Prolog中的总和计算

Question

Say I have two 'tables' in Prolog, eg:

item_value('Chair', 50).          
item_value('Table', 100).        
item_value('Plant', 75).

And another one:

shopping_cart('Max', ['Chair', 'Chair', 'Table']).            
shopping_cart('Sam', ['Plant', 'Table']).

I now want to write a predicate that calculates the sum of the items inside the shopping cart, something like total_sum(Person, Sum)

How would i do this? I can't wrap my head around coding this in Prolog.
Thanks in advance!

Answer 1

Try the following code:

item_value('Chair', 50).          
item_value('Table', 100).        
item_value('Plant', 75).

shopping_cart('Max', ['Chair', 'Chair', 'Table']).            
shopping_cart('Sam', ['Plant', 'Table']).

total_sum(Person, Sum) :-
    shopping_cart(Person, ItemList),
    maplist(item_value, ItemList, CostList),
    foldl(plus, CostList, 0, Sum).