Decision Structures
Chapter 4 Part 1
Chapter 4 Objectives

To understand
o
o
o
o
o
What values can be stored in a Boolean variable
What sequence structures are
What are decision structures
The difference between dual-alternative and singlealternative decision structures
Nested instructions
4-2
Boolean Variables

Boolean Variables hold only two values
o
o
o

True
False
Initial Value is usually
set to be “TRUE”
How it looks in variable definitions
4-3
Boolean Functions

Boolean Functions return one value from one of following
o
o

Example: Ask user for yes or no
o

True
False
User prompted to answer Yes (true) or No (false)
Asks the following question:
4-4
If/Else Decision Structure

All previous instructions have been consecutively
executed
One
After
The
Other

If/Else checks a Boolean condition then makes a
decision
o
If condition is


True: one set of instructions are executed
False: a different set of instructions is executed
4-5
If/Else Flow Chart

Flow charts help to visually see which instruction sequence
will be used next
4-6
Algorithm For Conditional Statement

BOTH actions will NOT be performed…only ONE can be!
If the hole is wider than the penguin
penguin falls down the hole
Else
penguin says “Drats!”
End If

TRUE
FALSE
The penguin falling or speaking are instructions that are
conditionally executed
o
o
They do NOT always execute
Executed only under certain conditions
4-7
If/Else Flowchart Compared to Alice
Penguin falls
down into the hole
4-8
If/Else Statement Location

Drag If/Else tile to Do Nothing
If/Else Tile
4-9
Using If/Else Instruction
Empty If/Else instruction
 Two empty “slots” in instruction
o If part
o Else part
 After dragging If/Else instruction to location choose true or
false as placeholder
 Replace placeholder with Boolean variable, Boolean function,
relational operator or logical operator
 Add instructions to If and Else parts
 More than one instruction can be added to If and Else parts
4-10
Single-Alternative Decision Structures

If/Else is a dual-alternative decision structure
o
There are two paths of execution



One when If condition is TRUE
Other when If condition is FALSE
Single-alternative decision structures are similar to
dual-alternative
o
o
Decides whether instructions in If path should be
executed or not
Else part is empty therefore nothing is executed
4-11
Single-Alternative Decision Structure
Start
Penguin turns to
face hole
Hole
wider than
penguin?
False
True
Penguin walks to
center of hole
Penguin falls
into hole
End
4-12
Nested If/Else Instructions


When an If/Else instruction is placed inside
another If/Else instruction
The inner If/Else
executes only if the
outer If/Else is true
4-13
Nested If/Else Instructions
Start
Penguin faces hole
False
“Too Far!”
Within
2 meters
False
“Drats!”
True
Hole wider?
True
Penguin walks
Penguin falls in
End
4-14
Homework



Read chapter 4 sections 4.1 – 4.3
Do labs in handout
Due one week after assigned
4-15