CIS162AD C

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CIS162AD C

• Variables and Calculations
• 02_variables.ppt

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Overview of Topics

• Operating System and Memory Allocation
• Declaring Variables
• Data Types
• Text Boxes - for input and output
• Arithmetic Operators
• Designing User-Friendly Interfaces

3
Computer Resources

• As we go through the course, we will want to
  conceptually understand
• Memory allocation
• Storage
• Well cover storage later, butlets look at
  memory now

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Software Two Major Categories

• Operating System (OS)
• Application Software

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Operating System (OS)

• Software that allocates and monitors computer
  resources including memory allocation, storage,
  and security.
• Various devices are controlled using system
  programs called drivers.
• OS ExamplesWindows, UNIX, DOS, VMS, etc.

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Main Memory

• Random Access Memory (RAM).
• Contents are lost when power is turned off.
• Measured in bytes.
• Each byte stores eight bits.
• BIT Binary Digit (0 1)
• Each byte is considered a location.
• Each byte has an address to identify its
  location.
• Application currently running and the data being
  manipulated must be loaded in memory (RAM).
• CPU only gets and stores data in RAM.

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Memory Location
Address 1 Address 2 Address 3 Address 4 Address
5 Etc All consecutive
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Trivia Question

• What is a nibble?
• A nibble is half a byte.
• A byte is 8 bits, so a nibble is 4 bits. ?

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Application Software

• Actual software we use to process raw data into
  information.
• Word, Excel, Powerpoint, Access, etc.
• Application software is used in many industries.
• Business Accounting, Sales
• Manufacturing Inventory, Labor
• Education Enrollment, Research
• Personal Budgeting, School
• Too many applications to list

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CS2 Dept Contact

• CS2 is a program that displays the contacts for
  various departments.
• The contact information is hard-coded in the
  program. (phoneLabel.Text 555-3434)
• To create a more flexible and dynamic program we
  can use variables.
• Variables are assigned storage locations in
  memory.
• The values in these memory locations can vary
  during the execution of the program.
• Hard-coded values can not vary.
• Beginning with CS3 we will be using variables.

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Variable Declaration

• Variables are declared by specifying the data
  type and name of the variable.
• Syntax dataType variableNameint
  intQtydecimal decPrice string strName
• The dataType specifies that only numbers or
  strings can be assigned to the named variable.

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Variable Initialization

• When a variable is declared they are assigned a
  default value.
• String variables are set to null or nothing.
• Numeric variables are set to zero, but the
  compiler will require that they be initialized
  before using them in a calculation or trying to
  display them.
• Treat local numeric variables as if they have not
  been initialized.
• Variables can be initialized to a value when
  declared or by assigning a value to them.

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Assigning Values to Variables

• Assigning values at declaration is called
  initialization.
• To initialize use the equal sign followed by a
  value.
• int hours //not initialized
• int hours 0 //initialized to zero
• Assignment statement is the use of the equal
  sign
• decPrice decimal.Parse (priceTextBox.Text)
• decGross intHours decRate

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When to Declare Variables

• Variables can technically be declared anytime
  before they are used.
• However, in structured programming, we define our
  variables at the beginning of a program or
  method.

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Variables Allocated Memory

• Within CS3s memory allocation, variables are
  assigned a memory location.
• Each time the program is ran, a different address
  may be assigned.
• The computer uses the memory address, but we
  programmers reference that location with the
  variable name.

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Memory Allocation
Applications
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Variables in Memory
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Memory Analogy
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Naming Rules

• Must begin with a letter.
• intHours intQty decPrice
• Rest can be letters, digits, or underscores.
• intHours intHours1 intHours_1
• C is case sensitive.
• inthours intHours intHOURS
• Each of these would reference a different memory
  location.
• Keywords or reserved words cannot be used.

