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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

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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