Variables, Calculations, Formatting Numbers, and Catching Errors

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Variables, Calculations, Formatting Numbers, and
Catching Errors

• Part 5 dbg

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Primitive Data Types
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Data Types (See table on page 85)
int whole numbers in the range 2.1billion 4 bytes
long whole numbers in the range 9.2E18 8 bytes
float floating point numbers (6 digits of precision) 4 bytes
double floating point numbers with greater range and precision (14 digits of precision) 8 bytes
bool Boolean values, TRUE or FALSE 1 byte
string alphanumeric data letters, numbers, punctuation varies
char single Unicode character 2 bytes
byte 0 to 255, binary data 1 byte
decimal monetary values with accurate rounding (28 digits of precision) 16 bytes
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Variables

• A variable is a programmer-assigned name
  associated with a storage/memory location the
  word variable implies that the associated value
  may change during the program lifetime.
• A variable must be declared before it can be
  used. The declaration must consist of the data
  type the variable will store and a legal
  identifier.
• decimal grossPay int age
• bool latePayment string firstName
• Use camel casing for variable identifiers, but do
  not use data type prefixes, as in textbook.

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Named Constants

• Use a named constant for a value that will not
  change during the program lifetime.
• Use of named constants adds meaning to your
  program and allows for a change in one location
  rather than throughout your code.
• Use the keyword const in front of your
  declaration and type the identifier in all caps.
• const decimal SALES_TAX_RATE 0.0825d
• const string STATE NY
• Named constants are typically declared before
  other variables in your program.

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Intrinsic Constants

• Intrinsic constants are constants defined in
  system class libraries.
• You may use them in your program.
• Color.Red, Color.Yellow, and Color.AliceBlue are
  color constants declared in the Color class.
• /the following statement sets the background
  color of form to Alice Blue color at run time/
• frmOriginal.BackColor Color.AliceBlue

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Initialization of Variables

• Assigning a starting value to a variable is
  called initialization.
• Declaring an initial value for a variable is
  optional, but a variable must have a value before
  it is used in an assignment or calculation.

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Suffixes For Real Numbers

• Numeric values assigned to variables of certain
  real types must have specific suffixes to avoid
  compiler errors.
• Any real (non integer numeric) number is
  automatically regarded as a double it is denoted
  with a d suffix.
• Add the f suffix to make the real number a float
  (single).
• Add the m suffix to make the real number a
  decimal.

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Variable Scope
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Variable Scope (See pg 93)

• Access to variables can be controlled by where
  you declare them.
• Variables declared within a class (ex. form), but
  outside any other programming block of the class
  are available anywhere in the class (class
  scope) declare class scope variables outside of
  functions.
• Variables declared within a programming block,
  such as within a function, are only available in
  that block (local or block scope).

? VariableScopes
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Hiding a Class Variable

• If a block scope variable has the same name as a
  class scope variable, the value of the local
  variable will hide the value of the class
  variable within the block.
• The original value of the class scope variable
  remains intact outside the block.
• This is not a recommended practice.

? VariableHiding
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Parsing Numbers from Strings

• Even though we may intend a TextBox to allow
  entry of a numeric value, anything typed in a
  TextBox becomes a string value.
• Numeric values may be extracted from the string
  with the Parse() method of one of the numeric
  types.

? ParseInteger
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Math

• Simple Operations

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Operators and Operands

• A math operation involves a math operator working
  with at least one operand.
• Operands are either variables or constants.
• The result of the operation is often assigned to
  another variable.
• The sign represents the assignment operator.

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Operators
Addition a b
- Subtraction a - b
Multiplication a b
/ Division a / b
Remainder a b
- Unary negative -a
Unary positive a
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Operator Precedence

• Operators within parentheses are applied first
  the innermost parenthesis of a nest.
• Unary operators applied next multiple unary
  operators applied left to right.
• Multiplication, division and remainder applied
  next multiple operators applied left to right.
• Addition and subtraction operators applied last
  multiple operators applied left to right.
• These rules are commonly called Order of
  Operations.

? Precedence
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Assignment Operators

• Instead of the following statement
• counter counter 1
• You can use the following statement
• counter
• Instead of the following statement
• number number 5
• You can use the following statement
• number 5
• Likewise there are other shortcut assignment
  operators
• - /

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Assignment Operators
Operator Sample Explanation
a 5 a a 5
- b - 4 b b - 4
c k 1 c c (k 1)
/ d / 2 d d / 2
k k k 1 (increment)
-- m-- m m -1 (decrement)
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Assignment Operators

• The assignment operator works with string
  variables, as well as with numeric variables.
• lblMessage.Text //empty string
• lblMessage.Text Hello,
• lblMessage.Text txtFirst.Text
  txtLast.Text
• lblMessage.Text . How are you today?
• If I type my name in the TextBoxes, the code
  above would display the following text in
  lblMessage Label.
• Hello, Dee Gudmundsen. How are you today?

? AssignOps
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Converting from One Numeric Data Type to Another

• Certain value types may be converted to others
  more directly using casting.
• In the example below, notice the different use of
  parentheses between casting with C and casting
  with C.
• Truncation may take place when floating point
  values are cast to integer values.
• ANSI values result when char variables are cast
  to int variables.

myInt (int) myDouble
? Casting
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Outputting Variable Values
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Displaying a Constant Value

• We can display the value of a string type
  constant by assigning it to the Text Property of
  a Label control at run time because the Text
  Property is of type string.

