1. Decision Structure
ISYS 350

2. Decision: Action based on condition Examples
Simple condition:
If total sales exceeds $300 then applies 5% discount; otherwise, no discount.
More than one condition:
Taxable Income < = no tax
3000 < taxable income <= % tax
Taxable income > % tax
Complex condition:
If an applicant’s GPA > 3.0 and SAT > 1200: admitted

3. Relational Operators
A relational operator determines whether a specific relationship exists between two values
Operator
Meaning
Expression
>
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 you? ==
Equal to
x == y
Is x equal to y? !=
Not equal to
x != y
Is x not equal to you?

4. A Simple Decision Structure
The flowchart is a single-alternative decision structure
It provides only one alternative path of execution
In C#, you can use the if statement to write such structures. A generic format is:
if (expression) {
Statements;
etc.; }
The expression is a Boolean expression that can be evaluated as either true or false
Cold
outside
True
Wear a coat
False

5. if Statement with Boolean Expression
sales > 50000
if (sales > 50000) {
bonus = 500; }
True
bonus = 500
False

6. Tuition Calculation (1)
Tuition is $ If you take more than 12 units, each unit over 12 will be charged $200.
private void button1_Click(object sender, EventArgs e) {
int units;
double tuition;
units = int.Parse(textBox1.Text);
tuition = ;
if (units > 12)
tuition = * (units - 12); }
textBox2.Text = tuition.ToString("C");

7. Example of if-else Statement
if (temp > 40) {
MessageBox.Show(“hot”); }
else
MessageBox.Show(“cold”);
temp >40
display “hot”
display “cold”
True
False

8. Tuition Calculation (2)
If total units <= 12, then tuition = 1200
Otherwise, tuition = per additional unit
private void button1_Click(object sender, EventArgs e) {
int units;
double tuition;
units = int.Parse(textBox1.Text);
if (units <= 12)
tuition = ;
else
tuition = * (units - 12);
textBox2.Text = tuition.ToString("C"); }
Note: If the if block contains only one line code, then the { } is optional.

9. Compute Weekly Wage
For a 40-hour work week, overtime hours over 40 are paid 50% more than the regular pay.
private void button1_Click(object sender, EventArgs e) {
double hoursWorked, hourlyPay, wage;
hoursWorked = double.Parse(textBox1.Text);
hourlyPay = double.Parse(textBox2.Text);
if (hoursWorked <= 40)
wage=hoursWorked*hourlyPay;
else
wage=40*hourlyPay + 1.5*hourlyPay*(hoursWorked-40);
textBox3.Text = wage.ToString("C"); }

10. Example: if with a block of statements
If total sales is greater than 1000, then the customer will get a 10% discount ; otherwise, the customer will get a 5% discount. Create a form to enter the total sales and use a button event procedure to compute and display the net payment in a textbox. And display a message “Thank you very much” if then total sales is greater than 1000; otherwise display a message “Thank you”.

11. private void button1_Click(object sender, EventArgs e){
double totalSales, discountRate, netPay;
string myMsg;
totalSales=double.Parse(textBox1.Text);
if (totalSales <= 1000)
discountRate = .05;
netPay = totalSales * (1 - discountRate);
myMsg = "Thank you!"; }
else
discountRate = .1;
myMsg = "Thank you very much!";
textBox2.Text = netPay.ToString("C");
MessageBox.Show(myMsg);

12. Throwing an Exception
In the following example, the user may entered invalid data (e.g. null) to the milesText control. In this case, an exception happens (which is commonly said to “throw an exception”).
The program then jumps to the catch block.
You can use the following
to display an exception’s
default error message:
catch (Exception ex) {
MessageBox.Show(ex.Message); }
try {
double miles;
double gallons;
double mpg;
miles = double.Parse(milesTextBox.Text);
gallons = double.Parse(gallonsTextBox.Text);
mpg = miles / gallons;
mpgLabel.Text = mpg.ToString(); }
catch
MessageBox.Show("Invalid data was entered."):

