I ran into a few problems trying to create a Dictionary of Lists in Unity JavaScript. So I thought I’d share some of the tips I learned here.

I’m interested in creating a Dictionary to hold Markov Chain information in order to generate text. So I’d like to be able to create a Dictionary holding a variable length of items for each of its Keys. I’d also like to be able to update the mapping at runtime.

It would look something like this:

• Key1: Item1, Item2, Item3
• Key2: Item4

At runtime, I’d like to be able to say: “Append Item5 to Key2.”

Here’s what I came up with in Unity JavaScript. I started out by initializing a table in my script prior to the Start function. The var table holds our mapping of Keys of type String to Values of type List.<int>, or Lists of integers.

1
2
3

import System.Collections.Generic;
// White space matters.
var table:Dictionary.<String,List.<int> > = new Dictionary.<String,List.<int> >();

The first hiccup I ran into is that white space matters when creating Generics in Unity JavaScript. The compiler will throw an error if it sees two >> in a row.

Then I fill the table with our test mappings.

1
2
3
4
5

// Fill our table with Lists containing ints using Arrays to intialize
// the Lists.
table["a"] = new List.<int>([1]);
table["b"] = new List.<int>([2]);
table["c"] = new List.<int>([3]);

As you can see, you can construct a List from an Array. Cool.

In the Start function, I iterate over the Keys in the Dictionary, Add the item 4 to the Value of Key c, and print out the first item of each of the values. Finally, we check to see whether 4 was appended to the desired entry.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19

function Start () {
    // Iterate over the Keys in our Dictionary.
    for(var str:String in table.Keys) {
        // Get the value for our Key.
        var value:List.<int> = table[str];

        // If the Key is our desired Key, append to its list.
        if (str == "c") {
            value.Add(4);
        }

        // Print the first item in each of the Lists.
        print(value[0]);
    }

    // Print the appended item to see that it worked.
    print(table["c"][1]);

}

Then I attach this script to the Main Camera of a fresh Scene for testing purposes. Bada bing, bada boom.

Attaching the script to any old object in the scene would work. I just chose the Main Camera because the camera object is there by default.

This was all relatively straightforward, after I understood some of the quirks of Unity’s JavaScript syntax. You can find an example script using this code here.