CODEWITHSUNDEEP
prolog
user3836982
user3836982

Reputation: 148

Prolog add function

I am a newbie in Prolog. I have this facts:

user(alice). 
user(bob).
money(alice,10).
money(bob,20).

The facts means that alice have 10 dollars and bob 20 dollars. In order to learn Prolog, i want to develop a function to transfer money between two users. My function should take in input the name of the receiver and the amount and add the quantity to the receiver. 

example: add(alice,20)

and the output should be 30. Actually my function is:

add(X,Y,Z) :- Z is money(X,M) + Y.

but it doesn't work.

How i can implement it? Thanks

Upvotes: 1

Views: 77

Answers (1)

Paulo Moura
Paulo Moura

Reputation: 18663

First, as you want to modify the facts for the predicate money/2 as the users exchange money, you need to declare the predicate dynamic:

:- dynamic(money/2).

Second, as Prolog is a relational language, not a functional language, you cannot write expressions as Z is money(X,M). Instead you need to write something like:

add(User, Money) :-
    % remove old clause and access how much money the user holds
    retract(money(User, Current)),
    % compute the updated amount of money the user will be holding
    Updated is Current + Money,
    % add updated fact
    assertz(money(User, Updated).

Upvotes: 1

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