Session 4 - Variables

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Programming with Alice
Computing Institute for K-12 Teachers
Summer 2011 Workshop
Session 4
Introduction to Variables

Variables are used in every aspect of programming. They
are used to store data the programmer needs later in the
program. There are many different types of variables in
Alice, but we will only be covering three in detail:

Numbers – A number variable can contain almost any
value. (e.g. 1, 0.01, -5,+/- 10 million if not more)

String – A string variable can contain letters, words, or
sentences.(e.g. “s”, “string”, “This is still a string”)

Boolean – A Boolean variable contains a true or false value.
Number Variable Overview – A
Video Game Outlook

Incrementing/Decrementing Number Variables – This is
useful when decreasing or increasing variables by a
specified number. (e.g. a player losing lives in a game or
increasing a player’s score).

Setting Number Variables – This tool is used to set the
variable’s value to a specified number. (e.g. setting the
score to zero after the game is restarted or setting the
players lives to max after the game is restarted)

Using Number Variables – Programmers use variables to
store data for later use.

Problems/Type differences – Similar to the issues related
with input, different types of data may have unexpected
outcomes.
World Variables and Object
Variables

Variables can either be world variables or object properties.

World Variables - Use these variables when all objects will
be sharing one variable. For example, in football, it would
be inefficient for each player to keep track of the time
remaining in the quarter using their own clock, it makes
sense to have one “global” clock that everyone can see.

Object Properties (Variables) – Use these variables when
an object requires its own variable. A good example might
Incrementing/Decrementing
Number Variables

As discussed in session 3, it is possible to use a variable
inside of a while loop in order to make it behave like a
regular loop. The steps required to construct this also help
us understand how to increment variables.

Create a new project and get a loop ready to repeat twice.
Now drag a camera “move” block and place it inside the
loop and choose a direction and distance.

Now drag a while loop and place it below the previous loop.
It doesn’t matter if you select true or false, we will be
changing that promptly.

Go to the properties tab of the camera object and create a
new variable named “index,” make sure it is a number
variable and the value is 1.
Variables in the Object Details
Area

There are three things we can realize about a variable
when looking at it in the object details area.

By this icon we can observe that this variable is of the type
“number”. String and Boolean variables have a similar
representation.

This represents the name of the variable. It is possible to
change the name by right clicking it and then entering a
new one.

Here we can see the value the variable contains when the
program starts. It is possible to change this value by
clicking the arrow next to it.
Incrementing/Decrementing
Number Variables

Make sure the value of the variable is 0 when the program
starts.

Now drag a < operator from the world’s functions, place it
into the conditional part of the while loop, mouse-over
expressions, mouse-over camera.index, and select 2.

Since this while loop will execute “while the index variable
is less than 2” and the index variable never changes inside
the loop, we need to increase it by one each time. This
gives it the same effect as a regular loop.

Drag the index variable and place it inside of the while loop
where the “Do Nothing” is located. Now choose to
increment that variable.

Alice can only increment/decrement variables by 1 but it is
possible to construct a snippet which will
increment/decrement a variable by a value other than one.
Closer Look

Look at the regular loop we have in our methods editor, there should
be a
button which you should click. This can help us
better understand that both of these loops are identical.

Loop/While Comparison – “index from 0” means the index starts at 0
(This number can be changed by clicking the arrow next to 0).“Up to
but not including” translates to “while index is < number.”
Incrementing by 1 is easy enough to understand.

Drag a camera move block to the while loop and choose the opposite
direction from the move block previously chosen. Choose the same
distance you chose previously.

Now let’s test our program. If you chose a large distance, it may take
longer for the camera to move but it should move one way using the
first loop and move the exact opposite way with the other loop. This
is to show these two loops can be constructed to behave the same
way.

Now try to construct another loop using a while loop that is identical
to the previous while loop except it decrements the index variable.
(Hint: You may need to play with values in the conditional part of the
while loop.
Setting Number Variables

Let’s now save the world and create a new one. The next
example will be very simple because it is so similar to an
example done in session 1.

