Expression and Decision Structure

ISYS 350

Performing Calculations• Basic calculations such as arithmetic calculation can be

performed by math operatorsOperator Name of the operator Description

+ Addition Adds two numbers

- Subtraction Subtracts one number from another

* Multiplication Multiplies one number by another

/ Division Divides one number by another and gives the quotient

% Modulus Divides one number by another and gives the remainder

Other calculations: Use Math class’s methods.Example: Math.Pow(x, y)

double x = 3.0, y = 2.0, z;z=Math.Pow(x,y);

Formula to Expression

CD

BA

DC

ABCD

BA

ABAAB

22

DC

AB

CD

BA

B

A

X

X

Y

X

Increment/Decrement Operators

++ Increment Adds 1 to the operand (x = x + 1).

-- Decrement Subtracts 1 from the operand (x = x - 1).

int x = 14;int y = 8;Int z = 10;int result7 = --y; // result7 = 7int result8 = ++x; // result8 = 15, x = 15++z; // z= 11

double a = 8.5;double b = 3.4;double result15 = --a; // result15 = 7.5double result16 = ++b; // result16 = 4.4

Compound (Shortcut) Operators

Operator

+= Adding the operand to the starting value of the variable.-= Subtracting the operand from the starting value of the variable.

*= Multiplying the operand by the starting value of the variable./= Dividing the operand by the starting value of the variable.%= Remainder after dividing the right operand by the value in the variable.

Statements that use the same variable on both sides of the equals sign

count = count + 1; // count is increased by 1 count = count – 1; // count is decreased by 1 total = total + 100.0; // total is increased by 100.0 total = total – 100.0; // total is decreased by 100 price = price * .8; // price is multiplied by 8 sum = sum + nextNumber; // sum is increased by value of nextNumber

Statements that use the shortcut operators to get the same results

count += 1; // count is increased by 1 count -= 1; // count is decreased by 1 total += 100.0; // total is increased by 100.0 total -= 100.0; // total is decreased by 100.0 price *= .8; // price is multipled by 8 sum += nextNumber; // sum is increased by the value of nextNumber

Decrement/Increment by 1

private void button1_Click(object sender, EventArgs e) { int myInt; myInt = int.Parse(textBox1.Text); // myInt-=1; // myInt = --myInt; --myInt; textBox1.Text = myInt.ToString(); } private void button2_Click(object sender, EventArgs e) { int myInt; myInt = int.Parse(textBox1.Text); //myInt += 1; //myInt = ++myInt; ++myInt; textBox1.Text = myInt.ToString(); }

Decrement/Increment by any Step Value

private void button1_Click(object sender, EventArgs e) { int stepValue; stepValue = int.Parse(textBox2.Text); int myInt; myInt = int.Parse(textBox1.Text); myInt -= stepValue; // myInt = myInt - stepValue; textBox1.Text = myInt.ToString(); } private void button2_Click(object sender, EventArgs e) { int stepValue; stepValue = int.Parse(textBox2.Text); int myInt; myInt = int.Parse(textBox1.Text); myInt += stepValue; // myInt = myInt + stepValue; textBox1.Text = myInt.ToString(); }

Prefix/Postfix Increment/Decrement Operators int a = 5;int b = 5;int y = ++a; // a = 6, y = 6int z = b++; // b = 6, z = 5

•When you use an increment or decrement operator as a prefix to a variable, the variable is incremented or decremented and then the result is assigned. •When you use an increment or decrement operator as a postfix to a variable, the result is assigned and then the variable is incremented or decremented.

Counter

• Example: Keep track the number of times a user clicks a button

• Need to declare a variable:int Counter=0;

• Need to increase the counter by 1 when the button is clicked:

Counter = Counter + 1; // Or

Counter+=1; // Or

++Counter;

Question: Where to declare this variable?

