lecture

Goto statement
Goto label

jmp instruction in assembler
cmp, followed by a jnz instruction

for loop, while loop, examples of syntactic sugar


in BASIC, my first program:
10 Print "Hello"
20 Goto 10

in Visual Basic:
here:
    Debug.Print("Hello")
    Goto here
    Debug.Print("Goodbye")

an infinite loop never ends
how would we do this without Gotos?

Do
    Debug.Print("Hello")
Loop

    I = 0
here:
    Debug.Print("Hello" & I)
    I = I + 1
    If i < 3 Then
        Goto here
    EndIf

I = 0
Do
    Debug.Print("Hello" & I)
    I = I + 1
Loop While i < 3

' this is called a post-test

what if we want a pretest?
I = 0
Do While i < 3
    Debug.Print("Hello" & I)
    I = I + 1
Loop

I = 0
goto there
here:
    Debug.Print("Hello" & I)
    I = I + 1
there:
    If i < 3 Then
        Goto here
    EndIf

So why don't we use Gotos?!
1) simpler
2) higher
3) more controlled
4) spaghetti code

http://en.wikipedia.org/wiki/Spaghetti_code

10 i = 0
20 i = i + 1
30 PRINT i; " squared = "; i * i
40 IF i >= 10 THEN GOTO 60
50 GOTO 20
60 PRINT "Program Completed."
70 END

FOR i = 1 TO 10
    PRINT i; " squared = "; i * i
NEXT i
PRINT "Program Completed."
END

a move to eliminate gotos
Goto statement considered harmful

http://www.u.arizona.edu/~rubinson/copyright_violations/Go_To_Considered_Harmful.html

Goto statement considered harmful considered harmful

rules for using goto:
1) it should always be fairly close
2) only goto above you, never below you



  Dim response As Integer, quotation As String = ""

  Do
    response = CInt(InputBox("Enter a number from 1 to 3."))
  Loop Until response >= 1 And response <= 3

functions - return a value, which we can use


x = Int(2.6)
y = Int(7.6) + 6

Public Class Form1
    Function Foo() As String
        Dim x As Integer
        x = 7
        Return "Hello"
    End Function

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim s As String
        s = Foo()
        Button1.Text = s
    End Sub
End Class


y = f(x)

money today is worth more than money a year from now
FV = PV * (1+RATE)^NPER


but can compount mult times per year. need divide by freq

HW: Chapter 4 Programming Projects
 1-4

notes from lecture

static, local, class-level variables
lifetime vs. scope
final is wendesday, 12/15, from 4-6 PM in our regular room

Midterm answers

Midterm for class 1
Midterm for class 2

lecture notes

Subs, part II
ByVal
ByRef

actual vs. formal parameters

changes to ByVal parameters do not "stick"
changes to ByRef parameters do "stick"

a reference is an arrow we can follow to the variable
a reference is an alias to the variable; an alias is another name for the variable

passing them in order
keyword parameters

we got up to end of example 2

Another bit of feedback

regarding one of the practice midterms:

The question:
On the tracing ex. Y’s final value is 4. It therefore should be that the statement should be the else clause because  <>4 or <>5 is false.
Is that correct bec. On the exam’s answer you wrote that the answer is it is there, which is the then statement.

My Answer:
https://docs.google.com/document/pub?id=1ukHo2jysIGz1wXkkpsk80-d9n9vQIpzNb4lMqEUscJA&pli=1

this is a very tricky question which, if i recall correctly, nobody got right.

Indeed, Y's final value is 4.
Consider: If y <> 4 Or y <> 5 Then

y <> 4 evaluates to False
y <> 5 evaluates to True, because 4 is not 5
So, False Or True is True
This becomes If True Then

So it will go into the If clause rather than the Else clause.

As I noted, this is a very tricky question, but if you copy the code into Visual Basic, you will see that this is what happens. People get it wrong because of the disconnect between human speech, such as English, and programming languages. You read it as "if it not 4 or five" but that is not what it means.

lecture notes

HW:
Sub Procedures, Part I
question 6, 8, 36, 38, 48-54 even
look at answer to one of the odd questions to get a sense of how to do it.

answer to q 38 for If blocks
If year mod 400 = 0 Then
'leap
ElseIf year mod 100 = 0 Then
' not leap
ElseIf year mod 4 = 0 Then
'leap
Else
' not leap
Endif


' leap year
If year mod 400 = 0 Or (year mod 100 <> 0 And year mod 4 = 0) Then
MsgBox("Leap year!")
MessageBox.Show("Leap year!")
Else
MsgBox("Not a leap year!")
Endif

Write a program that allows the user to use a button to toggle the color of the text in a text box between black and red.
' this will NOT work
If textbox1.ForeColor = Color.Black Then
textbox1.ForeColor = Color.Red
Endif
If textbox1.ForeColor = Color.Red Then
textbox1.ForeColor = Color.Black
Endif

' this WILL work
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        If TextBox1.ForeColor = Color.Black Then
            TextBox1.ForeColor = Color.Red
        Else ' If TextBox1.ForeColor = Color.Red Then
            TextBox1.ForeColor = Color.Black
        End If
    End Sub

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        TextBox1.ForeColor = Color.Black
    End Sub


    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        TextBox1.Visible = Not TextBox1.Visible
    End Sub

biggest:
Public Class Form1
    Dim biggest As Integer
    Dim secondBiggest As Integer

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim theNum As Integer
        theNum = Val(TextBox1.Text)

        If theNum >= biggest Then
            ' update my maxes
            secondBiggest = biggest
            biggest = theNum
        ElseIf theNum > secondBiggest Then
            secondBiggest = theNum
        End If
      
    End Sub

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        TextBox1.ForeColor = Color.Black
    End Sub

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        Button2.Text = "Biggest: " & biggest & vbTab & "SecondBiggest: " & secondBiggest
    End Sub
End Class