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From: vittal <vittal
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Date: Feb 22, 2006 11:53 AM
Subject: VB2
To: Vittal P <vittal
...@gmail.com>
Profsr.com <http://profsr.com/>
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LESSON 2 -- Writing code
The Code Editor
As we saw in the previous lesson, getting to the Code Editor is as simple as
hitting the proper button. You may have discovered that you can also call-up
the Editor by double-clicking on an object. It is also possible to select
"View code" with the right mouse button. You will note that the Editor has
all the functions of a text editor and then some. The most commonly used
functions will be *Cut ... Copy ... Paste* which you can call from the Menu,
from the Toolbar or from the right mouse button. You also have access to the
usual *Find* and *Replace* functions.
Getting helpThere is *a lot* of documentation available on VB. There is so
much, in fact, that it's easy to get lost in it. However, the on-line Help
available from the Menu should be used regularly. Very often just doing a
search on a word in particular will be sufficient to get you out of a jam.
If you want to go into more detail check out the Contents part of MSDN
(Microsoft Developers' Network) and surf through it.
Writing codeVB is not very particular about presentation - spaces, indents,
lower case or upper case, it doesn't make too much difference to the
compiler. But it may make a whole lot of difference to the programmer who
has to maintain your code in 2 years, after you've moved up to President.
* Apply "Best Programming Practices"
*When you work with RAD (Rapid Application Development) tools like VB in a
graphical interface environment, you become more than just a programmer, a
writer of code. You are a developer. We will cover that in the next lesson.
But at the moment, you are still a Programmer. And unless you are developing
an application for your own personal use, that nobody else will see, you
have to think of the environment, of the team you are working with.
* "No man (or woman) is an island!"
*Especially when it comes to programming. The code you write may have to be
checked by an Analyst. It will have to go through testing. It may have to be
modified by other team members and it almost certainly will go through
modifications, maybe several times, in the months and years ahead when you
probably won't be around to defend yourself. "The evil that men do lives
after them...". You do not write code for the VB compiler. You write it for
other developers and programmers. What you want others to say behind your
back is: "That Jane was blindingly efficient, brilliant, a genius with
comments ...". You do not want to be remembered as "...the Picasso of code,
master of the abstract".
If you are just starting out with the language, why not pick up a few good
habits right now and it may make your life a lot easier down the road.
1. Use comments when appropriate but not so many as to overwhelm the
code; the apostrophe ' is the comment identifier; it can be at the beginning
of a line or after the code.
*' This is a comment
'on 2 lines
DIM intNumber AS Integer 'This is a comment *
2. Use indents - code must be indented under control structures such
as If ... Then or Sub - it makes it so much easier to follow the logic.
*FOR i = 1 TO 5
value(i) = 0 ' Indent used in control structures
NEXT i *
3. Use standard capitalization - keywords like If, Dim, Option,
Private start with a capital letter with the rest in lower case; variable
names, control names, etc. are usually mixed case: ClientName, StudentId,
etc.
4. Write extra-long statements on 2 lines using the continuation
character _ (space underscore); in VB each line is assumed to be an
individual statement unless there is a continuation at the end of the first
line.
*Data1.RecordSource = _
"Select * From Titles" ' One statement on 2 lines is OK*
Naming conventionsThese are the rules to follow when naming elements in VB -
variables, constants, controls, procedures, and so on:
- A name must begin with a letter.
- May be as much as 255 characters long (but don't forget that somedy
has to type the stuff!).
- Must not contain a space or an embedded period or type-declaration
characters used to specify a data type ; these are ! # % $ & @
- Must not be a reserved word (that is part of the code, like Option,
for example).
- The dash, although legal, should be avoided because it may be
confused with the minus sign. Instead of Family-name use Family_name or
FamilyName.
