Ch (2)(presentation) its about visual programming

HOW TO DEVELOP A VB APPLICATION
 Design the Interface for the user
 Literally draw the GUI
 Drag buttons/text boxes/etc onto form
 Determine which events the controls on
the window should recognize
 Write the code for those
events
2

WHAT HAPPENS WHEN PROGRAM IS
RUNNING
1. VB monitors the controls for events
2. If event occurs, it runs procedures assigned to
that event
3. If no event exists, it goes back to #1.
3

PROPERTIES WINDOW
5
Properties Settings
Selected
control

CONTROL NAME PREFIXES
Control Prefix Example
button btn btnCompute
label lbl lblAddress
text box txt txtAddress
list box lst lstOutput
6

POSITIONING CONTROLS
7
Proximity
line

CODE EDITOR
9
Method
Name
box
Class
Name
box
Code Editor
tab
Form Designer
tab

SAMPLE CODE
Public Class frmDemo
Private Sub txtFirst_TextChanged(...)
Handles txtFirst.TextChanged
txtFirst.ForeColor = Color.Blue
End Sub
End Class
10

VARIABLES, INPUT, AND OUTPUT
 3.1 Numbers
 3.2 Strings
 3.3 Input and Output
12

ARITHMETIC OPERATIONS
 Numbers are called numeric literals
 Five arithmetic operations in Visual Basic
 + addition
 - subtraction
 * multiplication
 / division
 ^ exponentiation
13

NUMERIC EXPRESSIONS
 2 + 3
 3 * (4 + 5)
 2 ^ 3
14

DISPLAYING NUMBERS
Let n be a number or a numeric expression.
What does the statement
lstBox.Items.Add(n)
do?
15

EXAMPLE 1: CODE AND OUTPUT
Private Sub btnCompute_Click (...)
Handles btnCompute.Click
lstResults.Items.Add(5)
lstResults.Items.Add(2 * 3)
lstResults.Items.Add((2 ^ 3) – 1)
End Sub
What is the result?
17

NUMERIC VARIABLE
18
A numeric variable is a name to which a
number can be assigned.
Examples:
speed
distance
interestRate
balance

VARIABLES
 Declaration:
Dim speed As Double
19
Variable name
Data type
• Assignment:
speed = 50

INITIALIZATION
 Numeric variables are automatically initialized to 0:
Dim varName As Double
 To specify a nonzero initial value
Dim varName As Double = 50
22

NUMERIC EXPRESSIONS
Numeric variables can be used in numeric expressions
Dim balance As Double = 1000
lstBox.Items.Add(1.05 * balance)
23

ASSIGNMENT STATEMENT
Dim numVar1 As Double = 5
Dim numVar2 As Double = 4
numVar1 = 3 * numVar2
lstBox.Items.Add(numVar1)
24

INCREMENTING
 To add 1 to the numeric variable var
var = var + 1
 Or as a shortcut
var += 1
 Or as a generalization
var += numeric expression
25

BUILT-IN FUNCTIONS
 Functions return a value
Math.Sqrt(9) returns 3
Int(9.7) returns 9
Math.Round(2.7) is 3
26

INTEGER DATA TYPE
 Variables of type Double can be assigned both whole
numbers and numbers with decimals
 The statement
Dim varName As Integer
declares a numeric variable that can only be assigned
whole number values between about -2 billion and 2
billion
27

MULTIPLE DECLARATIONS
Dim a, b As Double
Two other types of multiple-declaration statements are
Dim a As Double, b As Integer
Dim c As Double = 2, b As
Integer = 5
28

PARENTHESES
 Parentheses should be used liberally in numeric
expressions
 In the absence of parentheses, the operations are
carried out in the following order:
^, * and /, + and -
29

THREE TYPES OF ERRORS
 Syntax error
 Run-time error
 Logic error
30

SOME TYPES OF SYNTAX ERRORS
 Misspellings
lstBox.Itms.Add(3)
 Omissions
lstBox.Items.Add(2 + )
 Incorrect punctuation
Dim m; n As Integer
Displayed as blue underline in VS
31

A TYPE OF RUN-TIME ERROR
Dim numVar As Integer = 1000000
numVar = numVar * numVar
What’s wrong with the above?
32

A LOGICAL ERROR
Dim average As Double
Dim m As Double = 5
Dim n As Double = 10
average = m + n / 2
What’s wrong with the above?
33

ERROR LIST WINDOW
 Dim m; n As Double
 lstResults.Items.Add(5
 lstResults.Items.Add(a)
34

– VARIABLES, INPUT, AND OUTPUT
 3.1 Numbers
 3.2 Strings
35

STRING LITERAL
A string literal is a sequence of
characters surrounded by quotation marks.
Examples:
"hello"
"123-45-6789"
"#ab cde?"
36

STRING LITERAL
A string literal is a sequence of
characters surrounded by quotation marks.
Examples:
Does this work?
“She said: “I’m tired.””
37

STRING VARIABLE
A string variable is a name to which a
string value can be assigned.
Examples:
country
ssn
word
firstName
38

