1. Lesson 04 Control Structures I : Decision Making
MIT 31043, Rapid Application Development
By: S. Sabraz Nawaz
Senior Lecturer in MIT
Department of MIT
Faculty of Management and Commerce
South Eastern University of Sri Lanka

2. Decision Structures
A control structure is a logical design that controls the order in which statements execute
A sequence structure is a set of statements that execute in the order that they appear
A decision structure execute statements only under certain circumstances
A specific action is performed only if a certain condition exists
Also known as a selection structure
By: S. Sabraz Nawaz

3. 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
By: S. Sabraz Nawaz

4. 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?
By: S. Sabraz Nawaz

5. Examples that use relational operators
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6. BRANCHING
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7. BRANCHING
Branching is the act of controlling which line of code should be executed next. The line to jump to is controlled by some kind of conditional statement. This conditional statement is based on a comparison between a test value and one or more possible values using Boolean logic.
This section describes three branching techniques available in C#:
The ternary operator
The if statement
The switch statement
By: S. Sabraz Nawaz

8. The Ternary Operator
The simplest way to perform a comparison is to use the ternary (or conditional) operator; this operator works on three operands.
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9. The if Statement
The if statement is a far more versatile and useful way to make decisions.
Unlike the ternary operator, if statements don’t have a result (so you can’t use them in assignments); instead, you use the statement to conditionally execute other statements.
The simplest use of an if statement is as follows, where <test> is evaluated (it must evaluate to a Boolean value for the code to compile) and the line of code that follows the statement is executed if <test> evaluates to true:
By: S. Sabraz Nawaz

10. The if Statement if (sales > 50000) bonus = 500; {
bonus = 500; }
sales > 50000
True
bonus = 500
False
By: S. Sabraz Nawaz

11. The if / else Statement
An if-else statement will execute one block of statement if its Boolean expression is true or another block if its Boolean expression is false It has two parts: an if clause and an else clause
You can also specify additional code using the else statement in combination with an if statement. This statement is executed if <test> evaluates to false:
Both sections of code can span multiple lines using blocks in braces:
By: S. Sabraz Nawaz

12. Example of if-else Statement
temp >40
display “hot”
display “cold”
True
False
if (temp > 40) {
MessageBox.Show(“hot”); }
else
MessageBox.Show(“cold”);
By: S. Sabraz Nawaz

13. Example
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14. GUI Design
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15. GUI – CheckBox
A small area on a computer screen which, when selected by the user, shows that a particular feature has been enabled. A CheckBox control allows users to select a single or multiple options from a list of options.
A typical CheckBox control has two possible states:
Checked state is when the CheckBox has check mark on
Unchecked is when the CheckBox is not checked
Typically, we use a mouse to check or uncheck a CheckBox
Checked property is true when a CheckBox is in checked state
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16. GUI – CheckBox…
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17. CheckedChanged Event
Like the Click event for a button, the checkbox has CheckedChanged as its default event.
The CheckedChanged event occurs when the value of the Checked property changes
By: S. Sabraz Nawaz

18. GUI – RadioButton
A RadioButton control provides a round interface to select one option from a number of options.
Radio buttons are usually placed in a group on a container control such as a Panel or a GroupBox and one of them is selected.
A typical RadioButton control has two possible states:
Checked state is when the button has check mark on
Unchecked is when the button is not checked
Typically, we use a mouse to check or uncheck
Checked property is true when a radiobutton is in checked state
At a time only one radiobutton can be selected within a container
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19. GUI – RadioButton…
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20. Checking More Conditions Using if Statements
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
By: S. Sabraz Nawaz

21. Checking More Conditions Using if Statements
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22. Improved Marks Processor
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23. 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?
By: S. Sabraz Nawaz

24. Sample Decision Structures with Logical Operators
The && operator
if (temperature < 20 && minutes > 12) {
MessageBox.Show(“The temperature is in the danger zone.”); }
The || operator
if (temperature < 20 || temperature > 100)
The ! Operator
if (!(temperature > 100))
MessageBox.Show(“The is below the maximum temperature.”);
By: S. Sabraz Nawaz

25. Nested Decision Structures
You can create nested decision structures to test more than one condition.
Nested means “one inside another”
In C#, a generic format is:
if (expression) {
statements; }
else
statements
By: S. Sabraz Nawaz

26. A Sample Nested Decision Structure
Salary >= 40000
yearsOnJob >= 2
Display “Minimum salary requirement not met.”
Display “You qualify for the load.”
Display “Minimum years at current job not met.”
End
By: S. Sabraz Nawaz

27. A Sample Nested Decision Structure
if (salary >= 40000) { if (yearOnJob >= 2) decisionLabel.Text = "You qualify for the loan." } else decisionLabel.Text = "Minimum years at current " + "job not met." decisionLabel.Text = "Minimum salary requirement " + "not met."
By: S. Sabraz Nawaz

28. Example
This program checks if the mark’s range is above 100 or below 0 (zero).
If the mark is out of range (0-100), an error message is show using the MessageBox.
If it is within the range, calculation is executed.
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29. Example
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30. switch

31. The switch Multiple-Selection Statement
C# provides the switch multiple-selection statement to perform different actions based on the possible values of an expression.
Each action is associated with the value of a constant integral expression or a constant string expression that the variable or expression on which the switch is based may assume.
A constant integral expression is any expression involving character and integer constants that evaluates to an integer value or a constant.
A constant string expression is any expression composed of string literals that always results in the same string.
A switch statement compares ONE variable against MULTIPLE possible values
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32. The syntax of the switch statement
Supported data types: bool, char, String, integral, or enum
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33. Sample switch Statement
switch (month) { case 1: MessageBox.Show(“January”); break; case 2: MessageBox.Show(“February”); case 3: MessageBox.Show(“March”); default: MessageBox.Show(“Error: Invalid month”); }
month
Display “January”
Display “February”
Display “March”
Display “Error: Invalid month”

34. switch Example I: Lottory
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35. switch Example II: Month Finder
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36. A switch statement that falls through the first case label
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37. GUI - PictureBox
You can display images on your form by using the PictureBox control.
It is a simple control which has a main purpose of displaying images.
All you have to do is browse for the desired image and Visual Studio will import it to your project. You can use several image formats such as JPEG, GIF, PNG, and BMP.
Properties
Description
Image
The image that will be displayed by the control.
ImageLocation
The path of the image to be displayed by the PictureBox.
SizeMode
Tells how the image will be displayed.
By: S. Sabraz Nawaz

38. GUI – PictureBox…
To display an image using the PictureBox control, there are multiple ways you can use. You can go to the Properties Window and find the Image property. Click the button to the right of it to bring out the Select Resource Dialog.
By: S. Sabraz Nawaz

39. GUI – PictureBox…
Once the image is displayed, it may not look like you want it to. If the loaded image is larger the size of the PictureBox, then the image will be clipped.
You can use the SizeMode property to change the way the image is positioned or resized inside the control.
PictureBoxSizeMode
Description
Normal
The image will be positioned in the upper-left corner of the PictureBox and if the image is larger than the PictureBox, the image will be clipped.
StretchImage
Resizes the image to match the size of the PictureBox.
AutoSize
Resizes the PictureBox to match the size of the image.
CenterImage
The image is centered inside the PictureBox. If the image is larger than the PictureBox, the image will be clipped.
Zoom
Fits the whole image inside the PictureBox while maintaining the image’s size ratio.
By: S. Sabraz Nawaz