Any way to check the existence of a variable?

[WhiteWolf]

Hi,

Is there any way to check if a variable has been created? Let's call it $var, and the logical test should yield true if there is a variable with name=var in the memory (regardless of its content), and yield false, if no such variable has been created yet.

Long story:

I wish to include a [text_input] created variable to the new version of my campaign, and would like to use the user-given variable a lot. The initialization happens at the very beginning.
Problem: With older savegames, this variable will not exist. So I can't give a default value for them.
Solution: Use a macro which checks at the beginning of each scenario if this input variable exists. If it doesn't --> we have an older player, so we can create a default value for him and give a warning that he should restart the campaign if he wants to specify this variable.

So far, I can't find anything that would make this possible. contains="" doesn't work, it even fires if the variable doesn't exist. boolean_equals="" doesn't work, it fires if the variable doesn't exist, but also fires if the variable equals a 'no' as a boolean.

Any ideas? Or just declare that the new version is simply incompatible with the earlier ones, so everyone who updated must...restart the whole campaign?

[zookeeper]

The most common way seems to be to compare it to a non-existent variable, so for example equals=$empty would yield true if the given variable doesn't exist (and, of course, assuming that $empty doesn't exist either).

[WhiteWolf]

This seems to work fine, thanks

[Sapient]

In WML, an empty scalar variable is equivalent to an undefined scalar variable.
In Lua, I believe you can check for wesnoth.get_variable("my_variable")==nil

[Shiki]

This sounds interesting.
In my case, I'm using two variables currently, one for checking If an previsouls event occured, and a second to unstore a unit which I will only have then. Basically, I'm testing if I have the 2nd Variable.

Code: Select all

        [if]
            [variable]
                name=WillEvent
                equals=1
            [/variable]
            [then]
                [unstore_unit]
                    variable=WillV
                    x,y=5,35
                [/unstore_unit]
                {CLEAR_VARIABLE WillV}
                 ... ... (lots of other WML stuff based on that [if], like dialogs and another [event] tag )
            [/then]
        [/if]
        {CLEAR_VARIABLE WillEvent}

Can I use the lua function with a [lua] tag shoehow as the conditional for the [if] ?
I suppose not (?), but maybe doing the [if] part in lua too, but still having the part after [unstore_unit] in WML?
The idea of refactoring away one variable looks for me now like rocket science...

Edit: So I tried zookeepers advice, and it doesn't look at all that hacky I thought.
But there is one trap. Comparing the array variable WillV with a nonexistent will everytime return true, no matter if it exists or not. You have to compare sth. like WillV.id with the nonexistent variable.

[enclave]

I'm not sure if I'm posting something related to your problem... or if somebody already gave you same answer (I couldn't understand if zookeeper's equals=$empty is the same thing or not.. so I thought I would add something)

Wiki says:

Boolean Values

When values are evaluated as boolean values they are checked to see if they are false or true.

These values are evaluated as true: "yes" and "true"
These values are evaluated as false: "no", "false" and ""(uninitialized)
However, in contexts where true is defined as the default value, an uninitialized value would be considered true.

Warning: Do not use "off", "on", integers, floating-point numbers or anything else than the above as booleans.

So from that I could only understand that if the value is uninitialized (empty/non-existant) it will return false in the check:

Code: Select all

[if]
    [variable]
        name=some_variable
        boolean_equals=false
    [/variable]
    [then]
        ## variable is empty
    [/then]
    [else]
        ## variable is not empty.. and possibly equals true?
    [/else]
[/if]

But I must mention that I think I tried it and it didn't work or it was some other issue in my code.. so I can not confirm that it really works, I don't remember. If you like alternative ways.. try this example.. and let us know.. Thanks..

[WhiteWolf]

No, this surely won't work. Let's say var equals false. {VARIABLE var false}. Then this type of logical test would trigger if the variable is uninitialized, but also triggers if var=false. This is not what we want.

Code: Select all

[if]
  [variable]
    name=var
    boolean_equals=false
  [/variable]
  [then]

  # it yields true all right

  [/then]
[/if]

# rest of code
{VARIABLE var false}
[if]
  [variable]
     name=var
     boolean_equals=false
  [/variable]
  [then]

  # it yields true, but var is initialized! It no longer works as wanted.

  [/then]
[/if]

Currently, only zookeeper's equals=$empy_nonexistnt is the solution that works best, in my experience.

[The_Gnat]

Hmm... not sure this is what you are looking for but i always use words for my variables (rather than boolean) that way when it equals "no" it is not because it is empty.

Code: Select all

    {VARIABLE var no}
    [if]
      [variable]
         name=var
         equals=no
      [/variable]
      [then]

      [/then]
    [/if]

This way the if statement will not run if there is no variable.

(hopefully this helps )

[enclave]

or I assume you could convert it into lua and test it there.. but it's not WML workshop anymore..
not sure if it works either, but it would be something like this:
for example you need to test if variable "doll_house" exists..

Code: Select all

[lua]
code = <<
if wesnoth.get_variable("doll_house") then wesnoth.set_variable("nil_test_variable","exists") end
>>
[/lua]
[if]
    [variable]
        name=nil_test_variable
        equals=exists
    [/variable]
    [then]
        ## then your variable exists..
    [/then]
[/if]

not 100% sure if I wrote the lua if then correctly, maybe it needs to look like this:
if wesnoth.get_variable("doll_house")==nil then wesnoth.set_variable("nil_test_variable","exists") end
I'm quite new to lua.. and prefer not to use it unless there is really no other way.

[Sapient]

A few points re:enclave's post.

1. Your second form was correct, not the first form. However I already stated that information (check my previous post in this thread).
2. Since you are only setting nil_test_variable conditionally, the value could be wrong after multiple uses.
3. The original poster of this question has already clarified that they don't really need to distinguish between empty and uninitialized variables, since they only care if the variable is effectively empty, i.e. equals=$empty

[Pentarctagon]

In 1.13.2+, you can also use the new syntax:

Code: Select all

$varname?default_value|

As a macro:

Code: Select all

#define DEFAULT VAR VAL

{VARIABLE {VAR} ${VAR}?{VAL}|}

#enddef

[Sapient]

In 1.13.2+, you can also use the new syntax:

Code: Select all

$varname?default_value|

I tried to lookup the documentation for that feature, but I couldn't find it on the SyntaxWML page.

So does it have the same effect for empty string ("") variables or does it only supply a default for uninitialized (i.e. Lua nil) variables?