STRING VARIABLE
 Declaration:
Dim firstName As String
39
Variable name
Data type
• Assignment:
firstName = "Fred"

STRING VARIABLE
You can declare a string variable and
assign it a value at the same time.
Dim firstName As String = "Fred"
40

ADD METHOD
Let str be a string literal or variable. Then,
lstBox.Items.Add(str)
displays the value of str in the list box.
41

STRING VARIABLE
You can assign the value of one string variable to another
Dim strVar1 As String = "Hello"
Dim strVar2 As String = "Goodbye"
strVar2 = strVar1
lstOutput.Items.Add(strVar2)
42

VARIABLES AND STRINGS
Private Sub btnDisplay_Click(...) Handles
btnDisplay.Click
Dim president As String
president = "George Washington"
lstOutput.Items.Add("president")
lstOutput.Items.Add(president)
End Sub
43

OPTION STRICT
 Visual Basic allows numeric variables to be assigned
strings and vice versa, a poor programming practice.
 To prevent such assignments, set Option Strict
to On in the Options dialog box.
44

OPTION STRICT -CONTINUED
 Select Options from the Tools menu
 In left pane, expand Projects and Solution
 Select VB Defaults
 Set Option Strict to On
45

OUTPUT
 The contents of a text box is always a string
 Input example
strVar = txtBox.Text
 Output example
txtBox.Text = strVar
46

DATA CONVERSION
 Because the contents of a text box is always a
string, sometimes you must convert the input or
output
dblVar = CDbl(txtBox.Text)
txtBox.Text = CStr(numVar)
47
Converts a String to a Double
Converts a number to a string

WIDENING AND NARROWING
 Widening: assigning an Integer value to a Double variable
 Widening always works. (Every Integer is a Double.)
 No conversion function needed.
 Narrowing: assigning a Double value to an Integer variable
 Narrowing might not work. (Not every Double is an
Integer.)
 Narrowing requires Cint.
 Will loose information (everything after the decimal place)
 Strings can be given a different initial value as follows
48

WITH OPTION STRICT ON
Dim dblVar As Double, intVar As Integer
Dim strVar As String
Not Valid: Replace with:
intVar = dblVar intVar = CInt(dblVar)
dblVar = strVar dblVar = CDbl(strVar)
strVar = intVar strVar = CStr(intVar)
50

CONCATENATION
Combining two strings to make a new string
quote1 = "We'll always "
quote2 = "have Paris."
quote = quote1 & quote2
txtOutput.Text = quote & " - Humphrey Bogart"
Displays
We'll always have Paris. - Humphrey Bogart
51

APPENDING
 To append str to the string variable var
var = var & str
 Or as a shortcut
var &= str
52

APPENDING EXAMPLE
Dim var As String = "Good"
var &= "bye"
txtBox.Text = var
53

STRING PROPERTIES AND METHODS
"Visual".Length is 6.
.length calculates the length of the string.
Varname = “blah”
Varname.length
54

"Visual".ToUpper is VISUAL
.ToUpper makes everything upper case.
Varname = “blah”
55

"123 Hike".ToLower is “123 hike”
.ToLower makes everything lower case
Varname = “Blah”
56

"a" & " bcd ".Trim & "efg" is “abcdefg”
.trim removes leading/trailing spaces
Varname = “ blah “
Varname.trim
57

STRING PROPERTIES
 Can apply a method onto a method
 What does this do?
Dim varname As String = "Tim Hortons"
varname.ToUpper.Replace("I", "O").ToLower()
58

POSITIONS IN A STRING
Positions of characters in a string are
numbered 0, 1, 2, ….
Consider the string “Visual Basic”.
Position 0: V
Position 1: i
Position 7: B
Substring “al” begins at position 4
59

SUBSTRING METHOD
Let str be a string
str.Substring(m, n)
is the substring of length n, beginning at position
m in str
“Visual Basic”.Substring(2, 3) ?
“Visual Basic”.Substring(0, 1) ?
60

INDEXOF METHOD
Let str1 and str2 be strings.
str1.IndexOf(str2)
is the position of the first occurrence of str2
in str1
(Note: Has value -1 if str2 is not a substring
of str1.)
"Visual Basic".IndexOf("is") is 1.
"Visual Basic".IndexOf("si") is 9.
"Visual Basic".IndexOf("ab") is -1.
61

THE EMPTY STRING
 The string "" (NOT " "), which
contains no characters, is called the empty
string or the zero-length string
 The statement lstBox.Items.Add("")
skips a line in the list box
 The contents of a text box can be cleared
with either the statement
txtBox.Clear()
or the statement
txtBox.Text = ""
62

INITIAL VALUE OF A STRING
 By default the initial value is Nothing
 Strings can be given a different initial value as
follows:
Dim name As String = "Fred"
63

COMMENTS
Private Sub btnCompute_Click (...)
Handles btnCompute.Click
'Calculate the balance in an account
Dim rate As Double 'Annual rate of interest
Dim curBalance As Double 'Current balance
64