Data types *Data type* *Storage size* *Range* Byte 1 byte 0 to 255 Boolean 2
bytes True or False Integer 2 bytes -32,768 to 32,767 Long (long integer) 4
bytes -2,147,483,648 to 2,147,483,647 Single (single-precision
floating-point) 4 bytes -3.402823E38 to -1.401298E-45 for negative values;
1.401298E-45 to 3.402823E38 for positive values Double (double-precision
floating-point) 8 bytes -1.79769313486232E308 to -4.94065645841247E-324 for
negative values; 4.94065645841247E-324 to 1.79769313486232E308 for positive
values Currency (scaled integer) 8 bytes -922,337,203,685,477.5808 to
922,337,203,685,477.5807 Decimal 14 bytes
+/-79,228,162,514,264,337,593,543,950,335
with no decimal point; +/-7.9228162514264337593543950335 with 28 places to
the right of the decimal; smallest non-zero number is +/-
0.0000000000000000000000000001 Date 8 bytes January 1, 100 to December 31,
9999 Object 4 bytes Any Object reference String (variable-length) 10 bytes +
string length 0 to approximately 2 billion String (fixed-length) Length of
string 1 to approximately 65,400 Variant (with numbers) 16 bytes Any numeric
value up to the range of a Double Variant (with characters) 22 bytes +
string length Same range as for variable-length String User-defined (using
Type) Number required by elements The range of each element is the same as
the range of its data type.
In all probability, in 90% of your applications you will use at most six
types: String, Integer, Long, Single, Boolean and Date. The Variant type is
often used automatically when type is not important. A Variant-type field
can contain text or numbers, depending on the data that is actually entered.
It is flexible but it is not very efficient in terms of storage.
Top <http://www.profsr.com/vb/vbless02.htm#Start>
Declaring variablesDeclaring a variable means giving it a name, a data type
and sometimes an initial value. The declaration can be *explicit* or *
implicit*.
An *explicit declaration:* variable is declared in the Declarations Section
or at the beginning of a Procedure. An explicit declaration looks like:
*Dim MyNumber As Integer*
Now the variable *MyNumber* exists and a 2-byte space has been reserved for
it.
An *implicit declaration:* the variable is declared "on the fly", its data
type is deduced from other variables. For example:
*Dim Total1 As Integer* 'Explicit declaration
*Dim Total2 As Integer* 'Explicit declaration
*Total3 = Total1 + Total2 'Implicit declaration
*
Total3 is not formally declared but is implied, it is "arrived at" from the
other declarations.
It is never a good idea to have implicit declarations. It goes against the
rules for clarity, readability and ease of use of the code.
To make sure that this rule is followed, start the Declarations with
the *Option
Explicit* clause. This tells the compiler to consider implicit declarations
as errors and forces the programmer to declare everything explicitly.
Other examples of declarations:
*Dim MyName As String
Dim StudentDOB As Date
Dim Amount5, Amount6, Amount7*
In the last example the type assigned to each variable will be: Variant. It
is the default type when none is specified.
There can be multiple explicit declarations in a statement:
*Dim EmpName As String, SalaryMonth As Currency, SalaryYear As Currency*
In this final example, what are the types assigned to the three variables:
*Dim Amount1, Amount2, Amount3 As Single*
All Single-precision floating point, you say. *Wrong!* Only Amount3 is
Single. Amount1 and Amount2 are considered Variant because VB specifies that
each variable in a statement must be explicitly declared. Thus Amount1 and
Amount2 take the default data type. This is different from what most other
languages do.
ConstantsA constant is a value that does not change during the execution of
a procedure. The constant is defined with:
*Const ValuePi = 3.1416*
The Scope of variablesThe term *Scope* refers to whether the variable is
available outside the procedure in which it appears. The scope is *
procedure-level* or *module-level*.
A variable declared with Dim at the beginning of a procedure is only
available in that procedure. When the procedure ends, the variable
disappears. Consider the following example:
Option Explicit
Dim Total2 As Integer
Private Sub Command1_Click ()
Dim Total1 As Integer
Static Total3 As Integer
Total1 = Total1 + 1
Total2 = Total2 + 1
Total3 = Total3 + 1
End Sub
Private Sub Command2_Click ()
Dim Total1 As Integer
Total1 = Total1 + 1
Total2 = Total2 + 1
Total3 = Total3 + 1
End Sub
Every time Button1 is clicked, Total1 is declared as a new variable during
the execution of that clicked event. It is a *procedure-level* variable. It
will always stay at 1. The same for the Button2 event: Total1 is a new
variable in that procedure. When the procedure ends, Total1 disappears.
