Variables: Difference between revisions

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A '''variable''' is a "storage container" or "named placeholder" for data. You can read and modify the data once this container is created.
A '''variable''' is a "storage container" or "named placeholder" for data. You can read and modify the data once this container is created.


== Requirements ==
== Requirements ==
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The following links guide to the basics to understand this article:
The following links guide to the basics to understand this article:


* [[ArmA: Introduction to Scripting]]
* [[Introduction to Arma Scripting]]
* [[Identifier]]
* [[Identifier]]


== Initialization ==
== Initialization ==


The first thing you have to do is to find a name for your variable. This name is called [[identifier]]. Read the article [[Identifier]] for more information.
The first thing to do to create a variable is to find its name, also called [[Identifier|identifier]]; this name must be speaking to the reader.
 
Once you find a proper name, you have to tell the game engine that you are going to use this identifier. This is called '''initialization''' or '''declaration'''. This is how it is done:
Once you know a name, you have to tell the game engine that you are going to use this identifier. This is called '''initialization'''. Then you may do anything with this variable.
 
Don't worry: This sounds more dramatic than it is. You can initialize any variable by assigning a value to it.


  myVariable = 0;
  myVariable = 0;


Querying undefined (or uninitialized) variables returns [[nil]] (undefined value). When converted to [[String|Strings]] with [[str]], they will always return [[scalar bool array string 0xe0ffffef]] (in [[Armed Assault]]) or [[scalar bool array string 0xfcffffef]] (in [[Operation Flashpoint]]).
Before [[{{arma2}}]], querying undefined (or uninitialized) variables returns [[nil]] (undefined value); from [[{{arma2}}]] and later, it returns an "Error Undefined variable in expression" error.
 
{{Informative | An undefined ([[nil]]) variable converted to [[String]] with [[str]] will return [[scalar bool array string 0xe0ffffef]] (in [[Armed Assault]]) and [[scalar bool array string 0xfcffffef]] (in [[Operation Flashpoint]]).<br>Unless trying to emulate [[isNil]], '''always''' declare your variable before trying to access it.}}
myString = str(undefinedVar);
myString => "scalar bool array string 0xe0ffffef"


Thus you can't really apply any [[Operators|operations]] on variables before they are initialized with a start value.


== Deletion ==
== Deletion ==


Once created, variables will take up space in your computer memory. This is not drastic for small variables, but if you use a big number of very large variables, you should undefine any variables that are not needed anymore. This can be done with the command [[nil]].
Once created, variables will take up space in the computer's memory. This is not drastic for small variables, but if a big number of very large variables is used, some unneeded variables should be undefined in order to free up memory. This can be done by setting their value to [[nil]].


  hugeVariable = nil;
  hugeVariable = nil;


This effectively destroys a variable as if it had never existed.
This effectively destroys a variable as if it had never existed.


== Scopes ==
== Scopes ==
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Private variables are only visible in a specific [[Script (File)|script]], or [[function]], or [[Control Structures]].  
Private variables are only visible in a specific [[Script (File)|script]], or [[function]], or [[Control Structures]].  


The [[identifier]] of private variables in a [[Script (File)|script]] always has to started with an underscore.  
The [[Identifier|identifier]] of private variables in a [[Script (File)|script]] always has to started with an underscore.  


  _myPrivateVariable = 0;
  _myPrivateVariable = 0;
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In functions you should additionally mark variables as private using the command [[private]]. Otherwise you may modify private variables of the calling script that are visible in the function.
In functions you should additionally mark variables as private using the command [[private]]. Otherwise you may modify private variables of the calling script that are visible in the function.


  private "_myPrivateVariable";
  [[private]] "_myPrivateVariable";
  _myPrivateVariable = 0;
  _myPrivateVariable = 0;


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If a private variable is initialized within a [[Control Structures]] (i.e. [[if]], [[for]], [[switch]], [[while]]) its scope will stay ''within'' this structure (i.e. outside of the structure it will still be seen as undefined).
If a private variable is initialized within a [[Control Structures]] (i.e. [[if]], [[for]], [[switch]], [[while]]) its scope will stay ''within'' this structure (i.e. outside of the structure it will still be seen as undefined).


<code>if (alive player) then {private _living=true}; hint format["%1",_living];</code>
<code>[[if]] ([[alive]] [[player]]) [[then]]
{
[[private]] _living = true;
};
[[hint]] [[format]] ["%1", _living];</code>
Returns an error, since the private variable was not initialized ''before'' being used within a control structure.
Returns an error, since the private variable was not initialized ''before'' being used within a control structure.


<code>private _dead=true; if (alive player) then {_dead=false}; hint format["%1",_dead];
<code>[[private]] _living = false;
</code>
[[if]] ([[alive]] [[player]]) [[then]]
Returns "false", since the variable was initialized before the if...then.<br>
{
To initialize private variables, so that they are available throughout the whole script (including any control structures), either initialize it via the [[private]] command (e.g. ''private ["_varname"];'') at the begining of the script.
_living = true;
};
[[hint]] [[format]] ["%1", _living];</code>
Returns [[true]] since the variable was initialized before the [[if]]..[[then]] statement.<br>
To initialize private variables so that they are available throughout the whole script (including any control structures), initialize it via the [[private]] command (e.g. {{Inline code | [[private]] _varname;}}) at the beginning of the script.


