Cs 102
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CS 102. Variables/Data Types. Overview. Data Types Variables (sec 3.2) Operations on variables Numeric Text Arrays All operations have order of precedence – remember that!! 2 + 3 *4 + 2 = 16. Overview. Formatting (sec 3.5) Msgbox (section 4.8) Debugging

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CS 102

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Cs 102

CS 102

Variables/Data Types


Overview

Overview

  • Data Types

  • Variables (sec 3.2)

  • Operations on variables

    • Numeric

    • Text

    • Arrays

  • All operations have order of precedence – remember that!!

    • 2 + 3 *4 + 2 = 16


Overview1

Overview

  • Formatting (sec 3.5)

    • Msgbox (section 4.8)

  • Debugging

    • Using print/msgbox statements

    • Simple debugging/tutorial

    • Exception handling


Data types

Data Types

  • Variables are always declared with a data type

  • Variable must follow rules for that data type

  • Very different than earlier versions of VB

    • In earlier versions, it was untyped

  • Basic data types:

    • String (Surrounded with double quotes)

      • “Hello world”

    • Boolean (True/False)

    • Byte (0-255)

    • Integer (-32,768 – 32,767)

      • Long (-2,147,483,648 – 2,147,483,647)


Data types1

Data Types

  • Basic data types:

    • Single (16 bit floating point)

    • Double (32 bit floating point)

    • Currency

    • Dates (Surround with # character)

      dtBirth = #5/1/2009#

  • Many, many others defined

    • Won’t need most/any of them for this class


Variables

Variables

  • “Boxes” that hold data of the proper type

  • Declared with the “Dim” statement

    • Short for “Dimension” – legacy Basic command

  • Examples:

    Dim strFirstName As String

    Dim intLoop As Integer

  • “As” tells VB the data type


Variables1

Variables

  • Variable names typically start with three letters of the data type

    • int, str, lng…

  • Variable names use camel-casing

    • Each word starts with a capital

    • No spaces

  • Variable names are meaningful!!!

  • These are guidelines, but most programmers use them

    • Make it easier to read programs

    • For this class, use this convention!!


Numeric operations

Numeric Operations

  • Basic math functions:

    • sumVal = sumVal + 2

    • intProd = 4 * 12

    • fVal = fVal / 3

  • Shorthand can combine the assignment and math

    • sumVal += 2

    • fVal /= 3


String operations

String Operations

  • Assign literal text with “=“

    Dim strTest as string

    strTest = “Hello world”

    Msgbox(strTest)

  • There are a huge number of string manipulation functions available:

    • strTest = “Hello” & “ there”

    • strTest = (4 / 3).ToString()

      • ToString converts from the data type of the variable to a string


Arrays

Arrays

  • An array is a collection of items of the same data type

    • Can access the array with an integer index

    • Arrays are all ZERO-BASED

      • Array goes from 0 to index - 1

        Dim strMessage(3) As String

        strMessage(0) = "First string"

        strMessage(1) = "Second string"

        strMessage(2) = "Third string"

        MsgBox(strMessage(1))


Arrays1

Arrays

  • Arrays can have multiple dimensions

  • Creates a two-dimensional grid of values

    • Can have more than two dimensions

      Dim strPronoun(3, 2) As String

      strPronoun(0, 0) = "Me"

      strPronoun(0, 1) = "You"

      strPronoun(0, 2) = "He"

      strPronoun(1, 0) = "Moi"

      strPronoun(1, 1) = "Tu"

      strPronoun(1, 2) = "Ils"

      MsgBox(strPronoun(1, 2))


Formatting

Formatting

  • There are many ways to format numbers/strings

    • ToString (saw this before)

      • Can specify a format for the conversion. Otherwise it makes the best guess it can

        Dim dblSample As Double

        Dim strResult As String

        dblSample = 1234.5

        strResult = dblSample.ToString("c")

        MsgBox(strResult)

        Answer in box is: $1234.50


Formatting1

Formatting

  • Some of the formats available:

    • n or N (number)

    • f or F (floating point)

    • c or C (currency)

    • p or P (percentage)

  • Formatting dates/times

    • d – Short date format (8/2/2009)

    • D – Long date format (Monday, August 2, 2009)

    • t – Short time format (3:22 PM)

    • T – Long time format (2:12:34 PM)

    • F – Long date and time format


Formatting2

Formatting

  • Format date and time example:

    Dim dtTest As Date = #8/23/2009#

    MsgBox(dtTest.ToString("D"))

  • Trim spaces from strings

    • TrimStart()

    • TrimEnd()

    • Trim()

      Dim strTest As String

      strTest = " X Hello Y "

      MsgBox(strTest.TrimStart)

      MsgBox(strTest.TrimEnd)

      MsgBox(strTest.Trim)


Formatting3

Formatting

  • substring – returns a part of a string

    • substring(x) – returns the string, starting with position number x

    • substring(x, y) = returns the string, starting with position x, for y characters

      Dim strTest As String

      strTest = "abcdefghijklmnop"

      MsgBox(strTest.Substring(3))

      MsgBox(strTest.Substring(4, 2))


Formatting4

Formatting

  • indexOf – Searches a string for a substring and returns the posisiotn of where it is

    • Returns -1 if string is not found

      Dim strTest As String

      strTest = "abcdefghijklmnop"

      MsgBox(strTest.IndexOf("j"))


Msgbox

Msgbox

  • Simple function that can be used almost anywyere

  • Pops up a dialog box with a string in it

    MsgBox(dtTest.ToString("D"))

  • More details in section 4.8

  • Msgbox(string) – prints out the string

  • Msgbox(string, caption) – prints out the string, with the caption as the title of the dialogue

  • Msgbox(string, caption, buttons) – Same as above, but with optional buttons


Msgbox1

Msgbox

  • MessageBoxButtons.OK – Only the OK button

  • MessageBoxButtons.OKCancel – Two buttons

  • Other combinations in section 4-8

  • Last argument is optional icon (see sec 4-8)

  • Example of Messagebox:

    Dim intRes As Integer

    intRes = MessageBox.Show(“Exit now?", "Exit program", MessageBoxButtons.YesNoCancel, MessageBoxIcon.Question)

    If intRes = Windows.Forms.DialogResult.Yes Then MsgBox("Pressed Yes!")

    If intRes = Windows.Forms.DialogResult.No Then MsgBox("pressed No!")

    If intRes = Windows.Forms.DialogResult.Cancel Then MsgBox("pressed Cancel!")


  • Exception handling

    Exception Handling

    • Can be VERY complicated subject

    • Really useful to learn/use

    • Basic idea: Figure out possible problems, and handle them before they cause an unexpected error in your program

    • What’s wrong with this program?

      If IsNumeric(TextBox1.Text) Then

      MsgBox(TextBox1.Text)

      Else

      MsgBox("That's not a number!")

      End If

      Assume that TextBox1 is a textbox we’ve added to the form.


    Exception handling1

    Exception Handling

    • Always think about the fact that they do not have to use the program the way that you intend them to (in fact, they usually won’t).

    • You MUST consider:

      • Error conditions (ex: divide by 0)

      • Empty data fields (they didn’t type in data)

      • Boundary problems (you assume the number is between 0-9. What if they enter 22?)

      • Other conditions

    • Must program DEFENSIVELY!!

      • We will cover how to do that in a later lecture


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