4 Decision Structures and Boolean Logic

CSWD1001
Programming Methods
Decision Structures and Boolean Logic
18/9/2018
CSWD1001 @ Kwan Lee First City Unversity Malaysia
(FCUC)
1

What we’ll learn
• Introduction to decision structures
• If
• If then
• If else
• Nested decision structures
• The case structures – switch case
• Logical operators – True or False
• Boolean variables – 0 or 1
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Decision Structures
program to perform actions only under certain conditions
allows a
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Three Structures
Sequence Structure Selection Structure or
Decision Structure,
Loop Structure
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Sequence Structure
• In order, you perform an action or task, and then you perform
the next action.
• Can contain any number of tasks, but there is no option to
branch off and skip any of the tasks.
• Once you start a series of actions in a sequence, you must
continue step by step until the sequence ends.
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Selection / Decision Structure
• Allow program to execute different lines of codes depending
on if a condition is true or false
• Also called if-then-else / dual-alternative ifs
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Loop Structure
• Continue to repeat actions while a condition remains true.
• The action or actions that occur within the loop are known as
the loop body.
• Also known as repetition or iteration or while loop
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Stacking Structures -
Combination all Three Structures
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The three structures can be combined in an
infinite number of ways
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Decision Structures
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Four Types of Decision Structures
• If-then
• If-else
• Nested If
• Switch case
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Structures If-then
• Use the If-then statement to write a single alternative decision
structure.
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Declaration of Structure If-then
If condition than
Statement
Statement
Etc
End If
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Flowchart of Structure If-then
Condition Statement
True
False
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Example Case
Kathryn teaches a science class and her students are required to take three
tests. She wants to write a program that her students can use to calculate their
average test score. She also wants the program to congratulate the student
enthusiastically if the average is greater than 95. Here is the algorithm:
1. Get the first test score.
2. Get the second test score.
3. Get the third test score.
4. Calculate the average.
5. Display the average.
6. If the average is greater than 95, congratulate the user.
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Pseudocode
Declare Real test1, test2, test3, average // Declare variables
Display "Enter the score for test #1." // Get test 1
Input test1
Input test2
Input test3
Set average = (test1 + test2 + test3) / 3 // Calculate the average score.
Display "The average is ", average // Display the average.
// If the average is greater than 95, congratulate the user.
If average > 95 Then
Display "Congratulations! Great average!"
End If
Program Output
Enter the score for test #1.
82 [Enter]
76 [Enter]
91 [Enter]
The average is 83
93 [Enter]
99 [Enter]
96 [Enter]
The average is 96
Congratulations! Great average!
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Flowchart Start
Declare Real test1,
test2, test3, average
Display “Enter the
Score for test #1”
Input test1
A
A
Input test2
Input test3
B
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Flowchart Cont…
B
Set average = (test1 + test2 + test3) / 3
Display “The average is”, average
Average >
95
Display "Congratulations!
Great average!"
Start
True
False
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Structure If-else
• Will execute one group of statements if its Boolean
expression is true, or another group if its Boolean expression
is false.
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Declaration of Structure If-else
If condition than
Statement
Statement
Etc
Else If
Statement
Statement
Etc
End
executed if the
condition is true.
executed if the
condition is true.
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Flowchart of Structure If-else
Condition Statement
TrueFalse
Statement
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Example Case
Chris owns an auto repair business and has several employees. If an
employee works over 40 hours in a week, Chris pays that employee 1.5
times his or her regular hourly pay rate for all hours over 40. Chris has
asked you to design a simple payroll program that calculates an
employee’s gross pay, including any overtime wages. You design the
following algorithm:
1. Get the number of hours worked.
2. Get the hourly pay rate.
3. If the employee worked more than 40 hours, calculate the gross pay with
overtime. Otherwise, calculate the gross pay as usual.
4. Display the gross pay.
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Example Case Cont…
You go through the top-down design process and create the hierarchy
chart shown in below:
Main()
gerHoursWorked(Real
Ref hours)
getPayRate(RealRef
Rate)
calcPayWithOT(Real
hours, Real rate, Real
Ref gross)
calcRegularPay(Real
hours, Real rate, Real
Ref gross)
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Example Case Cont…
As shown in the hierarchy chart, the main module will call four other
modules. The following are summaries of those modules:
• getHoursWorked - This module will ask the user to enter the number of hours
worked.
• getPayRate - This module will ask the user to enter the hourly pay rate.
• calcPayWithOT - This module will calculate an employee’s pay with overtime.
• calcRegularPay - This module will calculate the gross pay for an employee
with no overtime.
The main module, which executes when the program is run, will call
these modules and then display the gross pay
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Pseudocode
// Global constants
Constant Integer BASE_HOURS = 40
Constant Real OT_MULTIPLIER = 1.5
Module main()
// Local variables
Declare Real hoursWorked, payRate, grossPay
// Get the number of hours worked.
Call getHoursWorked(hoursWorked)
// Get the hourly pay rate.