13. Use Try/Catch to Detect Data Syntax Error{
double totalSales, discountRate, netPay;
string myMsg;
totalSales = double.Parse(textBox1.Text);
if (totalSales <= 1000)
discountRate = .05;
netPay = totalSales * (1 - discountRate);
myMsg = "Thank you!"; }
else
discountRate = .1;
myMsg = "Thank you very much!";
textBox2.Text = netPay.ToString("C");
MessageBox.Show(myMsg);
catch (Exception ex)
MessageBox.Show(ex.Message);

14. Practices
1. The average of two exams is calculated by this rule: 60% * higher score + 40% * lower score. Create a form with two textboxes to enter the two exam scores and use a button event procedure to compute and display the weighted average with a MessageBox.Show statement.
2. An Internet service provider offers a service plan that charges customer based on this rule:
The first 20 hours: $10
Each additional hour: $1.5
Create a form with a textbox to enter the hours used and use a button event procedure to compute and display the service charge with a MessageBox.Show statement.

15. Complex Condition with Logical Operators
The logical AND operator (&&) and the logical OR operator (||) allow you to connect multiple Boolean expressions to create a compound expression
The logical NOT operator (!) reverses the truth of a Boolean expression
Operator
Meaning
Description
&&
AND
Both subexpression must be true for the compound expression to be true || OR
One or both subexpression must be true for the compound expression to be true !
NOT
It negates (reverses) the value to its opposite one.
Expression
Meaning
x >y && a < b
Is x greater than y AND is a less than b?
x == y || x == z
Is x equal to y OR is x equal to z?
! (x > y)
Is the expression x > y NOT true?

16. Logical Operators: &&, ||, !
Cond1 Cond2 Cond1 && Cond2
T T
T F
F T
F F ||
Cond1 Cond2 Cond1 || Cond2 !
Cond ! Cond T F

17. Examples Write a complex condition for: 12 <= Age <= 65
Use a complex condition to describe age not between 12 and 65.
X <= 15 is equivalent to: X<15 AND X =15? (T/F)

18. More Complex Conditions
University admission rules: Applicants will be admitted if meet one of the following rules:
1. Income >= 100,000
2. GPA > 2.5 AND SAT > 900
An applicant’s Income is 150,000, GPA is 2.9 and SAT is Admitted?
Income >= 100,000 OR GPA > 2.5 AND SAT >900
How to evaluate this complex condition?
AND has higher priority

19. Is a CIS student with GPA = 2.0 qualified?
Scholarship: Business students with GPA at least 3.2 and major in Accounting or CIS qualified to apply:
1. GPA >= 3.2
2. Major in Accounting OR CIS
Is a CIS student with GPA = 2.0 qualified?
GPA >= 3.2 AND Major = “Acct” OR Major = “CIS”
Is this complex condition correct?
Parenthesis, ( )

20. NOT Set 1: Young: Age < 30 Set 2: Rich: Income >= 100,000 Young

21. Young: Age<30 Rich: Income >100000
private void button1_Click(object sender, EventArgs e) {
double Age, Income;
Age = double.Parse(textBox1.Text);
Income = double.Parse(textBox2.Text);
if (Age < 30 && Income > )
MessageBox.Show("You are young and rich");
else
MessageBox.Show("You are not young or not rich"); }

22. Boolean (bool) Variables and Flags
You can store the values true or false in bool variables, which are commonly used as flags
A flag is a variable that signals when some condition exists in the program
False – indicates the condition does not exist
True – indicates the condition exists
Boolean good;
// bool good;
if (mydate.Year == 2011) {
good = true; }
else
{ good = false; }
MessageBox.Show(good.ToString());

23. Using Boolean Variables and &&
private void button1_Click(object sender, EventArgs e) {
bool Young=false, Rich=false;
double Age, Income;
Age = double.Parse(textBox1.Text);
Income = double.Parse(textBox2.Text);
if (Age < 30) Young = true;
if (Income > ) Rich = true;
if (Young && Rich)
MessageBox.Show("You are young and rich");
else
MessageBox.Show("You are not young OR not rich"); }

