Transcript Chapter 4 (I don`t have the title)

Chapter 3 – Examples
The examples from chapter 3, combining
the data types, variables, expressions,
assignments, functions and methods
with Windows controls (list boxes, text
boxes)
These examples illustrate the concepts
discussed earlier
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Example 3.1.2
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Example 3.1.2
This is a ListBox
called
lstResults
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Example 3.1.2
Variable declarations
Variable a is uninitialized so starts
with a value of 0.
Variable b is initialized to 3.
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Example 3.1.2
Calling methods of
the ListBox object’s
Items property
The Clear method empties the Listbox.
The Add method adds a row to the ListBox.
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Anatomy of a Method Call
The ListBox
When calling the
Add method, you
pass it the data as
an argument.
lstResults.Items.Add(a)
The Items property
contains the
collection of data
that are displayed
in the ListBox
The Items property’s
Add method is a
subroutine that places
an item into the
collection
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Example 3.1.2
Assigning a value into the a variable
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Example 3.1.2
Adding a third item to the list
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Method Call passing a complex
expression as an argument
lstResults.Items.Add(a * (2 + b))
An argument can be a complex expression. The expression will be fully
evaluated before the resulting data is sent. In this case, the following steps take
place in this order:
1) 2 + b
innermost parentheses
2) Multiply a times the result if (1)
5
 25
3) Pass the result of (2) to the Add method
 Order of operations from innermost to outermost based on parentheses
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Example 3.1.4
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Example 3.1.4
This is a
ListBox called
lstResults
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Example 3.1.4
Variable declarations
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Example 3.1.4
Variable assignments
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Example 3.1.4
Empty the list box items
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Example 3.1.4
Add three values to the
list box
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Order of operations
lstResults.Items.Add(Math.Sqrt(5 * b + 1))
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16
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Add to list
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Order of operations
lstResults.Items.Add(Int(a ^ b + 0.8))
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8.8
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Add to list
The Int function truncates the number…it will not round up, but rather
just chops off the fractional part.
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Order of operations
lstResults.Items.Add(Math.Round(a / b, 3))
0.6666666666…
0.667
Add to list
The Math.Round method rounds the number either up or down,
depending on which rounded value is nearer. It can take two arguments:
1) The number to be rounded
2) The total number of decimal places for the rounded number
If the second argument is not provided, 0 is assumed. There will be
no decimal places, so the result will be a whole number.
Multiple
arguments to
methods are
separated by
commas.
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Example 3.1.5
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Example 3.1.5
This example converts 41 inches into 3 feet, 5 inches
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Example 3.1.5
Variable
declarations…three
in one statement
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Example 3.1.5
Backslash is for
integer division.
Truncates the
fractional part. So,
feet = 3
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Example 3.1.5
Mod gives the
remainder of an
integer division. So,
inches = 5.
41 divided by 12 is
3, with a remainder
of 5
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Example 3.2.1
This is a string literal, which
will be displayed literally.
This is a string variable, so
its contents will be displayed
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Example 3.2.2
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Example 3.2.2
This example adds the values in two textboxes and
places them in a third
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Example 3.2.2
The CDbl function takes a
string value and attempts to
convert it into a Double
value.
Note: the string must
consist of digits (and
perhaps one dot for a
decimal point). Otherwise
an exception (run-time
error) would occur.
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Example 3.2.2
The CStr function takes a
numeric value and converts
it to a string. This is
necessary because a
TextBox’s Text property
requires a string value.
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3 Program Modes
• Design mode
• Run mode
• Debug or Break mode
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Stepping through program
• Execute one line of code at a time (Stepping
into)
• Execute one procedure (Stepping over)
• Execute remaining lines of code (Stepping
out)
• Execution will stop a pre-specified line of
code (Break point)
• Hoover to see value of particular variable or
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object
Stepping through program
• Each line is highlighted
before execution
• Hoover to see values
• Num1 & both Textboxes
• num2 is 0 – Why?
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Break points
• Set a break point & run
• Program stops
• Hoover to see values
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Example 3.2.3
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Example 3.2.3
Declaring three
String variables
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Example 3.2.3
Assigning String
literals into String
variables.
