Doing Strings in VB Part 1

Strings are an indispensable
part of almost all VB software; you will need to use strings in
almost all the software you ever make.

Let’s start with

What is a string and where do you use it?

In VB String is a length of text assigned to a variable of type
Variant or of type String. A string can store a maximum of around
2 billion characters between ASCII value 32 to 256. Strings mean
a lot to a programmer. They can hold important data, which the
user reads, intermediate values, comments, or can be used simply
to test if the software works correctly. People store text in
strings in .INI files, in the windows registry .RES files and
other text resources.

Strings in a file

You may often need to store and retrieve text from a file.
Here’s how

Retrieving text from a file

VB6 and VB5 introduced the new File object handling system but
moldy programmers like me still prefer the old Open Statement.
Here’s sample code that does that

Dim MyFileText
As String ‘Makes a String Variable Called MyFileText

Open "MYFILE.TXT" for input as #1 ‘Opens The File
And Names It #1

MyFileText = Input$(Lof(1),1) ‘Assigns The Text In The File
To MyFileText

Close #1 ‘Closes The Filesize="2" face="Verdana">

Open
"MYFILE.TXT" for Input as #1 ‘Opens The File And
Names It #1

This line does the actual
opening bit. Myfile.Txt is the name of the file to be opened. You
can open a file in many ways for many purposes. I’ve used
Input Mode here because I just want to read the contents of the
file. If you want to write to a file use Output, use
Append to add in the end of the file and Random if you have a
Database in the file. Binary Mode can be used to load Bitmap or
Sound Files. #1 is the number of the file. Whenever you want
to work on the file you will access it using that number.

MyFileText =
Input$(Lof(1),1) ‘Assigns The Text In The File To MyFileText

This line assigns the
contents of the file to MyFileText variable. Input$ Function
reads data from a file using the file number.

The first argument of Input$ is Lof(1).
The LOF function retrieves the length of a file in number of
characters. The second argument 1 is the number of file,
which has to be read. So in practice we tell VB to read the
entire length [LOF(1)] of file number 1 in the variable
MyFileText.

Close #1
‘Closes The File

This statement closes the
file and frees file number 1. It’s a good practice to close
the file immediately after you’ve read the contents in a
variable to free resources and avoid problems caused by a file
that remains open all the while the software is running.

Problems with Opening File

For most problems VB gives a self evident error message which
documents in detail the problem and allows the error to be
trapped and rectified. However there’s a special case which
forced me to rack my brains for quite a while when I was new to
programming.

VB won’t recognize and read
a file with a null terminated string in the normal input mode. Now in most editors like
NotePad etc., no null terminated string is added at the end of
the file but in some special cases, specially when the files has
been used for Binary purposes there may be a null terminated
string at the end of the file, and the file has to be opened in Binary mode in
VB, if you try to open it in input mode, there will be some cryptic error.Rectifying this problem is quite easy, just remove the last
character from the file and it gets opened fine.

Writing Strings to Files

To put your string in a file use Output instead of Input to
open the file. To save your string into the file you can either
use Write # or Print # in this way.

Write
#FileNumber, TheText

Or

Print
#FileNumber, TheText

Searching Stuff in Strings

You may often need to search for a word in lengths of text.
Visual Basic’s Instr function does this great.

Dim WordPos

WordPos = Instr(1, MyText, MyWord, VbTextCompare)

Here WordPos holds the position
of the first character of the word if it is found in the file.

The first argument
‘1’ specifies the character no. from where Instr
should start looking. This is useful when you need to do multiple
searches or search from the middle of the text. You can also
leave this option blank if you want to search from the beginning
of the text.

The second argument
‘MyText’ specifies the name of the string variable
that has to be searched. You can also use a string length like
this one ("I can use This String Instead Of MyText")
instead of the variable.

The third argument
‘MyWord’ is the word or character that has to be
searched in MyText. MyWord can also be a string instead of a
variable.

The fourth argument
‘VbTextCompare’ decides the mode of the comparison.
By default the mode is Binary. Here I am doing a comparison
between two strings, that’s why I have used VbTextCompare
instead of the default VbBinaryCompare.

VbTextCompare is inferior to
Binary compare in speed. In fact when I ran a test which tried
finding the letter ‘A’ in a string comprising of all
alphabets VbTextCompare took twice the time needed by
VbBinaryCompare to finish the searches. However I still prefer
using VbTextCompare in most cases because VbBinaryCompare thinks
Capital ‘A’ and small ‘a’ are different
characters and won’t provide a match if the case is
different in the searched word and original string.

If Instr is successful in
finding a match it returns the position of the first character in
the word. If it is unsuccessful the function returns 0.

Extracting parts from a
string

You may often to extract specific portion of a string and use
them. VB has three functions for extracting string parts. Left,
Mid & Right.

VB Pros and Code invigilators
recommend using Mid for all types of extraction. It is entirely
possible to do almost everything with Mid, but they won’t
have made Left & Right if they weren’t supposed to be
used.

