CODEWITHSUNDEEP
prolog
NickName
NickName

Reputation: 313

Replace a list in prolog

I want to add a element to a list and replace it with my list.

So i wrote

initialstate(0,[],[[1,0],[2,3],[1,2],[1,3]]).
add(X,L2,[X|L2]).

I would like to make something like that... 

?- initialstate(_,List,_),add(4,List,List)

and replace me the initialstate to this one

initial_state(0,[4],[[1,0],[2,3],[1,2],[1,3]]).

The maining question for me it's how to replace the list inside the indetifiers "initialstate".

I am new in prolog...Please help me... and sorry if it is something stupid.

Upvotes: 0

Views: 147

Answers (2)

CapelliC
CapelliC

Reputation: 60014

once provided the appropriate declaration

:- dynamic initialstate/3.

you could update your DB in this way

?- retract(initialstate(A,B,C)), assertz(initialstate(A,[4|B],C)).

Upvotes: 1

User
User

Reputation: 14873

If you want to change a list you get a different list. So you need a different name for it.

Prolog is not an imperative language but more a descriptive one. This means: you do not say do this and this. You say this is like that and when this is the case this is true/false in files (knowledge). Then you start the interpreter, consult the knowledge and ask questions wether something is true.

Writing A = B means that A is always B. 

?- initialstate(_,List,_),add(4,List,List)

means initialstate of a List is when 4 added to the list is the list itself. This is not true. if 4 is added there should be a 4 where no 4 was before.

You can do this:

append4(List, ListWithFourInside) :- append(List, [4], ListWithFourInside).

Which reads like when I append 4 to a list I get a list with 4 inside.

It can be done shorter if you want the 4 in the beginning

add4(List, ListWithFourInside) :- [4 | List] = ListWithFourInside.

or even shorter:

add4(List, [4|List]).

Upvotes: 0

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