to write out the Profit we made by taking those items,
and the overall weight that we have put in the bag.
*/
Profit is sum([X[I]*V : I in 1..NumOfItems, get(Values, I, V)]),
Weight is sum([X[I]*T : I in 1..NumOfItems, get(Weights, I, T)]),
nl,
write('Profit: '), write(Profit), nl,
write('Weight: '), write(Weight), nl
.
I am using B-Prolog Version 8.1, it can be downloaded on this link(<- click)
You can copy my code and place it in the BProlog folder on the location where you chose to install it.
When you open/start bp aplication:
cl('path_to_my_code.pro').
Exaple I have for this problem is the following:
knapsack(X, [0,1,2,3], [1,1,1,4], [50,10,5,1], [1000,2000,100,1], 63).
And that should give us:
Profit: 3003
Weight: 63
X = [1,1,0,3]
I get the following:
***illegal_local_variables(get([1,1,1,4], _f8, _fc))
I concluded that he doesn't recognize I as a number.
If you have any book or article or whatever related to this please share.
How should this be done? Please help...
Thank you for your time

Answer 1

At this point since you get the error you should test get/3 predicate to see if this working properly. The problem is in the part:
I>0, I1 is I-1, get(X, I1, Y).
Since you call get with I as a variable, I>0 yields instantiation error, instead you can write:
get([X|_], 0, X).
get([_|X], I, Y) :- get(X, I1, Y), I is I1+1, I>0.