Smalltalk and Object Orientation: an Introduction - Free

provides any dimension size but requires additional programmer support. For example, in C to create a
two dimensional array of 3 by 4 integer elements we could write:
int table [3] [4];
In Smalltalk we can either use a TwoDList or create the two dimensional structure manually. For
example, if we used the TwoDList class:
| aTwoDList |
aTwoDList := TwoDList columns: 3 rows: 4.
We could then place the string ‘John’ into position 1, 1 using the atPoint:put: message. We can then
retrieve this string using the atPoint: message. Note these messages take a point object as the position. A
point object can be created using the @ form. For example:
aTwoDList atPoint: (1 @ 1) put: 'John'.
Transcript show: (aTwoDList atPoint: (1 @ 1)).
As this class possesses list like behaviour it can be used in a similar manner to an instance of list. For
this reason it is often used with the user interface facilities provided in VisualWorks. For example, with
the table widget. However, we cannot change the size of a TwoDList once it is created. Nor do we have
the ability to use higher dimensions. It is sometimes therefore necessary to manually construct multi -
dimensional arrays. For example:
temp := Array new: 3.
temp at: 1 put (Array new: 4).
temp at: 2 put (Array new: 4).
temp at: 3 put (Array new: 4).
We could then access elements of the object in the following manner:
((temp at: 1) at: 1)
If you find yourself having to do this often then it probably means that you are thinking to
procedurally and need to reconsider your design.
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11.7 Dictionaries
A Dictionary is a set of Associations, each representing a key -value pair. It is a subclass of
Set. The elements in a Dictionary are unordered, but each has a definite name or key. Thus a
Dictionary can be regarded as an unordered collection of object values with external keys. Part of
the protocol for dictionaries is listed below:
• at: aKey returns the value associated with aKey.
• at: aKey put: aValue puts aValue into a dictionary with the external key aKey.
• associationAt: aKey answers with the association given by the key (i.e. the value and
the key).
• keyAtValue: aValue answers with the name (key) associated with aValue.
• keys answers with a Set of keys from the receiver.
• values answers with a Bag of the values in the receiver. Note that values are not necessarily
unique (hence they are returned as a Bag rather than as a Set).
Here is a simple Dictionary example you might like to type in and try out.
| x |
x := Dictionary new.
x at: 'John' put: 'jjh'.
x at: 'Myra' put: 'msw'.
x at: 'Chris' put: 'cjr'.
x at: 'Denise' put: 'dec'.
Transcript show: (x at: 'Chris') printString.
Transcript show: (x keyAtValue: 'jjh') printString.
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