INTERNAL DOCUMENTATION
1. Other people can easily understand the program
2. You can understand the program when you read it
later
3. Long programs are easier to read because the
purposes of individual pieces can be determined at
a glance
65

LINE-CONTINUATION CHARACTER
 A long line of code can be continued on another line
by using an underscore (_) preceded by a space
msg = "I'm going to make " & _
"him an offer he can't refuse."
66

SCOPE
 The scope of a variable is the portion of the program
that can refer to it
 Variables declared inside an event procedure are
said to have local scope and are only available in
the event procedure in which they are declared
67

SCOPE
 Variables declared outside an event procedure are
said to have class-level scope and are available to
every event procedure.
 Usually declared after
Public Class formName
(Declarations section of Code Editor.)
68

AUTOMATIC COLORIZATION
Comments – green
String literals – maroon
Keywords – blue
Note: Keywords are words such as Sub,
Handles, Private, With, and End that have
special meaning in Visual Basic. They
cannot be used as variable names.
69

COMMENTING
 Commenting is critical
 For yourself and others
 Have to do it right
70

– VARIABLES, INPUT, AND OUTPUT
 3.1 Numbers
 3.2 Strings
75

FORMATTING OUTPUT WITH
FUNCTIONS
76
Function String Value
FormatNumber(12345.628, 1) 12,345.6
FormatCurrency(12345.628, 2) $12,345.63
FormatPercent(0.183, 0) 18%

FORMATTING OUTPUT WITH ZONES
 Use a fixed-width font such as Courier New
 Divide the characters into zones with a format string.
Dim fmtStr As String = "{0, 15}{1, 10}{2, 8}"
lstOutput.Items.Add(String.Format(fmtStr, _
data0, data1, data2))
77

Dim fmtStr As String = "{0, 15}{1, 10}{2, 8}"
Debug.Print(String.Format(fmtStr, "abc",
"def", "ghi"))
 “ abc def ghi”
78

Dim fmtStr As String = "{0, -15}{1, 10}{2, 8}"
lstOutput.Items.Add(String.Format(fmtStr, _
data0, data1, data2))
Here, 15 was preceded by a minus sign. This
produces left justification in 0th
zone. There will
be right justification in the other two zones.
79

Dim fmtStr As String = "{0,-15}{1, 10}{2, 8}"
Debug.Print(String.Format(fmtStr, "abc",
"def", "ghi"))

 “abc def ghi”
80

READING DATA FROM FILES
 Data can be stored in text files and accessed with a
StreamReader object.
 We assume that the text files have one piece of data
per line.
81

SAMPLE FILE: PAYROLL.TXT
Mike Jones
9.35
35
John Smith
10.75
33
82
Name
Hourly wage
Number of hours worked

STEPS TO USE STREAMREADER
Execute a statement of the form
Dim readerVar As IO.StreamReader = _
IO.File.OpenText(filespec)
or the pair of statements
Dim readerVar As IO.StreamReader
readerVar = IO.File.OpenText(filespec)
83

STEPS TO USE STREAMREADER
Read items of data in order, one at a time,
from the file with the ReadLine method.
strVar = readerVar.ReadLine
After the desired items have been read from
the file, terminate the communications link
readerVar.Close()
84

EXAMPLE USING STREAMREADER
Dim name As String
Dim wage, hours As Double
Dim sr As IO.StreamReader = _
IO.File.OpenText("PAYROLL.TXT")
name = sr.ReadLine
wage = CDbl(sr.ReadLine)
hours = CDbl(sr.ReadLine)
lstBox.Items.Add(name & ": " & wage * hours)
OUTPUT: Mike Jones: 327.25
85
Mike Jones
9.35
35
John Smith
10.75
33

COMMENT ON EXAMPLE
Consider
lstBox.Items.Add(name & ": " & wage * hours)
The ampersand automatically converted
wage * hours into a string before concatenating.
We didn’t have to convert wage * hours with
CStr.
86

GETTING INPUT FROM AN INPUT
DIALOG
stringVar = InputBox(prompt, title)
fileName = InputBox("Enter the name " _
& "of the file containing the " & _
"information.", "Name of File")
87
Title
Prompt

USING A MESSAGE BOX FOR OUTPUT
MessageBox.Show(prompt, title)
MessageBox.Show("Nice try, but no
cigar.", _
"Consolation")
88
Title
Prompt

MASKED TEXT BOX CONTROL
Similar to an ordinary text box, but has a Mask
property that restricts what can be typed into the
masked text box.
89
Tasks button

MASKED TEXT BOX CONTROL
90
Click the Tasks button to reveal Set
Mask property.
Click Set Mask to invoke Input Mask
dialog box.

MASK
92
A Mask setting is a sequence of
characters, with 0, L, and & having special
meanings.
0 Placeholder for a digit.
L Placeholder for a letter.
& Placeholder for a character

SAMPLE MASKS
93
State abbreviation: LL
Phone number: 000-0000
Social Security Number: 000-00-0000
License plate: &&&&&&

Ch (2)(presentation) its about visual programming

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