Add an object and create a variable for that object named
“distance.” Choose 0 for the initial value. Drag the variable
to the methods editor, mouse-over “set value” and choose
1 for now.

Drag a “ask user for a number” block and place it in the
box after set value to, choose other and type “Enter a
distance for the object to move.”

When the program starts, the “distance” variable will be
set to whatever the user’s answer is. Now drag a move
block from the object’s methods tab, choose a direction
and distance to move. Now replace the distance you just
chose with the “distance” variable. Let’s test our program
and see what happens.
String Variables

Words can also be used and stored into variables. Examples could be
asking someone's name, or what their favorite color is, or if they
would like to continue with the program.

The example we will construct will modify the previous program to
also ask the user if they would like the object to continue moving, if
so, to enter the word “continue.” This will require us to create a new
variable named “answer.” Even though a world variable isn’t required
here, let’s go ahead and make this variable a world variable. When it
asks for a value, select other and type “continue.”

Drag a while loop under the current snippets already present, select
true. Now drag the “answer” variable we just created and place it in
the conditional part of the while loop, mouse-over ==, and select
“continue”. If “continue” isn’t present, choose other to input it
yourself.

Now drag the object “move” block to the methods editor and place it
inside the while loop. Drag the “answer” variable from the properties
tab of the world object and place it under the move block but still
inside the while loop and set the value (doesn’t matter what the
value is for now). Drag an “ask user for String” block and place it in
the box after the words “set value to” in the set answer block
(world.answer), type your own question, it could be something like
“Enter continue for the object to move again.”
String Variable Test

When the program reaches the while loop, it first checks if
the “answer” variable is equal to “continue.” This is why we
set the initial value of the answer variable as we did.

Once in the while loop, the object moves, then the answer
variable is set to the user’s response to your question.
Again, the conditional in the while loop checks if the
“answer” variable is equal to “continue” and acts as it did
before.
Problems that could Arise

In the program we have created, when the user is asked a
distance to move, try entering a word or letter instead of a
number. Test this and see what happens.

The cause? Alice is type sensitive meaning it has different
types which are incompatible with each other. Usually Alice
will not allow us to mix types; for example, if we were to
create a number variable, we couldn’t assign a string value to
that variable. However, if the user enters a number value
when asked for a string, Alice can’t really stop them from
doing it but it will be stored as a string instead of a number.

In Java or C++, this could cause an error that could easily be
fixed, but the designers of Alice accounted for the error so it
most likely will continue running. However, you may have
unexpected results.

The good thing about entering a number instead of a word is
Alice will actually print the numeric value as a word. This
could be quite useful.
Boolean Variables

Boolean variables can be placed in the conditional section of a
while loop and in the predicate of an if/else structure.

Right click the while loop and click disable. Drag another while
loop and place it above the disabled one. Create a world
variable named “boolean” of type Boolean and set the variable
directly above the enabled while loop and choose true.

Drag a “ask user for yes or no” block and place it into the
“set” block; for the question type “Would you like the object to
move again”. Now drag the Boolean variable and place it in
the conditional part of the enabled while loop. If the value of
this variable is true, the while loop will repeat forever, if it is
false, it will never enter the while loop.

Now drag the move block from the disabled while loop and
place it in the enabled while loop. What do you think will
happen when the program starts? If the user answers yes, the
object will move forever, if the answer is no, it won’t move at
all. This is exactly how a Boolean variable works in
programming as well.
Project 4

Create a Alice script that averages multiple numbers. You
can either ask the user to enter how many numbers they
would like to average and use a loop, or use a while loop
and wait for the user to enter “continue.”(There are many
other ways to do this, feel free to be creative, this is only
two ways of giving the user a way to stop the program.)

Create an Alice program that asks the user to enter 2
numbers, stores them as variables A and B, then switches
the values in the variables.
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