Example

int Counter = 0;private void button1_Click(object sender, EventArgs e) { Counter = Counter + 1; textBox1.Text = Counter.ToString(); }

private void button1_Click(object sender, EventArgs e) { int Counter = 0; Counter = Counter + 1; textBox1.Text = Counter.ToString(); }

Incorrect:

Correct:

Variable Scope• The scope of a variable determines its

visibility to the rest of a program.– Procedural-level scope: declared in a procedure

and visible only in the procedure.– Class-level scope: declared in a class but outside

any procedure; visible to all procedures in the class.

Decision Structure

Decision: Action based on conditionExamples

• Simple condition:– If total sales exceeds $300 then applies 5%

discount; otherwise, no discount.• More than one condition:

• Taxable Income < =3000 no tax• 3000 < taxable income <= 10000 5% tax• Taxable income > 10000 15% tax

• Complex condition:– If an applicant’s GPA > 3.0 and SAT > 1200:

admitted

Relational Operators

• A relational operator determines whether a specific relationship exists between two values

Operator Meaning Expression 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 you?

== Equal to x == y Is x equal to y?

!= Not equal to x != y Is x not equal to you?

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;

Statements;

etc.;

}

• The expression is a Boolean expression that can be evaluated as either true or false

Coldoutside

Wear a coat

True

False

if Statement with Boolean Expression

if (sales > 50000){ bonus = 500;}

sales > 50000

bonus = 500

True

False

Tuition Rules (1)• Tuition is $1200. 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 = 1200.00; if (units > 12) { tuition = 1200.00 + 200 * (units - 12); } textBox2.Text = tuition.ToString("C"); }

Example of if-else Statement

temp >40

display “hot”display “cold”

TrueFalse

if (temp > 40){ MessageBox.Show(“hot”);}else{ MessageBox.Show(“cold”);}

Tuition Rules (2)If total units <= 12, then tuition = 1200Otherwise, tuition = 1200 + 200 per additional unit

private void button1_Click(object sender, EventArgs e) { int units; double tuition; units = int.Parse(textBox1.Text); if (units <= 12) tuition = 1200.00; else tuition = 1200.00 + 200 * (units - 12); textBox2.Text = tuition.ToString("C"); }

Note: If the if block contains only one line code, then the { } is optional.

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"); }

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”.

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; netPay = totalSales * (1 - discountRate); myMsg = "Thank you very much!"; } textBox2.Text = netPay.ToString("C"); MessageBox.Show(myMsg); }

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."):}

Use Try/Catch to Detect Data Syntax Errortry { 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; netPay = totalSales * (1 - discountRate); myMsg = "Thank you very much!"; } textBox2.Text = netPay.ToString("C"); MessageBox.Show(myMsg); } catch (Exception ex) { MessageBox.Show(ex.Message); }

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.

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?

Logical Operators: &&, ||, !

• && • Cond1 Cond2 Cond1 && Cond2

T TT FF TF F

• ||• Cond1 Cond2 Cond1 || Cond2

T TT FF TF F

• !• Cond ! Cond

TF

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)

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 800. Admitted?– Income >= 100,000 OR GPA > 2.5 AND SAT >900

• How to evaluate this complex condition?– AND has higher priority

• 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, ( )

NOTSet 1: Young: Age < 30

Set 2: Rich: Income >= 100,000

Young Rich

Young: Age<30Rich: 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 > 100000) MessageBox.Show("You are young and rich"); else MessageBox.Show("You are not young or not rich"); }

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());

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 > 100000) Rich = true; if (Young && Rich) MessageBox.Show("You are young and rich"); else MessageBox.Show("You are not young OR not rich"); }

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 > 100000) Rich = true; if (Young || Rich) MessageBox.Show("You are young OR rich"); else MessageBox.Show("You are not young and not rich"); }

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 if (expression){}…else {}

int 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");}

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 > 100000) 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"); }

Working with DateTime

• Get current date:– 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;

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”);

Date Comparison

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