Difference between revisions of "HashMap"

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=== Scalar Key precision ===
  
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Revision as of 01:13, 29 December 2020

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Arma 3
HashMaps were introduced in version 2.01.

A HashMap is a container storing Key-Value pairs which is very efficient with many entries that have different keys.
Due to keys being stored by their hash, a HashMap provides constant-time lookup for keys, meaning it is very efficient for finding a key. More information on Wikipedia.
As a HashMap is a container, it stores some traits/properties with Arrays

Working with Hash Maps

Array properties

A HashMap variable is a reference to the HashMap(see Wikipedia reference page); this means that if the HashMap is edited, all the scripts/functions using a reference to this HashMap will see the edition.

private _myMap = createHashMapFromArray [["a",1], ["b",2], ["c",3]];
private _myNewMap = _myMap;
_myMap set ["z", 4];
_myNewMap get "z"; // will be 4

An array set through setVariable does not need to be assigned again if you modify it by reference:

player setVariable ["myMap", createHashMapFromArray [["a",1], ["b",2], ["c",3]]];
private _myMap = player getVariable "myMap";
_myMap set ["z", 4];
player getVariable "myMap"; // is [["a",1], ["b",2], ["c",3], ["z",4]]

Key Types

Due to the keys requirement to be hashable not all variable types are supported to be used as a key in the HashMap.

Supported types are:

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Creating a HashMap

// Example of an empty HashMap
private _myMap = createHashMap;
count _myMap;			// returns 0

// Example of a prefilled HashMap
private _myFilledMap = createHashMapFromArray [["a",1], ["b",2], ["c", 3]];
count _myFilledMap;	// returns 3

Setting an element

private _myMap = createHashMap;
_myMap set [1, "hello there"];	// _myMap is [[1, "hello there"]]

Inserting an element with a key that already exists inside the HashMap, will overwrite the existing key.

private _myMap = createHashMapFromArray [["a",1], ["b",2]];
_overwritten = _myMap set ["a", 1337];	// _myMap is now [["a",1337], ["b",2]] and _overwritten is true

Getting an element

Values are retrieved by their key:

private _myMap = createHashMapFromArray [["a",1], ["b",2]];
_myMap get "a";	// returns 1
_myMap get "z";	// returns Nothing
_myMap getOrDefault ["z", "NotFound"];	// returns "NotFound"

Checking if an element exists

You can check if a key is in the HashMap using the in command:

private _myMap = createHashMapFromArray [["a",1], ["b",2]];
"a" in _myMap;	// returns true
"z" in _myMap;	// returns false

Counting elements

The count command can be used to return the number of Key-Value Pairs stored in the HashMap:

private _myMap = createHashMapFromArray [["a",1], ["b",2]];
count _myMap ; // returns 2

Retrieving Keys

You can retrieve an Array of all keys in the HashMap using the keys command:

private _myMap = createHashMapFromArray [["a",1], ["b",2]];
keys _myMap; // returns ["a", "b"]

HashMap Copy

private _myMap = createHashMapFromArray [["a",1], ["b",2]];
private _myNewMap = _myMap;
_myMap set ["a", 1337];
_myNewMap get "a"; // will be 1337

In order to avoid this behaviour, copy the HashMap with + (plus):

// making copy
private _myMap = createHashMapFromArray [["a",1], ["b",2]];
private _myNewMap = +_myMap;
_myMap set ["a", 1337];
_myNewMap get "a"; // still 1

Arrays stored as Key or value in the HashMap will also be deep-copied.

Removing (deleting) elements

You can remove elements from the HashMap by using deleteAt with the elements key:

private _myMap = createHashMapFromArray [["a",1], ["b",2]];
_myMap deleteAt "b"; // _myMap is "a",1

Iterating through the HashMap

The simplest way to iterate through a HashMap is the forEach command:

private _myMap = createHashMapFromArray [["a",1], ["b",2]];
{ systemChat str [ _x, _y ] } forEach _myMap;

When iterating through a HashMap with forEach, the _x variable holds the key of the current element, while the _y variable holds its value.


Advanced usage

Common errors

Scalar Key precision

Template:WIP bla bla when using scalar key bla bla link to wikipedia floating point precision


See Also