Total2 is declared in the Declarations section. It is a
*module-level*variable, meaning it is available to every control in
this Form. When
Button1 is clicked, it increments by 1 and it retains that value. When
Button2 is clicked, Total2 is incremented from its previous value, even if
it came from the Button1 event.
Total3 shows another way of retaining the value of a local variable. By
declaring it with *Static* instead of Dim, the variable acts like a
module-level variable, although it is declared in a procedure.
Another scope indicator that you will see when you study examples of code is
*Private* and *Public*. This determines whether a procedure is available
only in this Form (module) or if it is available to any module in the
application. For now, we will work only with Private procedures.
Top <http://www.profsr.com/vb/vbless02.htm#Start>
Operators Mathematical and Text operators *Operator* *Definition * *
Example * *Result * ^ Exponent (power of) 4 ^ 2 16 * Multiply 5 * 4
20 / Divide 20 / 4 5 + Add 3 + 4 7 - Subtract 7 - 3 4 Mod
Remainder
of division 20 Mod 6 2 \ Integer division 20 \ 6 3 & String
concatenation "Joan" & " " & "Smith" "Joan Smith"
Note that the order of operators is determined by the usual rules in
programming. When a statement includes multiple operations the order of
operations is:
*Parentheses ( ), ^, *, /, \, Mod, +, -*
Logical operators *Operator * *Definition * *Example * *Result * = Equal
to 9 = 11 False > Greater than 11 > 9 True < Less than 11 < 9
False >= Greater or equal 15 >= 15 True <= Less or equal 9 <= 15
True <> Not equal 9 <> 9 False AND Logical AND (9 = 9) AND (7 = 6)
False OR Logical OR (9 = 9) OR (7 = 6) True
Top <http://www.profsr.com/vb/vbless02.htm#Start>
Control Structures If...Then...Else
*If* condition1 *Then*
statements1
*Else*
statements2
*End If*
If condition1 is True, then statements1 block is executed; Else, condition1
is not True, therefore statements2 block gets executed. The structure must
be terminated with the End If statement.
The Else clause is optional. In a simple comparison, statements1 get
executed or not.
*If* condition1 *Then*
statements1
*End If*
Select Case
Can be used as an alternative to the *If...Then...Else* structure,
especially when many comparisons are involved.
*Select Case *ShirtSize
*Case* 1
SizeName.Caption = "Small"
*Case* 2
SizeName.Caption = "Medium"
*Case* 3
SizeName.Caption = "Large"
*Case* 4
SizeName.Caption = "Extra Large"
*Case Else*
SizeName.Caption = "Unknown size"
*End Select*
Do...Loop Used to execute a block of statements an unspecified number of
times.
*Do While* condition
statements
*Loop*
First, the condition is tested; if condition is True, then the statements
are executed. When it gets to the Loop it goes back to the Do and tests
condition again. If condition is False on the first pass, the statements are
never executed.
For...Next When the number of iterations of the loop is known, it is better
to use the For...Next rather than the Do...Loop.
*For* counter = start *To* end
statements
*Next*
1) The counter is set to the value of start.
2) Counter is checked to see if it is greater than end; if yes, control
passes to the statement after the Next; if not the statements are executed.
3)At Next, counter is incremented and goes back to step 2).
More will be covered on Control strucures as it becomes necessary in
upcoming lessons. Meanwhile,if you want to know more, consult the VB
Language Reference.
Assignment 2
To practise your coding and editing skills, continue the development of
Assignment1 by adding buttons with appropriate code to record the other
scores possible in a football game for each team. In case football is not
your thing, here is what they are:
Touchdown 6 points Field goal 3 points Convert 1 point 2-point
Convert 2 points Safety 2 points
Also, add a button to Exit the Form when you're finished with it.
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