CODEWITHSUNDEEP
prolog
Kate
Kate

Reputation: 165

Sum elements of list in prolog

I want to sum element of a list like this:

sum([1,[2,3],4],S).

I used that but I have a problem:

sum([],0).
sum([T|R],M) :- sum(R,S), M is T+S.

I get the following error:

ERROR: is/2: Type error: `[]' expected, found `[2,3]' (a list) ("x" must hold one character)

Upvotes: 2

Views: 2811

Answers (3)

coder
coder

Reputation: 12972

The problem is that you can't add T if T is a list. You could easily solve using is_list/1 that succeds if T is a list:

sum([],0).
sum([T|R],M) :- (is_list(T) -> sum(T,N1),sum(R,S), M is N1+S
                 ; sum(R,S), M is T+S ).

Examples:

?- sum([1,[2,3],4],S).
S = 10.

?- sum([1,2,3,4],S).
S = 10.

?- sum([1,[2],[3],4],S).
S = 10.

?- sum([1,[[2],[3]],4],S).
S = 10.

A better approach would be using CLPFD:

:- use_module(library(clpfd)).

sum([],0).
sum([[]],0).
sum([T|R],M) :- (is_list(T) -> sum(T,N1),sum(R,S), M #= N1+S
                 ; sum(R,S), M #= T+S ).

Now you can query more general questions like:

?- sum(L,N).
L = [],
N = 0 ;
L = [[]],
N = 0 ;
L = [N],
N in inf..sup ;
L = [N, []],
N in inf..sup ;
L = [_1836, _1842],
_1836+_1842#=N ;
L = [_1842, _1848, []],
_1842+_1848#=N ;
L = [_2142, _2148, _2154],
_2142+_2192#=N,
_2148+_2154#=_2192 ;
L = [_2148, _2154, _2160, []],
_2148+_2204#=N,
_2154+_2160#=_2204 ;
L = [_2448, _2454, _2460, _2466],
_2448+_2504#=N,
_2454+_2528#=_2504,
_2460+_2466#=_2528 ;

and goes on...

Upvotes: 2

code_x386
code_x386

Reputation: 798

You can use flatten/2 plus foldl/4 or sum_list/2 library predicates in Swi-Prolog:

% flatten/2 + foldl/4 based implementation
sum_nested_list(List, Sum) :-
    flatten(List, Flat),
    foldl(plus, Flat, 0, Sum).

% flatten/2 + sum_list/2 based implementation
sum_nested_list(List, Sum) :-
    flatten(List, Flat),
    sum_list(Flat, Sum).

Upvotes: 0

Kirill Bulygin
Kirill Bulygin

Reputation: 3826

Another approach:

% Old code, with vars renamed:
%sum([], 0).
%sum([Num|Tail], TotalSum) :- sum(Tail, TailSum), TotalSum is Num + TailSum.

% New code:
sum([], 0).
sum([Elem|Tail], TotalSum) :- 
    sum(Elem, ElemSum),
    sum(Tail, TailSum),
    TotalSum is ElemSum + TailSum.
sum(Num, Num).

Demo: http://swish.swi-prolog.org/p/cmzcsXrJ.pl.

Upvotes: 1

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