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Preferred Naming Conventions

• Variables should have meaningful names.
• Precede each variable name with a prefix of three
  letters in lowercase to clarify the data type.
• Camel-case all lowercase with the first letter
  of significant words in upper case . intQty,
  decPrice, strName, intHoursWorked
• Variables defined as constants (will not change
  value during the execution) should be in all
  uppercase with significant words separated with
  an underscore. decTAX_RATE, decTUITION_RATE
• Use the const modifier for constants.const
  decimal decTAX_RATE 0.07M

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Tribune, 12/30/1999

• Programmers Double-check for Last-minute Y2K
  Bugs.
• Experts said early efforts focused on checking
  dates typically identified with a heading
  mm-dd-yy or date buried within computer
  code. But prankster programmers sometimes used
  unusual names that can make these data variables
  nearly impossible to find.

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Tribune, 12/30/1999 continued

• Data Integrity said it found a date variable
  called Shirley The programmer responsible, it
  turned out, was dating a woman named Shirley when
  he wrote the software.
• Air Force experts compete in a variable of the
  week contest to find the most obscure title for
  a date field.
• The name of a girlfriend, athlete, movie stars is
  unfortunately all too common as programmers
  express their creative free will.

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Other Significant Changes

• Area code in Phoenix required (2000).
• Year Two Thousand Y2K (1999).
• Zip4
• Income tax laws and tax rates.
• Sales tax rates different in each city.
• The point is that there will always be some
  maintenance on programs, so make it easy to
  maintain.

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Use Good Variable Names
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Variable Scope

• Variables can only be referenced within the
  section of code it was declared in.
• Namespace variables may be referenced in entire
  project (multi-form project).
• Class-level variables may be referenced in all
  methods of a form.
• Local variables may be referenced only within the
  method in which it was declared.
• Block variables may be referenced only within a
  block of code inside a method.
• if and do statements create a block of code as
  well as the open and close braces .

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Global vs Local

• These terms are used to group the type of
  variables that can be created.
• Global refers to the Class-level and Namespace
  variables.
• Local refers to the Method and Block variables
• When using class-level variables, an additional
  prefix is added to the name (c). decimal
  cdecTotalPay

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Lifetime of Global Variables

• Class-level and Namespace variables exist for the
  entire time a form is loaded.
• Use global variables to store constants.
• Use global variables to store a running total
  (accumulation) or count that is displayed at the
  end of a session (ie number of transactions
  posted, total sales).

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Class-Level Variables

• namespace CS3
• public class CS3Form int cintQuantitySum in
  t cintSaleCount const decimal cdecDISCOUNT_RATE
  0.15M private void calculateButton_Click()
  class-level variables can be referenced
  here private void summaryButton_Click()
  class-level variables can be referenced
  here

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Lifetime of Local Variables

• The lifetime of a variable is the period of time
  that the variables exists.
• Method and Block variables exist for one
  execution of the method.
• Each time a method is executed, the local
  variables are created again and initialized to
  its default value.
• When the method is exited, its variables are
  destroyed, and their memory locations are
  released.
• Any values that were stored in the local
  variables are gone.

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Sub Procedure Variables

• namespace CS3
• public class CS3Form int cintQuantitySum in
  t cintSaleCount const decimal cdecDISCOUNT_RATE
  0.15M private void calculateButton_Click()
  int intQuanity decimal decPrice
  private void summaryButton_Click()
  decimal decExtended decExtended
  intQuantity decPrice //Error - intQuantity
  and decPrice not defined in procedure

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Global Variable Misuse

• Do NOT use global variables to store values that
  change and could be declared as local variables
  in methods. Even if a variable of the same name
  and type is needed in various procedures.
• Using global variables to store local values
• leads to bad programming habits
• makes the code in methods less reusable in other
  programs (same variable names would need to be
  used in all programs).
• Later we will be defining our own methods, and we
  will understand better why Global variables are
  not necessarily good.

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Data Types

• Integer
• Floating Point
• Alphanumeric
• Boolean
• Date

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Integers whole numbers ( or -)
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Floating Point decimal values
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Alphanumeric, Boolean, and Date
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Input/Output

• We need a user interface to get their Input and
  to display processing results as Output.
• We use variables to store the values entered by
  the user and to store the results of the
  processing.
• In C, textboxes are the most common control
  object used to facilitate I/O.

