Review. October 20, 2006

Transcription

1 Review October 20,

2 A Gentle Introduction to Programming A Program (aka project, application, solution) At a very general level there are 3 steps to program development Determine output Determine input Determine how to process the input to get the output Input Process Output 2

3 Program Development Cycle 1. Analyze: define the problem 2. Design: plan solution 3. Choose interface: Select objects (text boxes buttons, etc) 4. Code: Translate to prog. language 5. Test and debug 6. Document We ll see this in action as we progress through the material 3

4 Algorithms A step-by-step procedure for solving a problem or achieving a goal Consider a recipe Food Cooking Dish 4

5 Pseudocode An abbreviated form of computer code written in natural language Uses structural conventions of programming languages Abstracts away from implementation specifics No language-specific syntax Focus is on steps to solution Pseudocode can be easily translated into programming language (VB) More compact that flowcharts 5

6 Pseudocode Example Program: Add 5 to a number Get number Add 5 to number Display number Note there are a variety of ways to express something in pseudocode 6

7 Start Flowcharts Input Graphical representation of the logical steps to carry out a task logical flow Shows how one task relates to another visually Path taken dependent on input The flowchart originated in computing science No Process A Decision Output Yes Process B End 7

8 Hierarchy Chart Calculator program Add numbers Subtract numbers Divide numbers Multiply numbers 8

9 Four Controls at Design Time Text box To select a control, click on it. Sizing handles will appear when a control is selected. 9

10 The three steps in creating a Visual Basic program: 1. Create the interface; that is, generate, position, and size the objects. 2. Set properties; that is, configure the appearance of the objects. 3. Write the code that executes when events occur. 10

11 Event An event is an action, such as the user clicking on a button Usually, nothing happens in a Visual Basic program until the user does something and generates an event. What happens is determined by statements. 11

12 Examples of Events btnshow.click txtbox.textchanged txtbox.leave General Form: controlname.event 12

13 Structure of an Event Procedure Header Private Sub objectname_event(...) Handles objectname.event statements End Sub (...) is filled automatically with (ByVal sender As System.Object, ByVal e As System.EventArgs) 13

14 Data Types Integer Type Holds integers between ~ -2 billion to 2 billion Double Type Holds whole, or fractional numbers String Type Holds strings of characters Boolean Type Holds true or false 14

15 Three Types of Errors Syntax error E.g. lstbox.items.add(2 + ) Run-time error numvar = & Logic error Average = m + n / 2 15

16 Data Conversion dblvar = CDbl(txtBox.Text) intvar = CInt(txtBox.Text) txtbox.text = CStr(numVar) 16

17 String Properties and Methods "Visual".Length is 6. "Visual".ToUpper is VISUAL. "123 Hike".Length is 8. "123 Hike".ToLower is 123 hike. "a" & " bcd ".Trim & "efg" is abcdefg. hello world".substring(0,2) is he. hello world".indexof( e ) is 1. 17

18 Formatting Output with Zones Use a fixed-width font such as Courier New Divide the characters into zones with a format string (add - for left justification). Dim fmtstr As String = "{0,15:N1}{1,10:C2}{2,8:P0}" lstoutput.items.add(string.format(fmtstr, _ data0, data1, data2)) 18

19 Reading Data from Files Data can be stored in files and accessed with a StreamReader object. We assume that the files are text files (that is, have extension.txt) and have one piece of data per line. Dim readervar As IO.StreamReader readervar = IO.File.OpenText(filespec) 19

20 Getting Input from an Input Dialog Box stringvar = InputBox(prompt, title) filename = InputBox("Enter the name " _ & "of the file containing the " & _ "information.", "Name of File") Title Prompt 20

21 Using a Message Dialog Box for Output MsgBox(prompt, 0, title) MsgBox("Nice try, but no cigar.", 0, _ "Consolation") Note the 0 is a code for the type of button displayed. Title Prompt 21

22 Masked Text Box Similar to an ordinary text box, but has a Mask property that restricts what can be typed into the masked text box. State abbreviation: LL Phone number: Social Security Number: License plate: &&&&&& 22

23 Restricting Behaviour Option Strict: Turn strict typing on (VB will not automatically cast from one type to another) Option Strict On Option Explicit: Must pre-define variables before they are used (using dim) O/w VB uses variant type -- a catchall type Option Explicit On 23

24 Devices for modularity Visual Basic has two devices for breaking problems into smaller pieces: Sub procedures Function procedures 24

25 Sub Procedures Perform one or more related tasks General syntax Sub ProcedureName() statements End Sub Invoked via a call statement: ProcedureName() 25

26 Example lstbox.items.clear() ExplainPurpose() lstbox.items.add("") Sub ExplainPurpose() lstbox.items.add("program displays a sentence") lstbox.items.add("identifying a sum.") End Sub 26

27 Arguments and Parameters Sum(2, 3) arguments parameters Sub Sum(ByVal num1 As Double, ByVal num2 As Double) displayed automatically 27

28 Example with Variables Dim n1 As Integer Dim n2 As Integer Private Sub btnadd_click(...) Handles btnadd.click Sum(n1, n2) End Sub Sub Sum(ByVal num1 As Double, ByVal num2 As Double) DisplayPurpose() lstbox.items.add("the sum of " & num1 & " and " _ & num2 & " is " & (num1 + num2) & "." End Sub 28

