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Using Dictionaries
Using Dictionaries
Essentially, a dictionary is like a collection but with two additional advantages:
✦ Searching for that data in a dictionary is much faster.
Dictionaries use a data structure known as a hash table (which you read
more about later in this chapter). Every item stored in a dictionary must
include a unique key.
Collections don’t require keys because searching for data in a collection
is sequential. Therefore, the computer must start at the beginning of the
collection and examine each item one by one to retrieve a specific item.
The more items in a collection, the slower the search process.
If a programming language offers collections, it usually also offers dictionaries.
If a programming language doesn’t offer collections, it probably
doesn’t offer dictionaries either.
✦ In a dictionary, the key can be any value, including strings or numbers,
which gives you greater flexibility in assigning keys to data.
If a collection uses a key to identify data, that key must be a string.
Dictionaries are also called associative arrays. When you store data and a
key, that’s known as a key-value pair (refer to Figure 3-5).
Like a collection, a dictionary is a data type, so you must first declare a variable
as a dictionary. Then you must create a new dictionary and store data
in that dictionary.
To create a dictionary in the Smalltalk programming language, you could use
the following:
blackbook := Dictionary new.
This code declares the blackbook variable as a dictionary data type. The
new command simply creates an empty dictionary.
Adding data to a dictionary
After you declare a variable as a dictionary data type, you can start
adding data to that dictionary by defining both the data and the key you
want to associate with that date. So if you want to add the name Dick Ross
to a dictionary and assign it a moron key, you could use the following:
blackbook := Dictionary new.
blackbook at: ‘moron’ put: ‘Dick Ross’.