TheText =
Left(MyText,NoOfCharacters)

Left function retrieves
specified number of characters from the left of the specified
string for e.g. if you wrote size="2" face="Verdana">MyText = Left("ABCD",3)size="2" face="Verdana"> then left would give you
"ABC".

Right returns the specified
number of characters from the rightmost part of the string.

Mid is by far the most versatile, useful function which can serve
the function of both Left, Right and also extract text from the
middle of the document.

MyText =
Mid(TheText,StartPos,LenOfText)

The first argument
‘TheText’ is the name of the string from which the text
has to be extracted.

The second argument ‘StartPos’ is the character
position from which Mid should start taking the text.

The third argument ‘LenOfText’ is the no of characters
that have to be picked up.

Replacing Text In Strings

You can include this feature in your software using the Left,
Right, Mid and Instr functions. Let’s see some sample code
which ‘B’ with ‘F’ in a string ABCD in this
fashion.

Dim TheText as
String = "ABCD"

Dim WordPos as Integer

Dim MyTextLeft as String

Dim MyTextRight as String

First find the text using Instr

WordPos =
Instr(TheText, "B") ‘returns 2

Use Left to take text before the
searched character or word

MyTextLeft =
Left(TheText, WordPos-1)

Use Right to take text after the
searched character

MyTextRight
= Right(TheText, len("ABCD")-WordPos)

Or

MyTextRight
=
Mid(TheText,WordPos+len("B"),len(TheText)-WordPos+len("B"))

Put The Two Strings Together
with the replaced character

TheText =
MyTextLeft & "F" & MyTextRight

The Modus Operandi here is quite
simple. We look for the string in the text, take all the text
that is prior to the string with the left function, and all the
text that is present after the string using the Right or Mid
function. The two strings are then put together with the
replacement text or no text if the part of the string has to be
deleted.

Replacing Easily

If you were intimidated by the long length and seemingly
complex code, you can do this much more easily if you have VB6.
The new Replace function eliminates several lines of code with a
single line.

For e.g. if I want to replace all "BBBB" with
"C" I would use

Replace("BBBB","B","C")

Here the first argument is the
original text, Second is the text to be searched and the third is
the alternative text.

You can also specify the number of found words to be replaced
using an extra Count argument, i.e. set count as 1 if you want to
replace only the first find and none other or leave it to the
default to replace all finds.

Encyrpting Strings

If you’ve ever though about storing passwords or other
sensitive data in a file or a string you must have thought
Encrypting it. Several algorithms of encryption exist in the
market and some of them are very complex. You can make a simple
algorithm of your own by replacing the ASCII value of the
characters, however the approach provides a weak form of
encryption and can be broken very easily. However you can do
quality encryption very easily using the VB Xor function.
Here’s a Function Which Encrypts text using the numerical
keys provided by the user.

Public Function
XorEncrypt(Byval TheText As String, Byval Key1 As Integer, Byval
Key2 As Integer) As String

For I = 1 to Len(TheText)

XorEncrypt = XorEncrypt & Asc(Mid(TheText, I, 1)) Xor Key1
Xor Key2 & "."

Next

End Function

This extremely small function
uses the unique features of Xor to provide good quality
Encryption. First the ASCII value of the character is Xor’d
with Key1 and then the resultant value is Xor’d with Key2
resulting in a random number that’s very hard to decrypt,
the number is delimited by the period sign to distinguish two
characters from each other. Xor performs a bitwise calculation.
If you perform a Xor on two numbers and then Xor the resultant
figure with any of the two numbers Xor returns the other number.

Public Function
XorDecrypt(Byval TheText As String, Byval Key1 As Integer, Byval
Key2 As Integer) As String

Dim PeriodPos as Integer

Do

PeriodPos = instr(TheText,".")

If Not PeriodPos=0 Then

TheXordNum=Mid(TheText,1,PeriodPos-1)

XorDecrypt = XorDecrypt & Chr(Xor(Xor(TheXordNum, Key2),
Key1))

TheText = Mid(TheText, PeriodPos+1)

Else

Exit Do

Endif

End Function

There’s still a lot more to
strings, in fact a lot-lot more, we could talk about storing
Strings in .INI files, strings in registry, strings in Random
Access Files, Strings Compiled in .EXEs with resources and a
whole lot of other types of strings, but, I guess we won’t
be covering all that in this article. If you found this of any
help please drop me a mail and I’ll try to write all the
other parts as quick as possible.

Searching for Stuff

The most common functionality needed by any user is searching. You can use
the 'Instr' statement for performing searched in VB. This is how a typical instr
looks.

SearchPos
= instr(1,"ABCD","C",vbTextCompare)

Most of you should already be familiar with the
instr statement, so I am not going to explain it here. The thing that needs a
though is the last parameter, vbTextCompare. What parameter you pass to this
option decides how fast your search will be. If you use vbTextCompare, instr
ignore case and search strings in both upper case and lower case, but the speed
will be slowed tremendously. If you use vbBinaryCompare, it speeds up the search
more than 10 times, but will match case will searching. Personally I recommend
you use vbBinaryCompare, if you can, the speed gained is tremendous. 

Thanks

Razoredge

E-mail: size="2" face="Verdana">psl@nde.vsnl.net.in