Smalltalk and Object Orientation: an Introduction - Free

assigned an initial value before the condition expression. This is because the while message begins by
evaluating the control expression, if no initial value is provided for n it will default to nil. Comparing nil
with a numeric value will result in an exception being raised.
As you can see the equivalent C code is not that diffe rent, but the semantic meaning is completely
different.
The whileFalse: message has exactly the same format as the whileTrue: message. The only
difference is in the condition used to decide whether to evaluate the second block or not.
n := 1.
[n > 10] whileFalse:
[Transcript show: n.
n := n + 1].
In some cases we want to do all the work in the receiver block. These versions of the while message
can be viewed as being similar to the do statement in C. To support such a feature there are two
versions of the while loop which do not require the second block to be provided. These are the
whileTrue and whileFalse messages (note the absence of the ‘:’). For example:
C Version
do {
scanf(“%d”, &n);
printf(“%d\n”, n);
} while (N < 1000);
Smalltalk Version
[n := Transcript getNumber.
Transcript show: n; cr.
n < 1000] whileTrue.
Both of these segments of code are expected to read in a number, print it back out and then check to see
if it is less than 1000. If it is th e sequence of steps is repeated, otherwise the loop is terminated. For
example, this could be used to ensure that some input is in the desired range.
9.5 Summary
You now know virtually everything about Smalltalk except the classes which make up the very large
library of reusable components available to the developer. You have learnt the basics of the language in
chapter 6 and in this chapter you have learnt about cascading, blocks and their use in iteration
constructs. You have also learnt about conditional sta tements and their use of blocks. You are now
ready to explore the class structure in Smalltalk.
9.6 Further Reading
As ever, the Hopkins and Horan book [Hopkins and Horan 1995] is a good reference source for the
Smalltalk language. Other useful books are [Lalonde and Pugh 1991] and [Goldberg and Robson 1989].
10. The Collection Classes
10.1 Introduction
This chapter discusses probably the most used class hierarchy in Smalltalk; the collection class
hierarchy. The collection classes are the basis upon which data struc tures are constructed in Smalltalk.
Section two introduces the collection class hierarchy and the common functionality provided by all
collection classes. Section three presents a decision tree to help identify the most appropriate collection
class to use. Sections four, five and six present the Bag class, the Set class and the
86