Learn to Program with Small Basic

Changing the Value of a Variable
When you create a new variable in your program, you assign it an initial
value using the assignment operator. You can also use the assignment
operator to change the value of an existing variable, which wipes out the
old value, like this:
ferrets = 5
ferrets = 15
TextWindow.WriteLine("There are " + ferrets + " ferrets in my bed!")
The first line in this example creates a new variable named ferrets and
assigns 5 to it, but the second line changes its value to 15. So the WriteLine
statement will output There are 15 ferrets in my bed!
Behind the scenes, the variable ferrets points to a storage area in the
computer’s memory. When you write ferrets = 15, you tell Small Basic to go
into the space in the memory reserved for ferrets and replace the 5 with a
15. When you display the value of ferrets, you grab whatever’s stored in the
space at that moment.
You can also add to a variable to change its value. Imagine you’re programming
a game in which the player has to shoot attacking airplanes. When
the player shoots an airplane, you want to increase their score (stored in a
variable named score) by five points. How would you update the score variable?
Here’s one way:
score = 10 ' Assumes the player already has 10 points
temp = score + 5 ' temp = 10 + 5 (= 15)
score = temp ' Now the player has 15 points
The second line uses a temporary variable named temp to store the
result of adding five to the current value of score. The value of temp is then
assigned to score.
But you can do this faster in one statement:
score = score + 5
Do you see how the same variable, score, is on both sides of the assignment?
This statement adds 5 to the current value of the variable score and
then stores the result back into the same variable. The old value of 10 is
replaced by the new value, 15. See Figure 4-1.
score = score + 5
15
New value
of score
= 10 + 5
Old value
of score
Figure 4-1: Illustrating the statement
score = score + 5
46 Chapter 4