Call getPayRate(payRate)
// Calculate the gross pay.
If hoursWorked > BASE_HOURS Then
Call calcPayWithOT(hoursWorked, payRate, grossPay)
Else
Call calcRegularPay(hoursWorked, payRate, grossPay)
End If
// Display the gross pay.
Display "The gross pay is $", grossPay
End Module
// The getHoursWorked module gets the number of hours worked
and stores it in the
// hours parameter.
Module getHoursWorked(Real Ref hours)
Display "Enter the number of hours worked."
Input hours
End Module
// The getPayRate module gets the hourly pay rate and stores it in
the rate parameter.
Module getPayRate(Real Ref rate)
Display "Enter the hourly pay rate."
Input rate
End Module
// The calcPayWithOT module calculates pay with overtime. The
gross pay is stored
// in the gross parameter.
Module calcPayWithOT(Real hours, Real rate, Real Ref gross)
// Local variables
Declare Real overtimeHours, overtimePay
// Calculate the number of overtime hours.
Set overtimeHours = hours - BASE_HOURS
// Calculate the overtime pay
Set overtimePay = overtimeHours * rate * OT_MULTIPLIER
// Calculate the gross pay.
Set gross = BASE_HOURS * rate + overtimePay
End Module
// The calcRegularPay module calculates pay with no overtime and
stores it in
// the gross parameter.
Module calcRegularPay(Real hours, Real rate, Real Ref gross)
Set gross = hours * rate
End Module
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Flowchart
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Structure Nested-If
• To test more than one condition, a decision structure can be
nested inside another decision structure.
• Same as if-else, the difference have another if-else inside
main if-else
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Declaration of Structure Nested-If
If condition1 then
If condition2 then
Statement
Statement
Statement
etc.
Else
Statement
Statement
Statement
etc.
Else
Statement
Statement
Statement
etc.
End If
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Flowchart of Structure Nested-If
Conditi
on
Conditi
on
Statement
Statement Statement
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Example Case
Consider a program that determines whether a bank customer
qualifies for a loan. To qualify, two conditions must exist:
1. the customer must earn at least $30,000 per year, and
2. the customer must have been employed at his or her current job
for at least two years.
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Pseudocode
// Declare variables
Declare Real salary, yearsOnJob
// Get the annual salary.
Display "Enter your annual salary."
Input salary
// Get the number of years on the current job.
Display "Enter the number of years on your"
Display "current job."
Input yearsOnJob
// Determine whether the user qualifies.
If salary >= 30000 Then
If yearsOnJob >= 2 Then
Display "You qualify for the loan."
Else
Display "You must have been on your current"
Display "job for at least two years to qualify."
End If
Else
Display "You must earn at least $30,000"
Display "per year to qualify."
End If
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Flowchart Sales >
50000
Sales >
50000
Display “Enter
the number of
hours worked.”
Display “Enter
the number of
hours worked.”
Display “Enter
the number of
hours worked.”
Display “Enter
the number of
hours worked.”
Display “Enter
the number of
hours worked.”
Return
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Structure Switch Case
• The case structure lets the value of a variable or an
expression determine which path of execution the program
will take.
• The case structure is a multiple alternative decision structure.
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Declaration of Structure Switch Case
Select testExpression
Case value_1:
statement
statement
etc.
Case value_2:
statement
statement
etc.
Insert as many Case sections as necessary
Case value_N:
statement
statement
etc.
Default:
statement
statement
etc.
End Select
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Flowchart of Structure Switch Case
Test
Expression
Case 1
statement
Case 2
statement
Case 3
statement
Case 4
statement
Statement
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Example Case
Lenny, who owns Lenny’s Stereo and Television, has asked you to write
a program that will let a customer pick one of three TV models and then
displays the price and size of the selected model. Here is the algorithm:
1. Get the TV model number.
2. If the model is 100, then display the information for that model.
Otherwise, if the model is 200, then display the information for that
model. Otherwise, if the model is 300, then display the information
for that model.
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Pseudocode
// Constants for the TV prices
Constant Real MODEL_100_PRICE = 199.99
// Constants for the TV sizes
Constant Integer MODEL_100_SIZE = 24
// Variable for the model number
Declare Integer modelNumber
// Get the model number.
Display "Which TV are you interested in?"
Display "The 100, 200, or 300?"
Input modelNumber
// Display the price and size.
Select modelNumber
Case 100:
Display "Price: $", MODEL_100_PRICE
Display "Size: ", MODEL_100_SIZE
Case 200:
Display "Price: $", MODEL_200_PRICE
Case 300:
Display "Price $", MODEL_300_PRICE
Default:
Display "Invalid model number"
End Select
Program Output
Which TV are you interested in?
The 100, 200, or 300?
