Variables: Difference between revisions
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Local variables in [[call]]able code (e.g [[Arma 3: Functions Library|Functions]]) should be scoped using the | Local variables in [[call]]able code (e.g [[Arma 3: Functions Library|Functions]]) should be scoped using the [[private]]/[[privateAll]] commands, | ||
otherwise you may modify local variables of the [[call]]ing script that are visible in the function.}} | otherwise you may modify local variables of the [[call]]ing script that are visible in the function.}} | ||
<sqf> | <sqf> | ||
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* [[Control Structures]] | * [[Control Structures]] | ||
* [[Magic Variables]] | * [[Magic Variables]] | ||
* [[private]] ({{arma3}}) | * [[private]]/[[privateAll]] ({{arma3}}) | ||
* [[local]] ({{arma2}}) | * [[local]] ({{arma2}}) | ||
[[Category: Scripting Topics]] | [[Category: Scripting Topics]] |
Latest revision as of 15:13, 11 July 2024
A variable is a "storage container" or "named placeholder" for data. You can read and modify the data once this container is created.
Its "name" is referenced as Identifier.
Requirements
The following links guide to the basics to understand this article:
Initialisation
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 - keep in mind that code is meant to be read by human beings (see Code Best Practices - Variable format).
Once a proper name is found, it can be used to declare (or initialise) the variable:
Before Arma 2, querying undefined (or uninitialised) variables returns nil (undefined value); from Arma 2 and later, it returns an "Error Undefined variable in expression" error.
Deletion
Once created, variables 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, it is recommended to undefine the unneeded ones in order to free up memory.
Variable deletion is done by setting its value to nil:
Scopes
Variables are only visible in certain scopes of the game. This prevents name conflicts between different variables in different scripts.
There are two main scopes:
Global Scope
A global variable is visible from all scripts on the computer on which it was defined. Names given to units in the Mission Editor are also global variables pointing to those units, which may not be redefined or modified.
Local Scope
A local variable is only visible in the Script, Function or Control Structure in which it was defined.
Local Variables Scope
If a private variable is initialised within a Control Structure (i.e. if, for, switch, while), its existence will be limited to it and its sub-structures - the variable does not exist outside of the structure and will be seen as undefined.
Variables lifespan |
Code | ||||||||
---|---|---|---|---|---|---|---|---|---|
BEGINNING OF FILE - e.g execVM "script.sqf"; | |||||||||
TAG_GlobalVariable = "Global variable"; // Global variable is accessible from any scope | |||||||||
private _myVariable = 0; | |||||||||
if (_condition) then | |||||||||
{ | |||||||||
_myVariable = 1; | |||||||||
private _myVariable = 2; | |||||||||
_myVariable = 3; | |||||||||
} else { | |||||||||
_myVariable = 4; | |||||||||
private _anotherVariable = 10; | |||||||||
}; | |||||||||
hint str _myVariable; // if _condition is true, _myVariable's value is 1; | |||||||||
// if _condition is false, _myVariable's value is 4. | |||||||||
[] call { | |||||||||
hint str _myVariable; // works, as called code runs in the same scope | |||||||||
}; | |||||||||
[_myVariable] spawn { | |||||||||
hint TAG_GlobalVariable; // works | |||||||||
hint str _myVariable; // throws an "undefined variable" error, | |||||||||
// as _myVariable does not exist in this new script | |||||||||
}; | |||||||||
// trying to use _anotherVariable here would result in an "undefined variable" error, | |||||||||
// _anotherVariable being only scoped in the else block. | |||||||||
END OF FILE - TAG_GlobalVariable keeps on living in missionNamespace |
Correct | Incorrect |
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Data Types
Variables may store values of a certain Data Types (String, Number, etc). The kind of the value specifies the type of the variable. Different operators and commands require variables to be of different types.
A variable is not strongly typed and changes its type according to the new data:
Multiplayer Considerations
Storing functions (or any callable code) into global variables without securing them with compileFinal (since Arma 3) is a very bad practice in multiplayer. The biggest security risk would be to see it being overridden by a malicious usage of publicVariable, setting potentially dangerous code in it.
The best option is to declare your function in CfgFunctions so the engine secures it for you.
If you want to manually create a global function anyway, the best practice is the following: