CODEWITHSUNDEEP
prolog
user3470812
user3470812

Reputation: 13

Quick PROLOG enquiry

I have predicates as follows 

dur(a,1).
dur(b,2).
dur(c,3).
dur(d,4).

Say I want to add the elements of a list [a,b,c] so sum = 6. How do I access the value associated with the variable?

Because something like this 

len([],0).
len([H|T], Sum) :-
   len(T, Rest),
   Sum is H + Rest.

doesn't work, it works fine for [1,2,3] but not at all for [a,b,c] which makes sense, but I don't have a clue how to make it work for list len([a,b,c],Sum).

Upvotes: 1

Views: 71

Answers (1)

CapelliC
CapelliC

Reputation: 60034

How do I access the value associated with the variable?

You must 'join' deep in the loop, where the actual computing take place

len([],0).
len([H|T], Sum) :-
   dur(H, V),  % 'hardcoded' join
   Sum is V + Rest.
   len(T, Rest),

now the true problem is apparent.

Since a join is such a basic operation in Prolog - really, it does very little otherwise -it's difficult to write - and so reuse - algorithms truly independent from the naming of the data.

Usually 'second order' programming can help, massaging the data to adapt to more generic context. Like

len(Keys, Len) :- maplist(dur, Keys, Nums), sum_list(Nums, Len).

SWI-Prolog autoloads maplist/3 from library(apply), some other Prolog could require you to explicitly load it...

Upvotes: 1

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