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Example 3.2.3
Concatenating the contents
of two String variables and
assigning them into another
String variable
Note: remember that the right side of an
assignment statement is an expression. In
this example, the expressions are String
expressions.
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Example 3.2.3
The final result placed in
the Text property of
txtOutput is the result of
another concatenation
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Example 3.2.5
This example illustrates the use of some String methods and
properties.
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Example 3.2.5
The Substring method takes two arguments:
1) The beginning position of the substring (first position of the string is 0)
2) The length of the substring (number of characters to return)
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Example 3.2.5
This IndexOf method returns the first position of a substring
within a string.
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Example 3.2.5
This ToUpper method
converts the characters of a
string to upper case. There is
also a ToLower method
In this case, the string expression is a concatenation. Note that (str1 & str2) is a
concatenation of two strings. Because this is in parentheses, the concatenation
occurs BEFORE the conversion to upper case. What would happen if you did not
have the parentheses around str1 & str2?
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Example 3.2.5
The Trim method removes beginning and end spaces from a string. Here,
the trim takes place for str1, and the result is concatenated with str2
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Example 3.2.5
The Length property of a string gives the total
number of characters in the string. As you can
see, str2 contains “a wink”, which has a total of
six characters
In this case, the Substring method is only taking ONE argument (str2.Length – 4)  2. If a
second argument is not provided Substring returns the remainder of the string, starting at the
specified beginning position
So, at the end of the assignment statement, str3 contains the string “wink”.
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Example 3.2.5
Note: These methods (SubString, IndexOf, Trim, ToUpper) return a string
expression that can be used for display or assignment. But they do NOT change the
original contents of the variables str1 and str2. Only an assignment changes them.
So, at the end of this program, the contents of str1 and str2 are still “Quick as ” and
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“a wink”.
Example 3.2.7
It is good programming practice to include comments in your code in
order to explain what is being done.
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Example 3.2.8
In previous
examples, the
variables were
declared INSIDE
the procedure.
Here, it is
declared
OUTSIDE any
subroutines.
Two things to note about class-level variables:
1) Their data remain in existence throughout the entire time that the Form is
running
2) Their data is accessible by ALL subroutines and functions of the form
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Example 3.2.8
Compare
previous to what
would happen if
numTimes were
declared INSIDE
the procedure.
In this case, numTimes would only exist as long as the subroutine was running, and
would disappear when it ended. So, EVERY TIME you run the routine, it will
reinitialize to zero. Wouldn’t be able to keep a count of how many times the button
was pushed!
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Example 3.3.1
Working with
dates
In this case, we have declared a variable as a Date.
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Example 3.3.1
Note that we have set a mask in the
mtbDayOfBirth MaskedTextBox control.
The MaskedTextBox’s Mask
property helps guide the
user and prevent invalid
input.
Here we guarantee that only
numbers are entered
separated by slashes
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Masked Text Box Control
Click on the Tasks button to reveal the
Set Mask property.
Click Set Mask to invoke the Input
Mask dialog box.
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Input Mask Dialog Box
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Mask
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
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Sample Masks
• State abbreviation: LL
• Phone number: 000-0000
• Social Security Number: 000-00-0000
• License plate: &&&&&&
• Date: 00/00/0000
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Example 3.3.1
The CDate function converts a String to a Date
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Example 3.3.1
The FormatDatetime takes a date and
formats it four output. Options are LongDate,
ShortDate, GeneralDate, Longtime, and
ShortTime
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Example 3.3.1
The DateDiff function gives the time gap
between two dates. You can specify which
intervals you want: days, months, etc.
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Example 3.3.1
FormatNumber allows you to display a number in a wide
variety of formats. In this case we are specifying to display
only the whole number part of the difference.
As always, use parentheses to determine the order of
operations that take place…inner to outer.
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Formatting Numbers
• Lots of approaches:
• FormatNumber is good for getting the correct
number of places to the right of the decimal, and
has other options for how to display negative
numbers, etc.
• FormatCurrency is a great way to show dollars
and cents. For example, try this:
• FormatCurrency(varName)
• Where varName is the name of a numeric variable
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