100 [Enter]
Price: $199.99
Size: 24
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Flowchart
modelNum
ber
Display
“Price: $”,
MODEL_100
_PRICE
Display “Price:
$”,
MODEL_200_
PRICE
Display
“Price: $”,
MODEL_300
_PRICE
Display
“Invalid model
number”
Display “Size:”
MODEL_100_
SIZE
Display “Size:”
MODEL_200_
SIZE
Display “Size:”
MODEL_300_
SIZE
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Relational Operators
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A specific relationship exists between two values
Definition
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Relational Operators Table
Expression Meaning Example Meaning
> Greater than x > y Is x greater than y?
< Less than x < y Is x less than y?
>= Greater than or equal to x >= y Is x greater than or equal to y?
<= Less than or equal to x <= y Is x less than or equal to y?
== Equal to x == y Is x equal to y?
!= Not equal to x != y Is x not equal to y?
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Example
Expression Meaning
x > y Is x greater than y?
x < y Is x less than y?
x >= y Is x greater than or equal to y?
x <= y Is x less than or equal to y?
x == y Is x equal to y?
x != y Is x not equal to y?
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Logical Operator
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• Expressions that evaluated as either true or
false
Three types: AND, OR, NOT
Definition
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Logical Operator Table
Operator Meaning
AND The AND operator connects two Boolean expressions as operands and creates
a compound Boolean expression that is true only when both subexpressions are true
OR The OR operator takes two Boolean expressions as operands and creates
a compound Boolean expression that is true when either of the subexpressions is true
NOT The NOT operator is a unary operator that takes a Boolean expression as
its operand and reverses its logical value. If other words, if the expression is true,
the NOT operator returns false, and if the expression is false, the NOT operator returns true
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Compound Boolean expressions using L
Logical Operators
Expression Meaning
X > y AND a < b Is x greater than y AND is a less than b?
X == y OR x == z Is x greater than y OR is x equal to z?
NOT (x > y) Is the expression x > y NOT true?
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Truth table for AND, OR, NOT
AND
Expression Value of
expression
True AND false False
False AND true False
False AND false False
True AND true True
Expression Value of
expression
True OR false True
False OR true True
False OR false False
True OR true True
OR
Expression Value of
expression
Not true False
Not false True
NOT
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Declaration
If conditionA AND conditionB then
Statement
Statement
Statement
etc
End If
If Not condition then
Statement
Statement
Statement
etc
End If
If conditionA OR conditionB then
Statement
Statement
Statement
etc
End If
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Example The Loan Qualifier Program
Revisited
The purpose of this decision structure is to determine that a person’s
salary is at least $30,000 and that he or she has been at his or her current
job for at least two years.
If salary >= 30000 AND yearsOnJob >= 2 Then
Else
Display "You do not qualify for this loan."
End If
Input salary
Display "Enter the number of years on your ","current job."
Input yearsOnJob
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Yet Another Loan Qualifier Program
Suppose the bank is losing customers to a competing bank that isn’t as
strict about whom it loans money to. In response, the bank decides to
change its loan requirements. Now, customers have to meet only one of
the previous conditions, not both.
If salary >= 30000 OR yearsOnJob >= 2 Then
Else
Display "You do not qualify for this loan."
End If
Input salary
Display "Enter the number of years on your"
Display "current job."
Input yearsOnJob18/9/2018
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Boolean Variables
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• A Boolean variable can hold one of two values: true or
false.
• Boolean variables are commonly used as flags, which
indicate whether specific conditions exist.
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Example
• Suppose a salesperson has a quota of $50,000. Assuming the sales
variable holds the amount that the salesperson has sold.
If sales >= 50000 Then
Set salesQuotaMet = True
Else
Set salesQuotaMet = False
End If
If salesQuotaMet == True Then
Display "You have met your sales quota!"
End If
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Good Practise as Programmer
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Programming style
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Reasons for Structure
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Clarity
• The number-doubling program is small. As programs get bigger, they get more
confusing if they are not structured
Professionalism
• All other programmers expect your programs to be structured. It is the way things are
done professionally.
Efficiency
• Most newer computer languages are structured languages with syntax that lets you
deal efficiently with sequence, selection, and looping.
Maintenance
• You and other programmers will find it easier to modify and maintain structured
programs as changes are required in the future.
Modularity
• Structured programs can be easily broken down into routines or modules that can be
assigned to any number of programmers. The routines are then pieced back together.
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Summary
• Every decision you make in a computer program involves evaluating a
Boolean expression.
• You can use if-then-else or if-then structures to choose between two
possible outcomes. You use if-then-else structures when action is
required whether the selection is true or false. Use if-then structures
when there is only one outcome for the question for which action is
required.
• You can use relational comparison operators to compare two operands
of the same type. Th e standard comparison operators are =, >, <, >=,
<=, and <>.
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Summary
• In an AND decision, two conditions must be true for a resulting action
to take place. An AND decision requires a nested decision, or a nested
if.
• In an OR decision, at least one of two conditions must be true for a
resulting action to take place. In an OR decision, fi rst ask the question
that is more likely to be true.
• When you combine AND and OR operators in an expression, the AND
operators take precedence, meaning their Boolean values are evaluated
first.
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4 Decision Structures and Boolean Logic

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