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Question 1.
a. Write down the exact message that you
expect will be displayed in the Dialog box
b. After clicking the button in the Dialog
Box, what message answer should appear in the Workspace Display
Pane?
c. Will the temporary variable myName
appear in the Workspace Variables list pane.
Go to Answers 1a-c.
Exercise 1. Evaluate ALL of the following
3 line expression series, below in 1 go.
I.e. Highlight all 3 lines and click evaluate.
| myName |
myName := 'John'.
Dialog warn: 'Hello', myName
Check that the answers to questions 1a-c are correct.
Question 1d. What button
appeared on the dialog warn box?
Note: Dialog warn boxes always have this single button.
Go to answer 1d.
Question 1e. It is required
that a space is inserted between Hello and John in the dialog
box.
Suggest 3 ways of altering the following to achieve this space!
| myName |
myName := 'John'.
Dialog warn: 'Hello', myName
Go to answer 1e.
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Question 2.
What results do you expect if you just evaluate the single line:-
| myName |
Go to answer 2.
Exercise 2.
Evaluate and check result is as per answer 2
Question 3.
You have just declared myName as a temporary variable in exercise
2. State what you think will be the result of evaluating the single
line
myName := 'John'.
Go to answer 3.
Carry on straight away with Exercise 3
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Local Variables
Exercise 3. Evaluate
and check result is as per answer 3. Clicking the Create it
button will declare myName as a local variables.
This will place myName in the variable list pane.
Check that myName is in the Variable list pane.
If this is not the case pay particular attention to answer 3,
and carry out exercise 3 again.
myName is now a local variable, its scope is different
from its previous use as a temporary variable. To demonstrate
the difference in the scope you will carry out exercises similar
to those in the previous temporary variable section.
Exercise 4. Evaluate
| myName |
You will be informed that myName is already defined (perhaps
in an outerscope).
Click the cancel button.
This is informing you that the variable is already declared. Thus
proving that the scope is much wider than that of the temporary
variable. In fact you can close this LearningBook, save it, then
re-open the same book, the local variable and its value
still exists. The value is still nil until the next exercise.
Exercise 4.
Evaluate
myName := 'John'.
Dialog warn: 'Hello', myName
Works exactly as expected, setting the value of myName to 'John',
and displaying the dialog box, and the Dialog warn: box answering
with nil.
Question 4.
State 3 ways you can find out the current value of the
variable myName.
Go to answer 4.
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Exercise 5.
You are now required to place a space so that the Dialog box displays
Hello John instead of HelloJohn.
If you have not tried the ways in answers 1-3, you may like to
do that now. These are fairly straight forward, and if you fully
understand the alterations made it is not necessary to carry them
out.
In the before you try the final alternative way.
Because this is a tutorial on variables we will use an additional
variable that will be named aSpace to provide a solution.
First you will remove the local variable myName.
- Click on myName in the Variable list, so that it
is highlighted.
- Click on the Delete button underneath the list, note that
myName is no longer in the list
Start with the original expression
| myName |
myName := 'John'.
Dialog warn: 'Hello', myName
- Amend the first line so that an additional variable aSpace
is declared
- Insert an additional line that assigns the string literal
' ' to the variable aSpace
- Amend the last line by concatenating the variable aSpace
into this line. Take note off how this was done with the
variable myName
Evaluate your solution, and correct it if required .
Go to Solution 2
Exercise 5.
Remove the declaration line, allow the 2 local variables to be
created.
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Instances of a class.
Local variables that reference instances of a class are created
using expressions, as follows, and clicking on the create it button
| aFrog :=Frog new |
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| myAccount :=Account new |
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| aString :=ByteString new: 5 |
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| myArray :=Array new: 5 |
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Class Variables
There are very few examples of using class variables in the M206
OU course. You are likely going to be using class methods and
possibly class variables in a TMA. This question appears to create
problems for students.
As you do the course note how you use class methods.
- Messages for class methods are sent to the class.
- You have already used a class method and sent
a message to a class in this tutorial. Have you spotted
this fact. Clue classes, class variables and global variables
start with an upperCase letter.
The message new has been sent to various classes.
The message warn: has been sent to the Dialog class.
In LB 22 you will create a class variable FrogCount.
- Pay particular attention to how you
- use this class method.
- send messages to it.
Also in LB 22 you will work with the Date class. The date class
has 5 class variables.
- Pay particular attention to...
- All the practicals
- The discussion to the initialise method, LB-22
d9, which discusses ...
- the class variables.
- the use of the # character. (notation for a collection
known as arrays)
- How instance methods use class methods/variables. LB-22
p11
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