VB Lecture Notes - Assignment Statements

Objective #1: Use assignment statements to calculate and store mathematical results and to change the properties of objects.

• An assignment statement is a statement that is used to store a value into a variable or object's property.
• Assignment statements evaluate the mathematical expression on the right-hand side of the assignment operator and assigns the resulting value into the variable on the left-hand side of the equals symbol (which is known as the "assignment operator").
• For example, the statement
  
  intNum = 5
  
  is an assignment statement that stores the value 5 into the variable named intNum.
  
  Instead of a simple numerical value like 5, a mathematical expression can be typed on the right side of the equals symbol as in
  
  dblAnswer = intNum * 1.99 + intNum * 1.99 * 0.06
  
  where the righthand side of the equals symbol must be simplified first before the final result is assigned to the variable dblAnswer. Never type an assignment statement with a mathematical expression on the left of the equals symbol ( e.g.    intNum + 5 = intAnswer ) since it causes a compile error.
  
  An assignment statement can also store a string value (which is a word or phrase) into a String variable or an object's property such as
  
  strName = "Minich"
  
  or
  
  lblAnswer.Text = strFirstName + " " + strLastName
  
  Notice that the + symbol is working as a concatenation operator rather than an addition operator in the example above. (Visual Basic also allows the & to be used as a concatenation operator so the statement above could be rewritten as
  
  lblAnswer.Text = strFirstName & " " & strLastName
  
  When you declare Integer and Double variables it is good style to initialize them to zero as in
  
  Dim dblAmount As Double = 0    or      Dim intNum As Integer = 0
  
  but you cannot initialize a String variable to zero since you can't store numbers in String variables. Therefore, it is safe to initialize a String variable to the empty string (aka null string). The empty string is a string with no characters and is represented as two double quote symbols typed next to each other with no space in between as in "". Here is a declaration statement that initializes a String variable to the empty string
  
  Dim strName As String = ""
  
  You can also assign the empty string to a String variable or the Text property of a textbox or label at anytime during a program. Examples:
  
  strName = ""
  
  lblWinner.Text =""
  
  
• Examples:
  

  dblRate = 0.035

  dblInterest = dblPrincipal * dblRate * intTime

  strName = "John Doe"

  REMEMBER, DO NOT CODE THE FOLLOWING:    x + 8 = y     which is "backwards" as an assignment statement.

• You must be careful distinguish between string data and numeric data (which includes Integer, Long, Single, & Double). For example, the string "13" is not equivalent to the numeric value, 13, that may be stored in a variable of the Integer data type. The double quotations are used to show that the string "13" cannot be added, subtracted, etc. from another numeric variable. The quotations acts as delimiters showing precisely where the string begins and ends. The string "13" is different from the strings " 13" and "13 " which have spaces before and after the 13.
  
  
  • With the Val function, one could change "13" into 13 as in
    
    intNum = Val("13")
    
    
  • The CInt function can also be used to change "13" into 13 as in
    
    intNum = CInt("13")
    
    There are some subtle differences between the Val and CInt functions however but you are not responsible for knowing these differences in this VB course.
    
    
  • The Str function can be used to change the numerical value 19610 into the string "19610" as in
    
    strZipCode = Str(19610)
    
    
  • The ToString method can also be used to change a numeric variable into a string. The statement
    
    Dim intNum As Integer = 19610
    Dim strZipCode As String = "00000"
    
    strZipCode = intNum.ToString()
    
    causes the string "19610" to be stored in strZipCode. Notice the required empty parentheses after the ToString method.
    
    
• If a statement is cut-off along the right edge of the paper when a program is printed out, it is necessary to break up the statement so that it appears over two lines of code. This can be done by typing the line continuation operator ( _ ) after a space anywhere next to an operator. The continued second line of code should be indented for good style. For example, rather than
  
  dblPrice = dblPRICE_PER_BOOK * intNumBooks + dblPRICE_PER_BOOK * intNumBook * PA_SALES_TAX_RATE
  
  you could type out
  
  dblPrice = dblPRICE_PER_BOOK * intNumBooks + dblPRICE_PER_BOOK * _
     intNumBook * PA_SALES_TAX_RATE
  
  
• Besides being used for mathematical purposes, assignment statements can be used to change the value of an object's property. The statement
  
  lblMessage.Text = "Hello World"
  
  causes whatever was stored in the Text property of lblMessage to be overwritten with the string "Hello World". You can even make an object disappear by using the Visible property in the following statement
  
  lblMessage.Visible = False
  
  The Visible property is used to make an object or invisible depending on certain conditions during a program's execution. Even though a textbox with a Text Property, "You have won the game.", may be present on a form, it may be made invisible for the duration of a game. Then, only if the user actually wins the game, would the Visible Property be changed to True in order for the user to know that he or she won the game.
  