24. Using Boolean Variables and ||
private void button1_Click(object sender, EventArgs e) {
bool Young=false, Rich=false;
double Age, Income;
Age = double.Parse(textBox1.Text);
Income = double.Parse(textBox2.Text);
if (Age < 30) Young = true;
if (Income > ) Rich = true;
if (Young || Rich)
MessageBox.Show("You are young OR rich");
else
MessageBox.Show("You are not young and not rich"); }

25. The if-else-if Statement
You can also create a decision structure that evaluates multiple conditions to make the final decision using the if-else-if statement
In C#, the generic format is:
if (expression) { }
else if (expression) …
else
double grade = double.Parse(textBox1.Text);
if (grade >=90) {
MessageBox.Show("A"); }
else if (grade >=80)
MessageBox.Show("B");
else if (grade >=70)
MessageBox.Show("C");
else if (grade >=60)
MessageBox.Show("D");
else
MessageBox.Show("F");

26. Compare the two programs
if (grade < 60) {
MessageBox.Show("F"); }
else if (grade < 70)
MessageBox.Show("D");
else if (grade < 80)
MessageBox.Show("C");
else if (grade < 90)
MessageBox.Show("B");
else
MessageBox.Show("A");
if (grade < 60) {
MessageBox.Show("F"); }
if (grade >=60 && grade < 70)
MessageBox.Show("D");
if (grade >= 70 && grade < 80)
MessageBox.Show("C");
if (grade >= 80 && grade < 90)
MessageBox.Show("B");
if (grade >=90 )
MessageBox.Show("A");

27. Tax Rate Schedule Rules to determine tax rate:
Taxable Income < = no tax
3000 < taxable income <= % tax
10000<Taxable income <= % tax
Taxable income> %
double taxableIncome, taxRate, tax;
taxableIncome = double.Parse(textBox1.Text);
if (taxableIncome <= 3000) {
taxRate = 0; }
else if (taxableIncome<=10000)
taxRate= .05;
else if (taxableIncome <= 50000)
taxRate = .15;
else
taxRate = .25;
tax = taxableIncome * taxRate;
textBox2.Text = tax.ToString("C");

28. if-else-if Example
private void button1_Click(object sender, EventArgs e) {
bool Young=false, Rich=false;
double Age, Income;
Age = double.Parse(textBox1.Text);
Income = double.Parse(textBox2.Text);
if (Age < 30) Young = true;
if (Income > ) Rich = true;
if (Young && Rich)
MessageBox.Show("You are young and rich");
else if (Young && !Rich)
MessageBox.Show("You are young but not rich");
else if (!Young && Rich)
MessageBox.Show("You are not young but rich");
else
MessageBox.Show("You are not young and not rich"); }

29. Working with DateTime Get current date: Get current date and time:
DateTime thisDate=DateTime.Today;
Get current date and time:
DateTime thisDate=DateTime.Now;
Calculate # of days between two dates:
myDate = DateTime.Parse(textBox1.Text);
DaysBetween = thisDate.Subtract(myDate).Days;

30. Compute Age Given DOB: Age<30, Young 30<=Age<65, Middle Age>=65, Old
DateTime myDate, thisDate=DateTime.Today;
double Age;
myDate = DateTime.Parse(textBox1.Text);
Age = (thisDate.Subtract(myDate).Days / 365);
MessageBox.Show(Age.ToString());
If (Age<30)
MessageBox.Show(“Young”);
elseif (Age<65)
MessageBox.Show(“Middle”);
else
MessageBox.Show(“Old”);

31. Date Comparison DateTime thisDate, currentDate = DateTime.Today;
thisDate = DateTime.Parse(textBox1.Text);
if (currentDate>thisDate)
MessageBox.Show("after");
else
MessageBox.Show("before");