Smalltalk and Object Orientation: an Introduction - Free

11.9 Iterating over collections
A number of messages are available which allow the sa me operation to be applied to each element in a
collection and in some cases the results of this operation can be automatically collected. These
messages are referred to as the iteration messages, they include do:, select: reject:, collect:
and detect:. For all the messages (except do:), the result is a new collection ‘just like the receiver’.
The elements of this object depend on the criteria specified in the block associated with the message.
Note the result can be an empty collection.
11.9.1 The do: message
The do: aBlock message evaluates aBlock for each of the elements in the receiver (a
Collection). aBlock should have one argument. (Note that you have not yet encountered blocks.
For the time being treat them as begin/ends as in other languages su ch as Pascal or ADA. We will look
at them in more detail later.)
The following example illustrates the use of do:
count := 0.
letters do: [:each | each == $a ifTrue: [count := count + 1]].
This expression counts the number of ‘a’s in the collection letters. The do statement iterates over the
elements in the letters collection. Each element is bound to the temporary variable “each” in turn. It
then compares each with the character “a”. If the result of this comparison is the object true (remember
all expression return an object of some form) then the ifTrue: message will execute the code in its
associated block.
Compare it with C style code for the same task:
count = 0; i = 0;
max_num = length(letters);
for (i = 1, i