  
• So let's say that you want to write a computer program that computes your grade for a class at school. You could use variables and one button. First you must be able to compute your grade by hand with a calculator or scratch paper. How can you train a computer to compute your grade if you do not know how to perform the algorithm yourself? So you add up the points earned on all of your homeworks and tests and divide by the sum of the points possible for those assignments. This gives you your grade percentage. Okay, so how many numbers do you end up scribbling onto your scratch paper as partial subtotals or intermediate calculations? Probably three. You added up the points earned as one subtotal. You added up the points possible as another subtotal. And, the quotient that you calculated by dividing those two numbers was a third important number that you wrote down. Well, what would be three good, descriptive variable names for those three values and are they whole numbers or decimal numbers? The variable names intPointsEarned and intPointsPossible would be good names for the first two and there data type should be Integer since they would always be whole numbers. Do not use short variable names like x and y or even intPE and intPP since you probably wouldn't remember what those cryptic names mean if you needed to update your program a few years later. In fact, ask your math teacher sometime why you've been using x and y as varaible names in math class all of these years. Those problems don't seem to relate to the real world like computer programs where every program has meaning and variable names that explain the purpose of a given program. The other variable could be named dblGradeAverage and it should be a Double rather than an Integer since it is likely to store decimal numbers. Next you need to declare these variables with declaration statements and initialize them to the appropriate values.
  
  Dim intPointsEarned As Integer = 90
  Dim intPointsPossible As Integer = 100
  Dim dblGradeAverage As Double = 0
  
  Then you write one or more assignment statements to perform the calculation
  
  dblGradeAverage = intPointsEarned / intPointsPossible
  
  Finally you decide that you want to display the answer in a message box with the statement
  
  MessageBox.Show(dblGradeAverage)
  
  All of this code could be typed into the Click method of a button. Can you describe any improvements to the user interface, user-friendliness, functionality, or efficiency of this program?
  
  
• It is usually possible to accomplish the same task by using no local variables that you could accomplish with several variables. For example, the following two code segments produce the same results
  
  Short Version
  
  lblAve.Text = Str((Val(txtScore1.Text) + _
               Val(txtScore2.Text) + Val(txtScore3.Text)) / 3)
  
  
  
  Long Version
  
  Dim intTest1 As Integer = 0     ' student's 1st test score
  Dim intTest2 As Integer = 0     ' student's 2nd test score
  Dim intTest3 As Integer = 0     ' student's 3rd test score
  Dim dblAverage As Double = 0    ' student's average test score
  
  intTest1 = Val(txtScore1.Text)
  intTest2 = Val(txtScore2.Text)
  intTest3 = Val(txtScore3.Text)
  
  intSum = intText1 + intTest2 + intTest3
  
  dblAverage = intSum / 3.0
  
  lblAve.Text = Str(dblAverage)
  
  There are a number of differences between the two code segments besides the obvious difference in length. The short version does not use any variables. Instead, it computes the results by directly working with the textboxes and label. This does save some memory since no variables are declared but using variables makes it easier to maintain and upgrade a program. The use of variables in the long version gives the programmer the opportunity to document and explain each of the variables and this makes the program easier for other programmers to understand. For example, in the long version, it is not understood that it is a student's test scores that are being averaged. The long version is nicely spaced out as well making the code easier to read and analyze.
  
  
• If you attempt to store a decimal number in an Integer variable, Visual Basic will round the value first. For example, the statement
  
  Dim intWhole As Integer = 3.6
  
  results in the number 4 being stored in the variable intWhole, not 3.6. However, due to the type of rounding used in Visual Basic if the number is 2.5, the value 2 would be stored in intWhole since 2 is an even number. If the number is 3.5, the value 4 would be stored. This type of rounding is called "Banker's Rounding". Search Google or see this link for more information.
  
  
• Assignment statements can be used to overwrite the values stored in variables as the program executes from top to bottom.
  
