Smalltalk and Object Orientation: an Introduction - Free

This repeatedly eval uates the Smalltalk code in the second block while the result of the first block
evaluates to true. Note that whileTrue: is a message to the first block, it is therefore a method defined
in class Context. Other control structures which use blocks include r epetition e.g. timesRepeat: and
do:.
34.5 Class Boolean
Class Boolean, although not exactly part of the Smalltalk syntax, is important enough to be
considered here. It may at first seem confusing that class Boolean is not part of the language and is in
fact a c lass with methods defined which provide the usual logical operations. However, given
Smalltalk’s commitment the the philosophy of object orientation, it is perhaps not that surprising that
facilities such as Boolean are actually provided by a class definition.
Once you get used to the idea it is not that strange and most of the time you do not need to worry
about the fact. Indeed, as long as you learn the “syntax” of boolean operators you need never actually
know that they are not part of the basic language.
The class Boolean defines the whole protocol for boolean operations. This class has two
subclasses True and False which determine what should happen when different messages are sent to
them. Thus the equivalent of C’s if statement in Smalltalk is :
anObject
ifTrue: [some code ].
There is also the equivalent of the unless statement in some languages:
anObject
ifFalse: [ some code ].
These can be combined to form an if-then-else construct.
anObject
ifTrue: [ some code ]
ifFalse: [some other code].
Note the period is only placed after the last line. The message selector used here is actually
ifTrue:ifFalse:.
The usual range of boolean functions have been defined such as (&) and (|) or as well as specialized
versions such as and: and or: which only evaluate the second operator if necessary. For example:
(a < 2) & (b > 2) And
(a < 2) | (b > 2) Or
(a > 0) and: [b > (6 / a)] And where the second expression is only evaluated
if the first expression evaluates to true. As here,
this construct may be use d as a check to ensure
that erroneous operations are not performed.
(a > 0) or: [B > 0] Or where the second expression will only be
executed if the first evaluates to false.
34.6 Collections
As with class Boolean, collections are not strictly part of the Smal ltalk language, however they are so
important that they will be mentioned here.
Collections are the elements used to construct data structures in Smalltalk. They allow any object to
be grouped together and manipulated. Complex data structures can be built up by combining collections
together. Abstract Data Types (ADT’s) can be instantiated by subclasses the default collection classes.
They therefore provide a very powerful data construction mechanism.
The most commonly used collection classes are:
Set
Bag
A collection of objects in any order. No duplicates are allowed.
A collection of objects in any order; duplicates are allowed.
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