How to create global variable in prolog

Question

I have a list that I create as follows: ``` tab([(top,left),(top,middle),(top,right),(center,left),(center,middle), (center,right),(bottom,left),(bottom,middle),(bottom,right)]). ``` I wish to create a global variable AllPosition that is a tab. So I did the following: ``` tab(AllPos). ``` Is this right? Then I have to follow problem: I have a function that receives one of the pair in tab. That I wish to remove. So I did this: ``` place(Line, Column, Tab) :- AllPos \== [_,_] /*while AllPos isn't empty - not sur if this is done this way*/ -> (member((Line,Column), AllPos) -> (erase(AllPos, (Line,Column), AllPos)). ``` where `erase(List, Element, NewList)` erases the element Element from List and creates a new list NewList equal to List but without Element. Both functions `member` and `erase` are working. The thing is... As you might have noticed I use `AllPos`everywhere. That's because I want to, I want to modify it so I can use it later (after having removed some elements from it), in…

Accepted Answer

In SWI-Prolog you can use: `b_setval(name, value)` and `b_getval(name, value)`. And in case you don't want the values change back in case of backtracking, you can make them actual global by using: `nb_setval(name, value)` and `nb_getval(name, value)`.

Thus for example if you have a program and you want to check how often it went through a certain path, you can use:

```
recursive(100).
recursive(X) :- add, Y is X + 1, recursive(Y).

add :- nb_getval(counter, C), CNew is C + 1, nb_setval(counter, CNew).

testRecursion
:-
    % Set counter to zero
    nb_setval(counter, 0),

% Run some code with 'add'
    recursive(0), !,

% Print the results
    nb_getval(counter, CounterValue),
    write('Steps: '), writeln(CounterValue).
```

This is good for some experimental cases, but in general you will want to avoid global variables in Prolog because Prolog means programming in logic.

How to create global variable in prolog

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