Page 147 - Learn To Program With Scratch

BooLeanS in the reaL worLD
The word Boolean is used in honor of George Boole, a 19th-century British
mathematician who invented a system of logic based on just two values:
1 and 0 (or True and False) . Boolean algebra eventually became the basis
for modern-day computer science .
In real life, we use Boolean expressions all the time to make decisions .
Computers also use them to determine which branch of a program to fol-
low . A robotic arm may be programmed to inspect moving parts on an
assem bly line and move each part to Bin 1 if goodQuality = true, or Bin 2 if
goodQuality = false . Home security systems are usually programmed to sound
an alarm if the wrong code is entered (correctCode = false) or deactivate when
we enter the correct code (correctCode = true) . A remote server may grant
or deny access when you swipe your credit card at a department store based
on whether your card was valid (true) or invalid (false) . One computer in your
vehicle will automatically deploy the air airbags when it decides that a collision
has occurred (collision = true) . Your cell phone may display a warning icon
when the battery is low (batteryLow = true) and remove the icon when the bat-
tery’s charge is acceptable (batteryLow = false) .
These are just few examples of how computers cause different actions to
be taken by checking the results of Boolean conditions .

Note that the blocks in Table 6-1 all have a hexagonal shape. As you
might recall from Chapter 5, that means the result of evaluating one of
these blocks is a Boolean value, which can be either true or false. For this
reason, these expressions are also called Boolean expressions.
For example, the expression price < 2000 tests whether the value of the
variable price is less than 2,000. If price is less than 2,000, the block returns
(or evaluates to) true; otherwise, it returns false. You can use this expres-
sion to construct your decision condition in the form, “If (price < 2000),
then buy the car.”
Before we look at the if block, which allows you to implement such a
test, let’s go over a simple example that illustrates how Boolean expressions
are evaluated in Scratch.

Evaluating Boolean Expressions
Let’s say that we set two variables, x and y, as follows: x = 5, and y = 10.
Table 6-2 shows some examples that use Scratch’s relational blocks.
These examples reveal several important points about relational opera-
tors. First, we can use them to compare both individual variables (such as
x, y) and complete expressions (such as 2 * x and x + 6). Second, the result
of a comparison is always true or false (that is, a Boolean value). Third, the
x = y block doesn’t mean “Set x equal to y.” Instead, it asks, “Is x equal to y?”
So when the statement set z to (x = y) is executed, the value of x is still 5.

Making Decisions 125

www.it-ebooks.info