== Data Types ==
== Data Types ==
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== Multiplayer Considerations ==
== Multiplayer Considerations ==
 
<!-- TODO: introduce Functions -->
Storing vars and functions into global vars without securing them with [[compileFinal]] (only after Arma-3) is a very bad practice in MP.
Storing vars and functions into global vars without securing them with [[compileFinal]] (only after Arma-3) is a '''very''' bad practice in MP.
Basically, if you have a script installed on a server in mission file that uses functions stored in unprotected variables then hackers can overwrite that function attached to a public variable and make it execute code for everyone and do a lot of nasty stuff. So please put all of the code for each function in a SEPARATE file and initialize them like this:
Basically, if you have a script installed on a server in mission file that uses functions stored in unprotected variables then hackers can overwrite that function attached to a public variable and make it execute code for everyone and do a lot of nasty stuff. So please put all of the code for each function in a SEPARATE file and initialize them like this:
<code>myGlobalVarFunction = compileFinal preprocessFileLineNumbers "Dir\where\function\is\fnc_globalVarFunction.sqf";</code>
myGlobalVarFunction = [[compileFinal]] [[preprocessFileLineNumbers]] "Dir\where\function\is\fnc_globalVarFunction.sqf";
That way, the function can be called and spawned without it being subject to hackers that attempt to overwrite the global variable "myGlobalVarFunction".
That way, the function can be called and spawned without it being subject to hackers that attempt to overwrite the global variable "myGlobalVarFunction".


Everyone happy :)
Everyone happy :)


== See also ==
== See also ==


* [[Data Types]], [[Magic Variables]]
* [[Introduction to Arma Scripting]]
* [[Identifier]]
* [[Data Types]]
* [[Magic Variables]]
 


[[Category: Scripting_Topics]]
[[Category: Scripting Topics]]

Revision as of 00:57, 4 November 2019

Template:SideTOC Template:Stub

A variable is a "storage container" or "named placeholder" for data. You can read and modify the data once this container is created.


Requirements

The following links guide to the basics to understand this article:


Initialization

The first thing to do to create a variable is to find its name, also called identifier; this name must be speaking to the reader. Once you find a proper name, you have to tell the game engine that you are going to use this identifier. This is called initialization or declaration. This is how it is done:

myVariable = 0;

Before Arma 2, querying undefined (or uninitialized) variables returns nil (undefined value); from Arma 2 and later, it returns an "Error Undefined variable in expression" error.

An undefined (nil) variable converted to String with str will return scalar bool array string 0xe0ffffef (in Armed Assault) and scalar bool array string 0xfcffffef (in Operation Flashpoint).
Unless trying to emulate isNil, always declare your variable before trying to access it.


Deletion

Once created, variables will take up space in the computer's memory. This is not drastic for small variables, but if a big number of very large variables is used, some unneeded variables should be undefined in order to free up memory. This can be done by setting their value to nil.

hugeVariable = nil;

This effectively destroys a variable as if it had never existed.


Scopes

Variables are only visible in certain scopes of the game. This prevents name conflicts between different variables in different scripts.

There are three scopes:

private (old name local)
A variable is only visible in the script, or function, or Control Structures in which it was defined.
global
A variable is visible on the whole computer where it was defined.
public
A variable is broadcasted over the network and visible on all computers connected to the network.

Private Variables

Private variables are only visible in a specific script, or function, or Control Structures.

The identifier of private variables in a script always has to started with an underscore.

_myPrivateVariable = 0;

In functions you should additionally mark variables as private using the command private. Otherwise you may modify private variables of the calling script that are visible in the function.

private "_myPrivateVariable";
_myPrivateVariable = 0;

You may also pass more identifiers to the private command using an Array.

private ["_myLocalVariable1", "_myLocalVariable2", ...];

Global Variables

Global variables are visible on the whole computer where they are defined. Names given to units in the Mission Editor are also global variables pointing to those units, which may not be redefined or modified.

Identifiers of global variables must not start with underscore. Otherwise there are the same rules as for all identifiers.

myGlobalVariable = 0;

Public Variables

Public variables are global variables, visible on several computers in the network. You can never have true public variables, but you can emulate their behaviour.

The value of a global variable gets broadcasted over the network using publicVariable. After the call of this command the variable will have the same value on all clients. Once you modify the variable though you have to broadcast it manually again with publicVariable.

More informations relative to private variable

If a private variable is initialized within a Control Structures (i.e. if, for, switch, while) its scope will stay within this structure (i.e. outside of the structure it will still be seen as undefined).

if (alive player) then { private _living = true; }; hint format ["%1", _living]; Returns an error, since the private variable was not initialized before being used within a control structure.

private _living = false; if (alive player) then { _living = true; }; hint format ["%1", _living]; Returns true since the variable was initialized before the if..then statement.
To initialize private variables so that they are available throughout the whole script (including any control structures), initialize it via the private command (e.g. private _varname;) at the beginning of the script.

Data Types

Variables may store certain values. The kind of the value specifies the type of the variable. Different operators and commands require variables to be of different types.

Read the article Data Types for more information about variable types.

Multiplayer Considerations

Storing vars and functions into global vars without securing them with compileFinal (only after Arma-3) is a very bad practice in MP. Basically, if you have a script installed on a server in mission file that uses functions stored in unprotected variables then hackers can overwrite that function attached to a public variable and make it execute code for everyone and do a lot of nasty stuff. So please put all of the code for each function in a SEPARATE file and initialize them like this:

myGlobalVarFunction = compileFinal preprocessFileLineNumbers "Dir\where\function\is\fnc_globalVarFunction.sqf";

That way, the function can be called and spawned without it being subject to hackers that attempt to overwrite the global variable "myGlobalVarFunction".

Everyone happy :)


See also