Transcript Variables - Bismarck Public Schools

Chapter 2
VARIABLES AND CONSTANTS
Variables
 Variable – a named memory location that stores a
value.
 A variable can be thought of as a named box
(memory location) containing a value.
 Variables are used in a program so that values can be
represented with meaningful names.
Declaration of a Variable
 A variable must be declared before it is used.
 Declaration Statement:
Dim variable_name As type
 Ex.
Dim intNumber As Integer
Dim Statement
 Dim variable_name As type
 Dim – stands for dimension. A declared variable has
a “dimension” because it has been assigned space in
memory.
 Variable – prefix of the variable
 Name – descriptive name of the variable
 Type – Data Type
Initialization of Variable
 A numeric variable is automatically initialized to 0 in
Visual Basic
 A declaration statement can also include a specific
value for initialization.
 Example:
Dim sngRadius As Single = 9.5
 Initializes a variable named sngRadius with the value
of 9.5
Multiple Declaration of Variables
 Multiple variables with the same data type can be
declared in a single statement.
 Example:
Dim sngRadius, sngDiameter As Single
 Grouping variables together in a single declaration is
good programming style when the variables
represent related items.
Variable Assignment
 The value stored in a variable can be changed at
runtime through assignment.
 A variable assignment must be written so that the
variable name is on the left side of the equal sign and
the value is on the right.
 Example:
Dim sngRadius As Single = 10
sngRadius = 12.3
Variable Assignment cont…
 One common error is to reverse the variable name
and the value in an assignment statement.
 12.3 = sngRadius ‘Error
Initialization of Variable to a Numeric
Expression
 A numeric expression can also be used on the right
side of an assignment statement.
 sngCircleArea = 3.14 * sngRadius ^ 2
 A variable can be used wherever a value can be used.
IMPORTANT
 A variable can store only one value at any given time.
Dim sngRadius As Single = 10
sngRadius = 5
sngRadius = 12.5
 The final statement is what sngRadius with be
assigned to. sngRadius = 12.5
Obtaining a Value from
the User
 A TextBox object is one way to allow users to enter
values.
 A label is often placed near a text box to describe its
contents or purpose. This label is also called the
prompt.
TextBox
 Name = prefix = txt
 Text = what is displayed in the text box.
 TextAlign = sets the alignment of text relative to
the text box.
 At run time, the TextBox Text property stores
whatever characters are currently in the text box.
 sngRadius = Me.txtRadius.Text
TextBox cont…
 If the text box does not contain data that matches the
variable type, a run-time error occurs and the
program is halted.
 To prevent this, the Val() function should be used
when assigning data from a text box to a numeric
variable.
 Function = a procedure that performs a task and
returns a value.
Value Function
 Val() takes a string and returns a number
corresponding to the numeric characters in the
string.
 If the first character of the sting is not a number,
Val() returns a 0.
Value cont…
 Dim sngHeight As Single
 sngHeight = Val(“62.5 inches”) = 62.5
sngHeight = Val(“Twenty inches”) = 0
sngHeight = Val(“Six ft. 2 inches”) = 0
Named Constants
 A constant is a named memory location which stores
a value that cannot be changed from its initial
assignment.
 Const sngPI As Single = 3.14
 A value of a constant is typed only in the declaration,
eliminating the possibility of typing the wrong value
elsewhere in a program.
Constants cont…
 This also means that if the program is modified later,
the constant value need only be changed in one
place.
 Const sngPI As Single = 3.14159265
 As a matter of good programming style, constants
should be in all uppercase except for the prefix that
indicates the data type.
Constants cont…
 Const sngPI As Single = 3.14
sngPI = 22/7 ‘Error
Choosing Identifiers
 There are several identifiers that Visual Basic.net
reserves for use as keywords.
 Keyword – has special meaning, and therefore
cannot be used for a variable or named constant
identifier.
 For example, Single, End, and Sub are keywords.
Keywords appear in different font.
Built-In Data Types
 A variable or constant declaration includes a data
type that corresponds to the data stored.
 Visual Basic includes several data types:
Built-in Data Types
Type
Prefix
Used to represent
Short
srt
Integers from -32,768 to 32,767
Integer
int
Integers from -2,147,483,648 to
2,147,483,647
Long
lng
large integers (no decimals)
Single
sng
numbers possibly containing a
decimal
Double
dbl
large numbers possibly containing
a decimal
Date
dtm
a date in m/d/yyyy format
Decimal
dec
numbers that may have many
significant digits
Char
chr
a single character
String
str
a set of characters
Boolean
bln
True or False
Type
Used to represent
Short
Integers from –32,768 to 32,767
Integer
Integers from –2,147,483,648 to 2,147,483,647
Long
Large integers (no decimals)
Single
Numbers possibly containing a decimal
Double
Large numbers possibly containing a decimal
Date
A day in m/d/yyyy format
Decimal
Numbers that may have many significant digits
Char
A single character
String
A set of characters
Boolean
True or False
Variable Initialization
 Visual Basic automatically initializes variables to a
default value when they are declared.
 Variables of numeric type, such as Short, Integer,
Long, Single, Double, and Decimal are
initialized to 0.
 Date variables are initialized to 12:00:00 AM.
Variable Initialization cont…
 Boolean variables are initialized to False.
 Char and String variables are initialized to
nothing, which is equal to the keyword Nothing.
 This keyword can be used in place of an empty string
(“”) for clearing labels, and so on.
Automatic Type Conversion
 In an assignment statement, Visual Basic
automatically converts data to match the type of the
variable it is being assigned to.
 Dim intX = 6.7
‘intX is assigned to the value of 7
Automatic Type Conversion cont…
 Visual Basic will try to convert from one data type to
another as long as the data is valid for the receiving
data type.
 For example, assigning 12.3 to an Integer variable
intX is valid because the number can be converted to
12. However, an error will be generated when abc is
assigned to an Integer variable.
Scope
 Scope of a Variable – is the set of statements
that can use the variable.
 Local Declaration – a declaration at the
beginning of a procedure where the variable can
only be used “locally”
 Global Declaration – a declaration placed at the
beginning of the program that can be used in any
procedure throughout the program.
Special Division Operators
Integer Division
 Integer Division ( \ ) – truncates the decimal
portion of the quotient, which results in an integer.
 Dim intX As Integer
intX = 20 \ 7 ‘IntX is assigned 2
 Only returns the whole portion of the quotient
Modulus Division
 Modulus Division ( Mod ) – returns the
remainder resulting from division.
 Dim intX as Integer
intX = 20 Mod 7 ‘intX is assigned 6
 Only returns the remainder of the quotient.
 Modulus division is used in applications
where the separate digits of a number are
needed, for finding the number of minutes left
over after hours have been accounted for, and
for other integer related tasks.
Programming Errors
 Syntax Error – a statement that violates the
rules of Visual Basic
 Logic Error – are caused by statements that are
syntactically correct, but produce undesired or
unexpected results.
 Run-Time Error – Syntax and logic errors that
go undetected may generate a run-time error when
the program is run. A run-time error halts the
program at the statement that cannot be executed.
Debugging an Application
 Debugging – the process of getting an application
to work correctly. One technique uses breakpoints.
 Breakpoint – a statement that has been marked as
a stopping point.
 Watch Window – used to examine values in break
mode.