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TextBoxes

• Contents of a textbox is always a String.
• Can get and assigned values to a textbox.
• Use the property Text. string
  strName strName txtName.Text //Input txtN
  ame.Text strName //Output

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Labels as Prompts

• It is important to display a prompt to the user
  so they know what input is expected.
• A prompt is a brief description or label for the
  data to be entered.
• The user interface must be friendly.
• Use control object Label. Name Address

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Converting with Parse Method

• Numbers are also stored as Strings in textboxes.
• The String must be converted to a number before
  being assigned to a numeric variable, and before
  being used in a arithmetic expression.
• Each datatype has a Parse method for
  conversion. int intQty decimal
  decPrice decimal decSubtotal intQty
  int.Parse(quantityTextBox.Text) decPrice
  decimal.Parse(priceTextBox.Text) decSubtotal
  intQty decPrice

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ToString Function

• To display a number in a textbox it is must be
  converted from a number to a String.
• Use built-in function ToString. decSubtotal
  intQty decPrice subtotalTextBox.Text
  decSubtotal.ToString(N)
• N is a Format Specifier Code see next slide

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Format Specifier Codes for ToString

• Use the codes to format the display of output.
• N stands for Number
• Adds comma and includes 2 digits to the right of
  the decimal point.
• C stands for Currency
• Adds dollar sign, comma and includes 2 digits to
  the right of the decimal point
• Specify a specify number of decimal positions by
  including a number in the string C0, N4
• The value is rounded to the specified number

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Arithmetic Operations

• Arithmetic operations are the mathematical
  operations we can perform in our programs.
• Add () and Subtract (-)
• Multiply () and Divide (/)
• Modulus (), returns the remainder of a division
  operation (intMin intTotal 60)
• Exponentiation use Pow method of Math class.
• Use parentheses to specify which operations
  should occur first.

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Order of Precedence

• Inner to outer parentheses
• Left to right
• Multiplication and Division
• Modulus
• Addition and Subtraction
• Which is correct for 5 discount
• decDiscountAmt decQty (decPrice decPrice
  .05)
• decDiscountAmt decQty decPrice decPrice
  .05

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Shortcut Operators

• The following assignment operators are shortcuts
  for the standard longer forms of some
  expressions.
• , -, , /,
• Accumulation
• cdecQtySum cdecQtySum decQty
• cdecQtySum decQty
• Count
• cintSaleCount cintSaleCount 1
• cintSaleCount 1

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Increment and Decrement Operators

• adds one to a variable
• cintSaleCount cintSaleCount 1
• cintSaleCount 1
• cintSaleCount
• -- subtracts one to a variable
• cintSaleCount cintSaleCount - 1
• cintSaleCount - 1
• cintSaleCount--

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Program Remarks

• Comments should be included in each program (see
  Grading Criteria handout).
• Comments are ignored by the compiler.
• Comments are for you and other programmers that
  will eventually need to come back to maintain the
  program.
• Use two slashes (//) for single line remark.
  //This is a single line comment
• Use the open (/) and close (/) remark for a
  multi-line remark./ This is a multi-line
  comment used for longer comments/

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Designing User-Friendly Interfaces

• How are access keys defined?
• How is the Default button Property set? (Enter
  Key)
• How is the Cancel button Property set? (ESC Key)
• What is meant by a control object having focus?
• What is Tab Stop and Tab Index properties used
  for?
• What are Tool Tips used for?
• Can focus and control properties (forecolor,
  text) be changed at runtime using C Code?(See
  last slide for answers)

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Summary

• Operating System and Memory Allocation
• Declaring Variables
• Data Types
• Input/Output
• Arithmetic Operations
• Designing User-Friendly Interfaces

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Designing User-Friendly Interfaces

• Text Property Exit for Exit and use Alt-x
• Set AcceptButton property on the form to a button
  name.
• Set CancelButton property on the form to a button
  name.
• It is the object that the user is currently on
  and can type into it or press enter to select it.
• TabStop determines if a user can tab to the
  control, and TabIndex determines the order the
  user will move from control to control when the
  tab key is pressed.
• Provides hints to the user when they mouse over a
  control.
• Yes.