Smalltalk and Object Orientation: an Introduction - Free

9.3.3 Block temporary variables
Blocks can also possess their own temporary variables. These variables are defined between two
vertical bars and after any input parameters. For example:
[:x |
|temp1 temp 2|
temp1 := x.
temp2 := temp1 * 3.]
A block can actually have up to 256 temporary variables in VisualWorks. However this figure does
vary from implementation to implementation.
9.3.4 Typical block usage
Finally, blocks are often used for control structures:
aNumber even
ifTrue: [aString := 'even']
ifFalse: [aString := 'odd'].
Effectively this means send the message valu e to one of the blocks, depending on the result of
testing aNumber to see if it is even or odd. We shall look at the use of blocks in condition and iteration
statements later in this chapter.
9.4 Control structures
9.4.1 Flow of control
As has previously been menti oned, the if -then constructs in Smalltalk are actually methods defined on
the class Boolean. However, ignoring that issue for a moment, the actual use of the structures is very
straight forward. The basic formats of the if-then expression are:
aBoolean
ifTrue: aBlock
ifFalse: anotherBlock.
aBoolean
ifFalse: aBlock
ifTrue: anotherBlock.
aBoolean
ifTrue: aBlock.
aBoolean
ifFalse: aBlock.
The boolean object is often generated dynamically via some form of logical test (e.g . a < b ).
That is, the first operation is to create the boolean object, which is then sent the message
ifTrue:ifFalse:. This is why the boolean test is often bracketed with round brackets. Then if the
value of the boolean is true, the code in the ifTrue block is executed, if it is false the code in the ifFalse
block is executed. Consider the following example:
C version
Smalltalk Version
if (count < 100) (count < 100)
count++; ifTrue: [count := count + 1]
else {
ifFalse: [Transcript show:
printf(“Overflow\n”);
'Overflow'.
count = 0;
Transcript cr.
} count := 0.]
In both cases the code increments the value of a counter if its ma ximum count has not been reached;
if the maximum count has been reached, the code resets the counter and prints an error message. Nested
if-then statements can be constructed as in any other language. For example, in Smalltalk:
(count < 100)
ifTrue: [(index < 10)
ifTrue: [....]
ifFalse: [.....]]
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