Programming 5. Arrays & Lists

5. Arrays & Lists

Arrays

  • Arrays are a collection of variables of the same type, which allocate neighboring memory locations
  • Declared with square brackets following the type of the elements: 
      int[] userIds;
  • Declaring an array does not yet allocate space from memory

Creating empty arrays

  • Initialize an empty array with a length of 3 using the following syntax: 
    int[] userIds = new int[3];
  • Assign a value to an element in an array by specifying the index in square brackets: 
    userIds[0] = 104;
  • Indexing starts from 0: the above line assigns a value of 104 to the first element of the array
    • if a length of array is 3, the array items are accessed with indices 0,1,2
  • Print the length of an array 
      Debug.Log(userIds.Length);

Creating and populating arrays simultaneously

  • You can also create an array containing values with one statement: 
    string[] names = new string[3] { "Johannes", "Rene", "Ville" };
  • The same works without specifying the length in the brackets: 
    double[] balances = new double[] { 1.3, 200.3, 9332.14 };
  • …and without the new double[] part, as well: 
    double[] balances = { 1.3, 200.3, 9332.14 };

Arrays: Unity example

  • Creating and accessing an array of GameObjects 
    public GameObject[] players;

void Start ()
{
  players = GameObject.FindGameObjectsWithTag("Player");
    
  for(int i = 0; i < players.Length; i++)
  {
    Debug.Log("Player Number "+i+" is named "+players[i].name);
  }
}

Multidimensional arrays

  • C# supports multidimensional arrays: 
    char[,] letters = new char[3, 4]
{
    {'a', 'b', 'c', 'd'},
    {'e', 'f', 'g', 'h'},
    {'i', 'j', 'k', 'l'}
};
Debug.Log(letters[1, 3]);       // Outputs "h"
  • Note: These do not show up in Unity’s inspector

Lists

Lists

  • a similar data structure to arrays
  • unlike arrays, the size of lists can be easily changed
  • more suitable for dynamic data
  • Declaration and initialization: 
    using System.Collections.Generic;

// ...
  
List<int> userIds = new List<int>();

  • Note: you can’t call new List() outside a method!

  • Value assignment: 
    userIds[0] = 22;

The List class

  • a key benefit of using Lists lie in the built-in functionalities of the List class
  • after declaring a list, type nameOfYourList. and VS Code shows what’s available: Array

List functionalities

  • some are fields (no brackets), some are methods (functions with brackets ())
    • list.Count tells the length of the list
    • list.Add(newItem) adds newItem to the end of the list
    • list.Remove(item) removes item from list
    • list.Contains(item) checks if item is in the list & returns boolean
    • list.Find(predicate) finds an item from the list that matches the given predicate. More about it later!

Exercise 1. Enemy names

Create a list of names for enemy characters.

Print to console There are [X] enemy names.

  1. Create a function addName(enemyName) that
    • adds a new enemy name to the list
    • prints out Added [enemyName]. Now there are [X] enemy names.
  2. Create a function removeLastName() that
    • removes the last enemy name from the list
    • prints out Removed [enemyName].
  3. Print out all the names from the list that start with the letter “S”.

Iterating through an array with for

  string[] starters = new string[] 
  {
      "Bulbasaur", 
      "Charmander", 
      "Squirtle"
  };

  for (int i = 0; i < starters.Length; i++)
  {
      Debug.Log(starters[i]);
  }

Iterating through a list with for

List<string> starters = new List<string>() 
{
      "Chikorita", 
      "Cyndaquil"
      "Totodile", 
};

for(int i = 0; i < starters.Count; i++)
{
    Debug.Log(starters[i]);
}
  • note: lists use .Count instead of .Length!

Better iteration with foreach

  string[] starters = new string[] 
  {
      "Bulbasaur", 
      "Charmander", 
      "Squirtle"
  };

  foreach (string starter in starters)
  {
      Debug.Log(starter);
  }
  • Use if a) you don’t need the index while looping through list/array b) you aren’t mutating, i.e, editing, the array/list elements (see example below)

Foreach mutation blocked

alt

Foreach Unity example

  foreach (GameObject enemy in GameObject.FindGameObjectsWithTag("Enemy"))
  {
      Debug.Log(enemy.name);
  }

Deleting an element from list

  • As lists are dynamic, their elements can be deleted
  • There are two ways to delete an element from a list: 1) list.RemoveAt(index);
    • RemoveAt removes the element at the given index 2) list.Remove(element);
    • Remove removes the first occurrence of the given element from the list

Reverse for loop

  • You can’t delete an element when looping through a list normally
    • The length of the list changes after deletion $\Rightarrow$ you’ll eventually try to refer to an nonexistent element!

  • Easy fix: loop through the list in a reverse order

      for (int i = list.Count; i >= 0; i--)
  {
      list[i] = 1337;
  }
  • This way, you won’t run into reference issues

Copying a list

  • Copying a list of value types 
      List<YourType> newList = new List<YourType>(oldList);
  • Copying a list of reference types 
      List<YourType> oldList = new List<YourType>();
  List<YourType> newList = new List<YourType>(oldList.Count);

  oldList.ForEach((item)=>
      {
          newList.Add(new YourType(item));
      });

Exercise 2. Looping through

Loop through the starters array, and print the starter and its number like this: #001 Bulbasaur

Now, edit the strings with a loop to include this number as well.

Exercise 3. Creative data

Create two-dimensional data structure with x,y coordinates.

Create a function populateLevel() that creates new enemy GameObjects in the scene based on these coordinates.

Also, create a function newCoordinate(x, y) that can be used to add new coordinates into the data structure.

Extra: What if you want to populate the level with enemies and collectibles, both of which saved in the same data structure?

Exercise 4. List of lights

Continue exercise 1 from Scripting gameobjects.

Instead of three lights, have nine lights, each with a tag “Light” (preferably as a prefab).

Control the lights from the light switch with a program that runs through a list of all the lights instead of switching them on and off individually.