|
|
The Collection class (and all its sub-classes) is
one feature of the Smalltalk language that makes it so powerful. Tedious
lines of code that are written over and over again in other languages are
just a single message in Smalltalk. Sometimes, the best way to learn about
something is just to play with it. The SortedCollection is no
exception.
|
|
|
This lesson introduces
SortedCollections and describes some of their specific characteristics.
|
|
|
1. If VisualWorks is not already
running, please start running it now.
2. From the VisualWorks main Launcher window, either click the
last button on the Toolbar or select the menu option Tools>>Workspace.
3. In the Workspace, enter the following:
|mySort |
mySort := SortedCollection new.
mySort add: 'dog'.
mySort add: 'apple'.
mySort add: 'cat'.
mySort inspect.
4. Highlight all of this text, <Operate-Click>
and select Do it.
Note that
a new (inspector) window will appear and the caption of the window is a
SortedCollection.
5. In the left pane of the Inspector window, click
(highlight) the word self.
You should
see SortedCollection
('apple' 'cat' 'dog')
6. In the Inspector window, click
the Elements tab. In the left pane of the window, click
(highlight) the 1.
You should
see 'apple'
|
|
|
|
|
Figure
S-1. Inspector
Window of our
SortedCollection
|
|
Figure
S-2. The
"first" item in our SortedCollection
|
7. Close the Inspector window
|
|
|
How does a Sorted Collection work?
By definition, a SortedCollection is just that, a collection of
whatever you put into it or whatever you decide to collect but its contents
are sorted or are in order. For out example, since we are placing Strings
into our collection, they are alphabetized. In short, it stores whatever you
place in it in a sorted order. However, if you ever wanted to reverse that
order, you would require what is called a sort block. Watch what
happens when you compare the results (above) to the one below.
|
|
|
8. In the Workspace, enter
the following:
|mySort |
mySort := SortedCollection sortBlock: [:a :b| a >= b].
mySort add: 'cat'.
mySort add: 'apple'.
mySort add: 'dog'.
mySort inspect.
9. Highlight all of this text, <Operate-Click>
and select Do it.
Note that
a new (Inspector) window will appear and the caption of the window is
still "a SortedCollection".
10. In the left pane of the Inspector window, click (highlight)
the word self.
You should
see SortedCollection
('dog' 'cat' 'apple')
11. In the left pane of the Inspector window, click (highlight)
the 1.
You should
see 'dog'
|
Figure S-3. Inspector Window of our SortedCollection
|
Figure S-4. The "first" item in our SortedCollection
|
12. Close the Inspector window
|
|
|
Can you see how a SortedCollection works now?
On the first line, we declared a temporary variable (mySort). On the
second line, we told Smalltalk to create a new SortedCollection and
assign it to our temporary variable. On the third line, we added the String
'cat' to our SortedCollection. On the next line, we added the String
'apple' to our SortedCollection and finally added the String
'dog'. Then we told Smalltalk to inspect our SortedCollection.
we add
something to a SortedCollection, Smalltalk inserts it into the
collection, in the sorted order. If none is given, it defaults to ascending.
However, you override the default sort order by using a sort block.
Try not to read too much into the syntax of the block [:a
:b | a >= b]. Think of it like this: "Of any 2 items in the SortedCollection
(a and b), make an item following
another item less than or equal to the other". If you were to change the
greater than sign to a less than sign, the sort order would be reversed. In
most programming languages, you would have to write (sort) code yourself. In
Smalltalk, it's built into the language.
|
|
|
Summary
SortedCollections are collections that keep items
in a particular sequence or order.
|
