Choosing the right collection type
Arrays and Collections, which should I use?
This article explains the basic uses of - and differences between - some of the various types of array-like containers you can use in Unity. If you haven't heard of arrays before, they are basically a single variable with a series of "compartments", where each compartment can contain a value. They're useful if you want to deal with a collection of values or objects together. In a game programming situation, you might use an array to store a reference to every enemy in your scene, or as a container for collected items in the player's inventory. Arrays can be 'iterated over', which is a fancy way of saying that you can go through each item in your array in sequence, and inspect or perform functions on each item in turn.
There are many types of containers, and a few of them use the name "array", but there are other types too. The broader term "collection" can be used to describe all these types of containers. These are the types of collection that I'll be describing in this article:
- Built-in array
- Generic List
- Generic Dictionary
- 2D Array
There are other types for more specialised situations, but I have selected these as being some of the basic staples of programming.
All types of collections share a few common features:
- You can fill them with objects, and read back the values that you put in.
- You can 'iterate' through them, which means you can create a loop which runs the same piece of code against each item in the collection.
- You can get the length of the collection.
- For most collections, you can arbitrarily add and remove items at any position, and sort their contents.
Below, I describe some of the most common types of collection, along with their pros and cons, and some (but not all) of their most useful properties and methods.
The most basic type of array, available in both JS and C#, is the built-in array. The main shortcoming of built-in arrays is that they have a fixed-size (which you choose when you declare the array), however this shortcoming is balanced by their very fast performance. For this reason, built-in arrays are the best choice if you need the fastest performance possible from your code (for example, if you're targeting iPhone). If there is a fixed and known number of items that you want to store, this is the best choice.
It's also common to use this type of array if you have a varying number of items to store, but you can decide on a 'maximum' for the number of objects that you'll need. You can then leave some of the elements in the array null when they're not required, and design your code around this. For example, for the bullets in a shooting game, you may decide to use an array of size 50, allowing a maximum of 50 active bullets at any one time.
This type of array is also the only type of array which shows up in Unity's inspector window. This means that a built-in array is the correct choice if you want to populate its contents in the Unity editor, by dragging and dropping references.
It's also usually the type of array you get back from Unity functions, if you use a function which may return a number of objects, such as GetComponentsInChildren.
Built-in arrays are declared by specifying the type of object you want to store, followed by brackets. Eg:
Basic Declaration & Use:
C# <csharp>TheType myArray = new TheType[lengthOfArray]; // declaration int myNumbers = new int; // declaration example using ints GameObject enemies = new GameObject; // declaration example using GameObjects int howBig = myArray.Length; // get the length of the array myArray[i] = newValue; // set a value at position i TheType thisValue = myArray[i]; // get a value from position i</csharp>
Some direct links to useful Functions/Methods of Built-in Arrays:
Basic Declaration & Use:
C# <csharp>ArrayList myArrayList = new ArrayList(); // declaration myArrayList.Add(anItem); // add an item to the end of the array myArrayList[i] = newValue; // change the value stored at position i TheType thisItem = (TheType) myArray[i]; // retrieve an item from position i myArray.RemoveAt(i); // remove an item from position i var howBig = myArray.Count; // get the number of items in the ArrayList</csharp>
Some direct links to useful Functions/Methods of the ArrayList:
A Hashtable is a type of collection where each item is made up of a "Key and Value" pair. It's most commonly used in situations where you want to be able to do a quick look-up based on a certain single value. The piece of information that you use to perform the look-up is the 'key', and the object that is returned is the "Value".
If you are familiar with web development, it's similar to the type of data in a GET or POST request, where every value passed has a corresponding name. With a Hashtable however, both the keys and the values can be any type of object. For most practical applications, it's usually the case that your keys are going to be all the same type (eg, strings) and your values are likely to be all of the same type too (eg, GameObjects, or some other class instance). As with ArrayLists, because Hashtable keys and values are untyped, you usually have to deal with the type casting yourself when you retrieve values from the collection.
Hashtables are designed for situations where you want to be able to quickly pick out a certain item from your collection, using some unique identifying key - similar to the way you might select a record from a database using an index, or the way you might pick out the contact details of a person using their name as the 'unique identifier'.
Basic Declaration & Use:
C# <csharp>Hashtable myHashtable = new Hashtable(); // declaration myHashtable[anyKey] = newValue; // insert or change the value for the given key ValueType thisValue = (ValueType)myHashtable[theKey]; // retrieve a value for the given key int howBig = myHashtable.Count; // get the number of items in the Hashtable myHashtable.Remove(theKey); // remove the key & value pair from the Hashtable, for the given key.</csharp>
Some direct links to useful Functions/Methods of the HashTable:
The Generic List (also known as List<T>) is similar to the JS Array and the ArrayList, in that they have a dynamic size, and support arbitrary adding, getting and removing of items. The significant difference with the Generic List (and all other 'Generic' type classes), is that you explicitly specify the type to be used when you declare it - in this case, the type of object that the List will contain.
