prolog sum list using atomic

Question

I am trying to perform a sumList function in prolog that takes the input: sumList([1,[2,3],[4],5],X). and returns X=15.

I understand how to sum a simple list and a list of lists(ie [1,2,3,4,5] & [[1,2,3],[4,5]]) but i am getting errors because i am not properly handing the multiple inner lists(because i do not yet know how to, i havent encountered this before), i was told i have to use atom or atomic by a college somehow but i haven't seen a clear example on how this is done. any sml code i could translate or prolog code that could help me sum the list [1,[2,3],[4],5] would be greatly appericated. thanks!

Eugene Sh. · Accepted Answer · 2014-12-04T22:32:57.963

0

In SWI Prolog (just not sure about ANSI) the atomic(X) predicate can tell you whether the X is list or a number (in your case). It can be used write the sum predicate by recursively calling itself when a list is encountered, or just add the element to a sum if a simple number:

sum([], 0).
sum([H|T], S) :-
    atomic(H),
    sum(T, S1),
    S is H + S1.
sum([H|T], S) :- 
    sum(H, S1),
    sum(T, S2),
    S is S1 + S2.

edited Dec 04 '14 at 22:32

answered Dec 04 '14 at 22:22

Eugene Sh.

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thanks so much! that is amazing i have been struggling trying to find any decent examples of atom or atomic(more then their general definitions) for half the afternoon. Yes i am working in SMI prolog. – Laura Dec 04 '14 at 22:38
i do not have the reputation to upvote @Eugene but i did accept the answer, i really appreciate the help in understanding atomic and the example. – Laura Dec 08 '14 at 18:33

score 0 · Answer 2 · answered Jul 11 '21 at 22:29

Prolog treats an [] as atomic. The implementation from Eugene Sh. doesn't handle the case when H is an empty list []. Checking if H is an empty list in the second clause solves the issue.

sum([], 0).
sum([H|T], S) :-
    atomic(H),
    H\==[], % fail incase H is an empty list 
    sum(T, S1),
    S is H + S1.
sum([H|T], S) :- 
    sum(H, S1),
    sum(T, S2),
    S is S1 + S2.

prolog sum list using atomic

2 Answers2