Smalltalk and Object Orientation: an Introduction - Free

An interesting exercise is to create a very large bag and an array (e.g. over 100,000 numbers) and time
the performance of the inject: message versus the do: message on the two objects. The result of doing
this for bags and arrays of 100, 1000, 10000, 100000 and 1,000,000 is interesting. The performance of
the array is noticeably faster, however the differences between the inject: and do: messages (for the
same class) are negligible.
The Smalltalk facility which allows the developer to time their code is millisecondsToRun:.
This is used to calculate the processor time taken to execute the code in the block passed to it. For
example:
| anArray time1 aNumber |
anArray := Array new: 10000000.
1 to: 10000000 do: [:i | anArray at: i put: i].
time1 := Time millisecondsToRun:
[anArray inject: 0 into: [:sum :item | sum + item ]].
Transcript cr; show: 'Inject '; show: time1 printString.
11.11 Conversion
An instance of one collection can be converted into a different sort of collection with certain conditions.
For example, it is possible to convert a Bag instance into a Set instance, but any duplicate objects in the
Bag instance will be removed. For example:
| a b |
a := Bag new.
a add: 'John'.
a add: 'Paul'.
a add: 'John'.
b := a asSet.
It is quite common to want to take a set or a bag and to sort it in some manner. The easiest way of doing
this is to convert it to a sorted collection. For example:
| aBag aSortedCollection |
aBag := Bag new.
aBag add: 'John'; add: 'Paul'; add: 'Denise'; add: 'Fiona'.
aSortedCollection := aBag asSortedCollection.
aSortedCollection do: [:item | Transcript cr; show: item].
This results in the list of names being printed alphabetically in the Transcript (see Figure 11.2).
Figure 11.2: Printing a sorted list in the Transcript
You should note that these conversion messages do no t affect the receiving object in anyway. That
is, it does not actually convert the collection, rather it creates a new collection of the correct class and
fills it with the objects in the receiving collection.
The other commonly used conversion messages include:
• asBag. This will return a bag collection
• asSet. This returns a set collection. Note if duplicates existed in the receiving collection, they
will have been ignored.
• asOrderedCollection . This returns an OrderedCollection with elements from the
receiver. The ordering is arbitrary.
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