  Dim intX As Integer = 1
  Dim intY As Integer = 2
  Dim intResult As Integer = intX + intY
  lblResult.Text = Str(intResult)    ' lblResult contains 3
  intX = 5
  lblResult.Text = Str(intResult)    ' lblResult still contains 3 not 7
  intResult = intX + intY
  lblResult.Text = Str(intResult)    ' lblResult now contains 7
  
  
  Some students mistakenly believe that intResult is initially tied to the formula intX + intY. This is incorrect since only the current values stored in intX and intY are used to create the sum that's initially stored in intResult.

Objective #2: Use the order of operations to evaluate expressions and to write proper expressions.

• Expressions are groups of operators and operands that can be evaluated.
  
  • Examples of expressions:
    

    9 + intNum - 12

    (intGame1 + intGame2 + intGame3) / 3
    

  • Operators
    • unary operators

    -     is the negative sign

    • binary operators

      ^    is the exponent operator in VB (for example, 2 ^ 3 evaluates to 8 since 2 to the third power is 8). Since few other computer languages allow the ^ to be used as the exponent operator, later in this course, you will learn how to use the Pow method from the Math class to perform exponentiation.

      *    is the multiplication operator

      /    is the division operator

      Mod    is the modulus operator. The modulus operator finds the remainder of a division problem. For example, 10 Mod 3 evaluates to 1 since the remainder of 10 divided by 3 is 1. Other examples are 12 Mod 5 is 2, 4 Mod 3 is 1, 4 Mod 2 is 0, 10 Mod 10 is 0.

      +    is the addition operator

      -    is the subtraction operator

  • Operands are the numbers or variables that are bound by the operators.
    • In the expression         x + 3         both x and 3 are operands.

• Mathematical notation must be written in lines of code. For example, in computer programming, you must represent the fraction one half as 1/2 rather than writing it with a fraction bar as you would in a mathematics classroom.
• Insert parentheses where necessary in order to preserve the correct evaluation. That is, you must realize that VB obeys the order of operations which some students learn as "Please Excuse My Dear Aunt Sally" . The rule-of-thumb is to add extra parenthesis when you are in doubt if they are required or not. But note, though, that later in the school year, your teacher may deduct points for unnecessary parentheses in your code. Also note that multiplication and division are actually tie and should be evaluated from left to right in the order that they appear. The expression 12 / 3 * 4 evaluates to 16 and not 1. The same goes with addition and substraction. The expression 4 - 2 + 2 evaluates to 4 and not 0. The Mod operation fits in between multiplication/division and addition/subtraction. The expression 10 Mod 4 * 2 evaluates to 2 since the multiplication is performed first before the modulus.
  
  More examples:
  •   10 / 5 * 2 evaluates to 4 (not 1)
  •    24 / 8 * 4 evaluates to 12 (not 0.75)
  •    20 / 2 * 5 evaluates to 50 (not 2)
  •    2 * 2 * 2 / 2 * 2 evaluates to 8 (not 2)
  •    TRY THIS ONE: 36 / 18 * 2 evaluates to ?
  •    10 - 5 + 5 evaluates to 10 (not 0)
  •    100 - 3 + 97 evaluates to 194 (not 0)
  •    50 + 60 - 3 + 100 evaluates to 207 (not 7)
  •    TRY THIS ONE: 100 - 5 + 5 - 5 + 5 evaluates to ?
    
    
• Note that VB computes the two examples below differently behind-the-scenes, even though the final answer may be the same in both cases:
  

  dblFahr = 9 * dblCel / 5 + 32

  dblFahr = 9 / 5 * dblCel + 32

Objective #3: Know how to set break points, examine values of variables and expressions in the Watch window, and single-step through programs.

• A break point is a spot in a program where you would like the compiler software to stop when it executes the program. Simply click the gray gutter the runs along the left of the code window at the line of code where you want to set the break point. When you execute the program use the Debug/Start menu command rather than the Debug/Start Without Debugging menu command.
• Visual Basic will stop executing the code at that line and allow you to view the values stored in variables and objects' properties in the Autos, Local, or Watch window that appears in the lower-left corner of the screen. Right-click the Values column and uncheck "Hexadecimal Display" if the values don't look correct.
• To single-step through the program once you have stopped its execution at a break point, click the Debug/Step Into menu command. You can continue to check the values of variable in the Autos window.