Numbers, Variables and strings

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Numbers, Variables and strings

• 3.3 Numbers
• Much of the data processed by computers consists
  of numbers. Frequently numbers are called numeric
  constants
• The five arithmetic operations in Visual Basic
  are addition, subtraction, multiplication,
  division, and exponentiation.

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• One way to show a number on the screen is to
  display it in a picture box. If m is a number,
  then the instruction
• picBox.Print m
• Displays the number m in the picture box picBox.
• Print is a reserved word and the Print operation
  is called a method. Another important method is
  Cls. The statement
• picBox.Cls
• Erases all text and graphics from the picture box
  picBox.

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• A variable is a name that is used to refer to an
  item of data. The value assigned to the variable
  may change during the execution of the program.
• In Visual Basic, variable names can be up to 225
  characters long, must begin with a letter, and
  can consist only of letters, digits, and
  underscores. Visual Basic does not distinguish
  between uppercase and lowercase letters used in
  the variable names.
• If var is a variable and num is a constant, then
  the statement
• Varnum
• assigns the number num to the variable var.
  Such statement is called an assignment statement.
  The computer sets aside a location in memory
  with the name var and places the number num in
  it.

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• The statement
• picBox.Print var
• Displays the value of the variable var in the
  picture box.
• A combination of constants, variables, and
  arithmetic operations that can be evaluated to
  yield a number is called a numeric expression.
  Expressions are evaluated by replacing each
  variable by its value and carrying out the
  arithmetic operations. For example 2distance
  75, (abc)/6.
• Example The following program displays the value
  of an expression.
• Private Sub cmdCompute_Click()
• picResults.Cls
• A5
• B4
• picResults.Print A(2B)
• End Sub

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• Run, and then click the command button. The
  following is displayed in the picture box.
• 30
• If var is a variable, then the statement
• varexpression
• First evaluates the expression on the right side
  and then assigns its value to the variable. For
  this reason a statement such as
• nn1
• Is meaningful. In terms of memory locations, the
  statement retrieves the value of n from ns
  memory location, uses it to compute n1, and then
  places the sum back into ns memory location.

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• A string is a sequence of characters that is
  treated as a single item. For example phrases,
  words, names, telephone numbers, social security
  numbers.
• A string variable is a name used to refer to a
  string. The allowable names of string variables
  are identical to those of numeric variables. The
  value of a string variable is assigned or altered
  with assignment statements and displayed with
  Print methods just like the value of a numeric
  variable.
• If x,y,..,z are characters and strVar1 is a
  string variable, then the statement
• strVar1xy..z
• Assigns the string constant xy..z to the
  variable.

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• The statement
• picBox.Print xy..z
• Or
• picBox.Print strVar1
• Displays the string xy..z in a picture box.
• If strVar2 is another string variable, then the
  statement
• strVar2strVar1
• Assigns the value of the variable strVar1 to the
  variable strVar2. The value of strVar1 will
  remain the same.
• Example (The form design consists of a command
  button and picture a picture box)
• Private Sub cmdCompute_Click()
• picBalance.Cls
• Interestrate0.0655
• principal100
• Phrase The balance after a year is
• picBalance.Print phrase(1interestrate)principal
  
• End Sub

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• Run, and then click the command button. The
  following is displayed in the picture box.
• The balance after a year is 106.65
• Declaring Variable Types
• There are several advantages to specifying the
  type values that can be assigned to a variable. A
  statement of the form
• Dim variableName As string
• Specifies that only strings can be assigned to
  the named variable. A statement of the form Dim
  variableName As Single
• Specifies that only real numbers of
  single-precision can be assigned to the named
  variable. A single-precision numeric variables
  can hold numbers of magnitude from as small as
  1.4x10-54 to as large as 3. 4x10-38 .

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• A Dim statement is said to declare a
  variable.From now on we will declare all
  variables. However, all the programs run
  correctly even if the Dim statements are
  omitted.Declaring variables at the beginning of
  each event procedure makes programs easier to
  read and helps prevent certain type of errors.
• Example
• Dim interestrate As Single
• Dim principal As Single
• Dim phrase As String
• Dim interestrate As Single, principal As Single,
  phrase As String
• Another type of numeric variable, called integer,
  can hold only integers from 32768 to 32767.

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• Example
• Dim intVar As integer
• Other types of numeric variable are Long, Double
  and Currency (see Appendix C of the textbook)
• Input and Output by means of Text Boxes
• The contents of a text box is always a string.
  Thus
• strVar txtBox.text
• And
• txtBox.TextstrVar
• Can be used to assign the contents of the text
  box to the string variable strVar and vice
  versa.
• Numbers are stored in text boxes as strings.
  Therefore, they should be converted to numbers
  before being assigned to numeric variables.

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• If str is a string representation of a number,
  then
• Val(str)
• Is that number.
• If num is a number, then
• Str(num)
• Is a string representation of the number.
• Example
• The statements
• numVarVal(txtBox.Text)
• And
• txtBox.TextStr(numVar)
• Can be used to assign the contents of the text
  box to the numeric variable numVar and vice
  versa.

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• The Conversion Program
• Scroll bars are useful controls and allow the
  user to enter information which can be used by
  the program. We are going to develop a program
  such that the user can specify a distance in
  miles which the program converts into kilometres.
  
•  

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•      
•  

Form Name frmConversion Caption
Conversion   Command Button Name cmdExit Capt
ion Exit Text Box Name txtMiles   Text
Box Name txtKilometers  
Label Name lblMiles Caption Miles
Label Name lblKilometers Caption
Kilometres Horizontal Scroll Bar Name hsbMiles
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• The code for the cmdExit button is the
  following 
• Private Sub cmdExit_Click()
• Beep
• End 
• End Sub
• Run the Project
• Firstly, we declare the two variables in the
  General Declarations section like this

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• Turning to the code for the hsbMiles scroll bar,
  we have
• Private Sub hsbMiles_Change()
• Miles hsbMiles.Value
• txtMiles.Text Miles
• Kilometers 1.6 Miles
• txtKilometers.Text Kilometers
•  End Sub
• Run the program

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•  Now use the following code for the hsbMiles
  scroll bar
• Private Sub hsbMiles_Scroll()
• Miles hsbMiles.Value
• txtMiles.Text Miles
• Kilometers 1.6 Miles
• txtKilometers.Text Kilometers
• End Sub
• Run the program

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