An array in Smalltalk is similar to an array in any
other language, although the syntax may seem peculiar at
first. To create an array with room for 20 elements, do(19):
x := Array new: 20 !
The Array new: 20 creates the array; the x := part
connects the name x with the object. Until you assign
something else to x, you can refer to this array by the name
x. Changing elements of the array is not done using the
:= operator; this operator is used only to bind names to
objects. In fact, you never modify data structures;
instead, you send a message to the object, and it will modify itself.
For instance:
(x at: 1) printNl !
which prints:
nil
The slots of an array are initially set to "nothing" (which
Smalltalk calls nil). Let's set the first slot to the
number 99:
x at: 1 put: 99 !
and now make sure the 99 is actually there:
(x at: 1) printNl !
which then prints out:
99
These examples show how to manipulate an array. They also
show the standard way in which messages are passed arguments
ments. In most cases, if a message takes an argument, its
name will end with `:'.(20)
So when we said x at: 1 we were sending a message to whatever
object was currently bound to x with an argument of 1. For an
array, this results in the first slot of the array being returned.
The second operation, x at: 1 put: 99 is a message
with two arguments. It tells the array to place the second
argument (99) in the slot specified by the first (1). Thus,
when we re-examine the first slot, it does indeed now
contain 99.
There is a shorthand for describing the messages you
send to objects. You just run the message names together.
So we would say that our array accepts both the at: and
at:put: messages.
There is quite a bit of sanity checking built into an
array. The request
6 at: 1
fails with an error; 6 is an integer, and can't be indexed. Further,
x at: 21
fails with an error, because the array we created only has
room for 20 objects.
Finally, note that the object stored
in an array is just like any other object, so we can do
things like:
((x at: 1) + 1) printNl !
which (assuming you've been typing in the examples) will
print 100.
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