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Fábio Pires
Fábio Pires

Reputation: 109

Prolog-Sum positive numbers from a List

I have the following Prolog fact to sum the positive numbers from a list but it´s not working. What am I doing wrong?

somarPositivos([],0).
somarPositivos([H|T],Soma):-H>0,somarPositivos(T,Soma1),Soma is Soma1+H.
somarPositivos([H|T],Soma):-H=<0 ,somarPositivos(T,Soma1).

Upvotes: 0

Views: 618

Answers (1)

Paulo Moura
Paulo Moura

Reputation: 18663

You have a typo in the third clause. It should be:

somarPositivos([H|T],Soma):-H=<0 ,somarPositivos(T,Soma).

Most Prolog compilers would print a warning that the variables Soma and Soma1 are singleton variables in that clause.

Some notes on programming style following Prolog coding guidelines. Indent your code. Use spaces around operators. Use underscores instead of camelCase for predicate names for readbility. Thus, preferably write:

somar_positivos([],0).
somar_positivos([H|T],Soma) :-
    H > 0,
    somar_positivos(T,Soma1),
    Soma is Soma1+H.
somar_positivos([H|T],Soma) :-
    H =< 0,
    somar_positivos(T,Soma).

As an improvement to this initial version of the predicate, can you rewrite it to make it tail-recursive using an accumulator?

Upvotes: 2

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