Once you've declared it, you can only add objects of the correct type - and because of this restriction, you get two significant benefits:
- no need to do any type casting of the values when you come to retrieve them.
- performs significantly faster than ArrayList
This means that if you were going to create an ArrayList, but you know that you will only be putting objects of one specific type of object into it, (and you know that type in advance) you're generally better off using a Generic List. For me, this tends to be true pretty much all the time.
The generic collections are not part of the standard System.Collections namespace, so to use them, you need to add a line a the top of any script in which you want to use them:
Basic Declaration & Use: (C# only) <csharp>List<Type> myList = new List<Type>(); // declaration List<int> someNumbers = new List<int>(); // a real-world example of declaring a List of 'ints' List<GameObject> enemies = new List<GameObject>(); // a real-world example of declaring a List of 'GameObjects' myList.Add(theItem); // add an item to the end of the List myList[i] = newItem; // change the value in the List at position i Type thisItem = List[i]; // retrieve the item at position i myList.RemoveAt(i); // remove the item from position i</csharp>
Some direct links to useful Methods of the Generic List:
The Generic Dictionary is to the Hashtable what the Generic List is to the ArrayList. The Generic Dictionary provides you with a structure for quickly looking up items from a collection (like the Hashtable), but it differs from the Hashtable in that you must specify explictly the types for the Keys and Values up-front, when you declare it.
Because of this, you get similar benefits to those mentioned in the Generic List. Namely, no annoying casting needed when using the Dictionary, and a significant performance increase compared to the Hashtable.
Because you need to specify the types for both the Keys and the Values, the declaration line can end up a little long and wordy. However, once you've overcome this they are great to work with!
Again, to use this, you'll need to include the Generic Collections package by including this line at the top of your script:
Basic Declaration & Use: (C# only) <csharp>// declaration: Dictionary<KeyType,ValueType> myDictionary = new Dictionary<KeyType,ValueType>;
// and a real-world declaration example (where 'Person' is a custom class): Dictionary<string,Person> myContacts = new Dictionary<string,Person>;
myDictionary[anyKey] = newValue; // insert or change the value for the given key ValueType thisValue = myDictionary[theKey]; // retrieve a value for the given key int howBig = myDictionary.Count; // get the number of items in the Hashtable myDictionary.Remove(theKey); // remove the key & value pair from the Hashtable, for the given key.</csharp>
Some direct links to useful Methods of the Generic Dictionary:
So far, all the examples of Arrays and Collections listed above have been one-dimensional structures, but there may be an occasion where you need to place data into an array with more dimensions. A typical game-related example of this is a tile-based map. You might have a 'map' array which should have a width and a height, and a piece of data in each cell which determines the tile to display. It is also possible to have arrays with more than two dimensions, such as a 3D array or a 4D array - however if you have a need for a 3D or 4D array, you're probably advanced enough to not require an explanation of how to use them!
There are two methods of implementing a multi-dimensional array. There are "real" multi-dimensional arrays, and there are "Jagged" arrays. The difference is this:
With a "real" 2D array, your array has a fixed "width" and "height" (although they are not called width & height). You can refer to a location in your 2d array like this: myArray[x,y].
In contrast, "Jagged" arrays aren't real 2D arrays, because they are created by using nested one-dimensional arrays. In this respect, what you essential have is a one-dimensional outer array which might represent your 'rows', and each item contained in this outer array is actually an inner array which represents the cells in that row.
Usually, "real" 2D arrays are preferable, because they are simpler to set up and work with, however there are some valid cases for using jagged arrays. Such cases usually make use of the fact that - with a jagged array - each 'inner' array doesn't have to be the same length (hence the origin of the term "jagged").
Basic Declaration & Use of "real" 2D arrays: (C# only) <csharp>// declaration: string[,] myArray = new string[16,4]; // a 16 x 4 array of strings
// and a real-world declaration example (where 'Tile' is a user-created custom class): Tile[,] map = new Tile[32,32]; // create an array to hold a map of 32x32 tiles
myArray[x,y] = newValue; // set the value at a given location in the array ValueType thisValue = myArray[x,y]; // retrieve a value from a given location in the array int width = myArray; // get the number of items in the Hashtable myDictionary.Remove(theKey); // remove the key & value pair from the Hashtable, for the given key.</csharp>