IS437: Fall 2004

Instructor: Dr. Boris Jukic

Data Types, Constants, Variables, Scope, Conversion

Learning Objectives

• data types - variables and constants - scope & lifetime - declaration (option explicit, option strict)• assignment - numeric - string - date• some useful functions & controls - type conversion

Frequently-used data types• integer short 2 bytes -32,768 to +32,767 integer 4 bytes about +/- 2 billion long 8 bytes a very large integer number• real decimal 16 bytes 0 to 28 digits of accuracy single 4 bytes 38 to 45 digits of accuracy double 8 bytes 308 to 324 digits of accuracy• Boolean 2 bytes true or false• string 2 bytes for each character up to 2 billion Unicode

characters• date 8 bytes• object 4 bytes

Constants

• Named constants: (programmer defined) e.g., const pi as single = 3.1415

const taxRate as single =0.07

• Intrinsic constants: (system provided) e.g., Color.White Color.Blue Color.Red Color.Yellow Color.Green ...

Structure of a VB Application

Project (s)

Form(s)

Controls

solution

Scope of variables & constants

dim i as integerconst m as short = 8

module-level (form): variables declared using the Private declarationprivate y as integerprivate const j as integer = 5

local (sub): variables declared using the Dim declaration

project level variables:variables declared using the Public declarationPublic z as integerPublic const k as integer = 15

block (within a loop)For i = 0 to 3 dim k as integer …Next i

Lifetime of Variables

-local: only alive during the execution of a procedure or a block of code

-For temporary results/values

- form-level: alive as long as the form remains loaded (typically the whole lifetime of a project)

-(namespace) project : project

-If values are to be preserved after the project itself is not active

-External data depositories

Declaration of Variables & Constants

• “option explicit on” as default to enforce declaration

• Why is declaration a good idea? - e.g., dim monument as integer rebates = 345678 …. rebtaes = rebates + 75

‘the error will be detected if declaration is mandatory ….• local declaration - e.g., Private Sub command1_click(…) … dim x as integer dim y const MARKUP as single = 0.14 … End sub

Declaration of Variables & Constants

• form-level declaration

where? In a declaration section of the form private x, y, z as integerprivate const rstmsg as string = “Please Reset all Values”

• project-level declaration where? It is good practice to declare them all in a single module. (show in class)public x as integer

public const COMP_NAME as string = “First Trust and Bank”

public const MAX_BONUS as decimal = 11500

Variable Declaration

Project-level Declaration

Structure of a VB Application

Project (s)

Form(s)

Controls

code module - declare public var. & constants- utility procedures such as sort, etc.

solution

Choosing the variable scope

•Choose the variable and constant scope as narrow as possible

•minimizes computational needs of the code•Makes code more modular, divided into useful, independent components

Recommended Naming Conventions

prefix• integer integer int long lng• real decimal dec single sng double dbl• Boolean bln• string str• date dat

Scope Prefix

project g

module (form) m

local none

For example:

gstrCustomerName

Assignment (numeric)• operators

+ addition - subtraction * multiplication / division ^ exponentiation \ integer division 5 \ 2 = 2 mod modulus 5 mod 2 = 1

• e.g., a = (5 * 4) / 2

Assignment (string variable)• empty string - e.g., dim x as string x = ““ text1.text = x

• assign one string to another - e.g., text2.text = text1.text

• string concatenation using “&” - e.g., dim firstname, lastname as string firstname = “John” lastname = “Smith” label1.text = firstname & “ “ & lastname• concatenate with itself - e.g., dim name as string name = “Roberts” name = name & “ Rogers”

John Smith

Assignment (Date)

• e.g., dim date1, date2 as Date

date2 = #3-6-98 2:15#

date2 = #March 6, 1998 2:15 pm#

date2 = #14:15# date1 = “1/20/97” with option strict off

date1 = “2:12 am” with option strict off

Option Strict

“Option Strict Off” is the default setting. Change the setting by typing “Option Strict On” at

the top of the code window. With “Option Strict On”, the system strictly enforces

data type checking. Data conversion from “wider” to “narrower” data type is not

allowed Conversion between string and numeric types is not

allowed This prevent many hard to find bugs and execution errors

Option Strict

With “Option Strict Off”, it is okay to write: Dim s as string, i as single

s = “34”

i = s / 2

• With “Option Strict On”, the code above will generate an error message:

Option Strict On disallows implicit conversion from string to single.

Widening & narrowingOption Strict Off

Option Strict On

Widening OK OK

Narrowing OK, but may result in over-flow exception in runtime

No

Widening: store a variable value into another variable with greater precision or magnitude

Narrowing: the reverse

Dim i as integer

Dim j as long

i = 35

j = i

Widening

Narrowing (Option Strict Off)

Dim i As Integer ‘4-bytesDim j As Long ‘8-bytesj = 3000000000i = j

The above code will generate a runtime exception, but not a warning in design time.

Data type conversion functions

CStr string CInt integer CLng long CSng single CDbl double CDec decimal CDate date …

Convert its argument toFunction Dim x as integer

Dim s as string

x = 5

s = CStr(x)

s = “5”

Dim d as date

Dim s as string

s = “01/15/2003”

d = CDate(s)

d = #01/15/2002#