CS 102

1. CS 102 Variables/Data Types

2. 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

3. Overview • Formatting (sec 3.5) • Msgbox (section 4.8) • Debugging • Using print/msgbox statements • Simple debugging/tutorial • Exception handling

4. 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)

5. 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

6. 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

7. 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!!

8. 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

9. 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

10. 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))

11. 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))

12. 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

13. 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

14. 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)

15. 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))

16. 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"))

17. 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

18. 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!")

19. 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.

20. 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