Introduction to Objects

Imagine that you are going to be tested on the meaning of a word that you didn't know. How would you find out the meaning of this word? You could ask a friend or make up a meaning, but the best approach would be to look up the word in a dictionary. Dictionaries are useful because they pair up a word with a definition. To find the definition of a particular word, you open the dictionary, turn to the page that has that word, and read the definition for the keyword.

In computer programming, this type of data structure is very useful. Not surprisingly, programmers call this data structure a dicinoatry as well, though you will often heard them referred to as objects or maps.

Keys and Values

Like a dictionary of words in a book, a dictionary data structure contains pairs of "keywords" and "definitions." However, in programming these are called "keys" and "values."

Keys are Unique

In a book dictionary, each keyword usually only appears once -- though there may be multiple definitions for one word, there are not two separate entries for the word. You would not normally see this in a dictionary:

Each word, or key is unique. Rather than have the keyword listed multiple times, you would simply have more than one definition:

This same structure holds for computer dictionaries. Each key can only appear once.

Creating an Object, Map, or Dictionary

To create a programming dictionary, you first declare a dictionary variable:

var myDictionary = {};

Once you have an empty dictionary, you can add items to it as pairs of keys and values. There is a specifc way of doing this: nameOfDictionary['nameOfKey'] = 'value';

We can create a dictionary of Karel's favorite things like so:

var karelsFavoriteThings = {};

karelsFavoriteThings['food'] = 'doggie treats';
karelsFavoriteThings['toy'] = 'tennis ball';
karelsFavoriteThings['sound'] = 'dogs barking';
karelsFavoriteThings['hobby'] = 'coding';
karelsFavoriteThings['number'] = 42;

This gives us a map of Karel's favorite things that would look like:

Getting Items out of a Dictionary

Now that we have an object containing Karel's favorite things, we may want to look up some information about Karel. This is how to look something up in a programming dictionary: nameOfDictionary['nameOfKey'];. This will return the value associated with that key, which you can store this into a variable, print, and so on.

Let's get some of Karel's favorite things out of the dictionary:

println("Some of Karel's favorite things: ");

var favToy = karelsFavoriteThings['toy'];
println("Karel's favorite toy is: ");
println(favToy);

println("Karel's favorite food is: " + karelsFavoriteThings['food']);

println(karelsFavoriteThings['number']);
println("That is Karel's favorite number.");

This program would print the following to the console:

Some of Karel's favorite things:
Karel's favorite toy is:
tennis ball
Karel's favorite food is: doggie treats
42
That is Karel's favorite number.