🚀 go-pugleaf

zyang@ccu.umanitoba.ca (Z. Yang) writes:

>Hi, netters,

>I am new in prolog and have some difficulty in list processing. What I want to
>do is write a predicate, ie. called 'subsum', which does following,

>?- subsum([L1,L2,...],NewList).
>NewList = [Sum_of_L1,Sum_of_L2,...]

>where L1, L2 etc. are lists themselves(could be nested).

>Any pointers will be appreciated.
>Thanks in advance to all kind souls.

>-zgy

I kind of figured it out(see following) by calling some pre-defined
predicates. Still not satisfied, though. Can it be done in a simplier way??
Or, is there a written predicate out there doing the same thing?

Any better ideas appreciated.
Thanks.

-zgy

--- predicate 'subsum' begins -----
% consider a list of lists LL = [L1,L2,...]. obtain the list of 
% individual listsums, ie. LLS = [L1sum,L2sum,...]

subsum([],[]).				% boundary condition
subsum([L1|Rest],LLS):-
	flatten(L1,FlatL1),		% flatten L1
	listsum(FlatL1,L1sum),		% sum of L1: L1sum
	subsum(Rest,LLS1),		% sums of L2, L3, ...
	add(L1sum,LLS1,LLS).		% add L1sum in front of [L2sum,...]

flatten([Head|Tail],FlatList):-		% flatten a list
        flatten(Head,FlatHead),
        flatten(Tail,FlatTail),
        concat(FlatHead,FlatTail,FlatList),
        !.
flatten([],[]).
flatten(X,[X]).

concat([],L,L).				% concatenate two lists into one
concat([X|L1],L2,[X|L3]):-
        concat(L1,L2,L3).

listsum([],0).				% sum of all entries in a list
listsum([H|T],Sum):-
        listsum(T,Sum1),
        Sum is H + Sum1.

add(X,L,[X|L]).				% add an element in front of list L