? StringConstant
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Displaying a Variable Value

• We can display initial values of variables.
• We can assign new values to variables and display
  them as well.
• String variables can be assigned directly to the
  Text Property of a Textbox or Label.

? StringVariable
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Displaying Numeric Values

• Values stored in variables with types other than
  string must be converted to strings before
  assigning them to a string variable or a Text
  Property.
• Variables of other types all have a ToString()
  function for this conversion.

? NumericVariables
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Rounding

• Use the Round(), Ceiling() or Floor() methods of
  the Math class for various types of rounding.
• The Round() method uses a technique called
  rounding toward even.

? Rounding
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Changing the Format of Numeric Values While
Outputting

• You can specify a specific format for the output
  string by placing a format specifier within () of
  the ToString() method.
• decimal extendedPrice 109.8765d
• lblPrice.Text extendedPrice.ToString(C)
• These statements display the extended price as
  money in the Price label.
• displays as 109.88

? FormatSpecifiers1
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Format Specifiers (See pg 102)
C Currency using local settings with 2 decimal places
E Scientific notation in powers of 10
F Fixed decimal places 2 by default
G General E or F chosen on basis of length
N Thousands separated by commas 2 decimal places by default
P Percent
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Lets Try Some Output Specifiers

• Page 103, Feedback 3.6, questions 1-3

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Finding Errors

• Writing Values to the Output Window

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Tracking Calculations with Debug

• The Debug object of the System.Diagnostics
  namespace has some useful methods for examining
  code while it is running.
• Running the WriteLine() method allows you to
  write information to the Output Window while the
  program is running.

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Tracking Calculations with Debug

• This represents a very quick way to peek into a
  running program and view the actual contents of
  variables without having to show them with
  controls.

? DebugWriteLine
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Making Code User Error Proof

• Try/Catch

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Trapping Run Time Errors

• Certain problems with data may cause a program to
  abnormally terminate or crash at run time.
• Code that could cause such problems can be
  trapped.
• Trapped code can be run in trial mode and
  rejected if actually completing the instructions
  could lead to a crash.

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Try/Catch Trap

• This is a simple structure that encloses the code
  within braces following the keyword Try.
• If an error condition is encountered, code
  enclosed in braces following the keyword Catch is
  executed.
• The code runs normally if no error condition
  exists.

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Try/Catch Trap

• Although coding the Catch portion of the trap is
  not required, a standard technique is to use a
  MessageBox or Label text to alert the user that
  the Try has resulted in an error.
• Parsing a string for a numeric value is an
  excellent candidate for a Try/Catch trap.

? ParseTryCatch
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Remember Try/Catch

• It is always a good idea to use Try/Catch traps
  when doing calculations on values from user
  input.
• You can retrieve a meaningful error message if
  you declare an object of type Exception in the
  Catch.
• Run the Message() method of the Exception object
  and display in a MessageBox.

? Exceptions
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Providing Feedback to User

• The MessageBox

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Message Box Class

• Another class from the Framework Class Library.
• The object produced is a small, modal form,
  containing at least one button.
• A modal form can not lose focus until it is
  closed.
• The MessageBox is closed with one of its buttons.

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MessageBox Class Interface

• Run the Show() method to display a MessageBox
  object.
• Pass an informational message to the Show()
  method as a string type argument.

? SimpleMessage
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MessageBox Class Interface

• No Properties are revealed for the MessageBox
  class but we can customize the MessageBox by
  supplying additional arguments or parameters.
• It is possible to display a title bar caption, a
  meaningful icon, and a set of buttons, depending
  on the arguments passed to the Show() method.
• Intellisense displays a descriptive prompt for
  the list of optional arguments.

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Argument Lists

• Any time we run a function with arguments,
  Intellisense will display a prompt that describes
  them.

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Multiple Argument Lists

• Sometimes, as in the case of the MessageBox
  Show() method, the underlying function may have
  more than one argument list.
• We say that the method presents multiple
  interfaces or signatures.
• Intellisense allows us to choose which argument
  list we want to use for the method.

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Overloaded Functions

• The term overloaded function is used when there
  are multiple versions of a function (with the
  same name, but with different argument lists).
• All the signatures of all of the overloaded
  function appear in Intellisense.

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Adding a Caption

• A second argument sends a string type caption
  that will appear in the title bar of the
  MessageBox form.

? MessageCaption
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Returning a Value

• So far, the versions of the Show() method we have
  run have represented void functions.
• Including the buttons argument to the list
  requires that the MessageBox be used as a
  function that returns a value.

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Returning a Value

• The user causes a different constant to be
  returned, depending upon the button used to close
  the MessageBox.

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Buttons Arguments

• Several different patterns of buttons may be
  added to the MessageBox.
• Intellisense provides a list of the available
  values for the argument.

? MessageButtons
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Icon Arguments

• Special icons can be added to emphasize the
  meaning of a MessageBox.
• Requires an additional argument.
• MessageBox icons can be identified as providing
  general information, questions, important
  information and warnings.

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Icon Arguments

• Intellisense provides a list of the available
  values for the argument.

? MessageIcons