29 Passing By Value and By Reference Memory Location 1 This refers to the value of whatever is in this memory location not the location itself ByVal Procedure Parameter This refers to anything stored in this memory location ByRef ProcedureParameter Memory Location 1 Any changes to this, do not effect what is stored in the memory location 1 it s just a copy Any changes to this do effect what is stored in the memory location 1 it s not a copy 29

30 Local & Class-level Variables Dim num As Integer = 4 Private Sub btnone_click(...) Handles btnone_click Dim num As Integer = 4 SetFive() num only exists txtbox.text = CStr(num) within this sub End Sub procedure -- this called it s scope Sub SetFive() Dim num As Integer = 5 End Sub Output: 4 this num is completely unrelated to the one above 30

31 Function Procedures Function procedures (aka user-defined functions) always return one value Function FunctionName(ByVal var1 As Type1, _ ByVal var2 As Type2, _ ) As datatype statement(s) Return expression End Function Call: myvar = FunctionName(var1,var2, ) 31

32 Top-Down Design General problems are at the top of the design Specific tasks are near the end of the design Top-down design and structured programming are techniques to enhance programmers' productivity 32

33 Top-Down Design Criteria 1. The design should be easily readable and emphasize small module size. 2. Modules proceed from general to specific as you read down the chart. 3. The modules, as much as possible, should be single minded. That is, they should only perform a single well-defined task. 4. Modules should be as independent of each other as possible, and any relationships among modules should be specified. 33

34 Detailed Hierarchy Chart 34

35 Structured Programming Control structures in structured programming: Sequences: Statements are executed one after another. Decisions: One of two blocks of program code is executed based on a test for some condition. Loops (iteration): One or more statements are executed repeatedly as long as a specified condition is true. 35

36 Object-Oriented Programming an encapsulation of data and code that operates on the data objects have properties, respond to methods, and raise events. Like a taxonomy Living Things Plants Animals 36

37 ANSI Characters For n between 0 and 255: Chr(n) For a string str, the ANSI code of the first character: Asc(str) EXAMPLES: Chr(65) is "A" Chr(162) is " " Asc("A") is 65 Asc(" 25") is

38 Relational Operators < less than <= less than or equal to > greater than >= greater than or equal to = equal to <> not equal to ANSI values are used to decide order for strings (works left to right in case of equal letters). E.g. AB < BC becase 65 < 66 AB < AC becase 66 < 67 38

39 Logical Operators AND OR NOT ((6<7) AND (10>2)) OR 1=1 A=8; B=10 ((87=A) OR (B>A)) AND (A=8) 39

40 If Statement If condition1 Then action1 ElseIf condition2 Then action2 ElseIf condition3 Then action3 Else action4 End If 40

41 Select Case Block The general form of the Select Case block is Select Case selector Case valuelist1 action1 Case valuelist2 action2 Case Else action of last resort End Select 41

42 Rules for Select Case Case Else (and its action) is optional Each value list contains one or more of the following types of items separated by commas: 1. a literal; 2. a variable; 3. an expression; 4. an inequality sign preceded by Is and followed by a literal, variable, or expression; 5. a range expressed in the form a To b, where a and b are literals, variables, or expressions. 42

43 Flags A flag is a variable that keeps track of whether a certain situation has occurred. The data type most suited to flags is Boolean. 43

44 Example 4: Form The file WORDS.TXT contains words from a spelling bee, one word per line. Count the words and determine whether they are in alphabetical order. 44

45 Example 4: Partial Code Dim word1 As String = "" Dim orderflag As Boolean = True Do While (sr.peek <> -1) word2 = sr.readline wordcounter += 1 If word1 > word2 Then orderflag = False End If word1 = word2 Loop 45

46 More About Flags When flagvar is a variable of Boolean type, the statements If flagvar = True Then and If flagvar = False Then can be replaced by If flagvar Then and If Not flagvar Then 46

47 For Next Loops Used when we know how many times we want the loop to execute A counter controlled loop (this is i below) Dim i As Integer For i = 1 To 5 lsttable.items.add(i & " " & i ^ 2) Next 47

48 Example 2 Control variable Type Decl Start value Stop value Amount to add to index For index As Integer = 0 To n Step s Next lstvalues.items.add(index) 48

49 Do Loops A loop is one of the most important structures in programming. Used to repeat a sequence of statements a number of times. The Do loop repeats a sequence of statements either while or until a certain condition is true. 49

50 Do While Loop Syntax Do While condition statement(s) Loop Condition is tested, If it is True, the loop is run. If it is False, the statements following the Loop statement are executed. These statements are inside the body of the loop and are run if the condition above is True. 50

51 Do Until Loop (Post Test) Syntax Do statement(s) Loop Until condition Loop is executed once and then the condition is tested. If it is False, the loop is run again. If it is True, the statements following the Loop statement are executed. 51

52 Example Dim i As Integer = 1 Do For j As Integer = 10 To 0 _ Step -1 ListBox.Add.Items(j) ListBox.Add.Items(i) Next i += 1 Loop Until i > 10 Output: