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File Input Output

File Input Output Append

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  Handling Files in Visual Basic

   
By Alex Allan
   
In, Out, 
   Shake It All About
   
First off, let's take a look at the file modes input, output and 
   append. You use these three types to read or write plain text - such 
   as that found in .txt, .bat and .ini files.
   
But when should you use each mode? Well, it depends on what you 
   want to do. Use the following list to help you decide...
   

    
The Output mode creates a blank file and allows you to 
    write information into it. 
    
The Append mode is similar to the Output mode but 
    appends (adds to) an existing file. 
    
The Input mode opens a file for reading. 
   
Top Tip: You may hear these file modes being referred 
   to as 'sequential files'. That's 'cause once you have read or 
   written to a line, you can't go back to it unless you close and 
   re-open. In other words, the modes are one way – sequential.
   
So, for example, to open a file for output, you'd use the 
   Open statement like this:
Open "c:\windows\faq.txt" For Output As #1

   
That's all fine and dandy, but how do you use each mode 
   after you’ve opened the file?
   
To write to a file we use (Output and Append 
   only):
Print #filenumber, expression

   
To read from a file we use (Input only):
Input #filenumber, variablelist

   
This probably looks completely confusing at the moment, so let's 
   figure out what it all means.
   
Let's imagine you've opened a file for output, like 
   this...
Open "c:\groovy\myfile.txt" For Output As #1

   
...we now want to put information into this file using the Print 
   statement, like this:
Print #1, "Hello World!"

   
This inserts the information you pass it direct to the file in 
   #1.
   
If you'd opened a file for Input, like this...
Open "c:\groovy\myotherfile.txt" For Input As #1

   
...you can read information from the file, like 
this...
Input #1, MyVariableName

   
This reads information from the file in #1 and puts it into your 
   variable. 
   
Let's use an example - it's easier to explain that way!
   
Building a 
   Sample
   
Let's build a sample to demonstrate accessing files:
   
Open Visual Basic and double-click on "Standard EXE"
   
You should be left with a blank form.
   

    
Throw a simple Text Box onto the form 
    
Go to the Properties window and change MultiLine to True 
    
Create two Command buttons, setting the caption of the first 
    to Read and the second to Write. 
   
So far, it should look something like this:
   

   
Now, double-click on Read and insert the following 
   code:
Private Sub Command1_Click()
 'Outline:  - Asks the user for a file.
 '    - Reads all the data into Text1.
 
 Dim FilePath As String
 Dim Data As String
 
 FilePath = InputBox("Enter the path for a text file", "The Two R's", _
 "C:\WINDOWS\WINNEWS.TXT")
 'Asks the user for some input via an input box.
 
 Open FilePath For Input As #1
 'Opens the file given by the user.
 
 Do Until EOF(1)
 'Does this loop until End Of File(EOF) for file number 1.
  Line Input #1, Data
  'Read one line and puts it into the varible Data.
  Text1.Text = Text1.Text & vbCrLf & Data
  'Adds the read line into Text1.
  MsgBox EOF(1)
 Loop
 
 Close #1
 'Closes this file.
End Sub

   
Read the comments (anything prefixed by an apostrophe).
   
Next, double-click on Write and insert the following 
   code:
Private Sub Command2_Click()
 'Outline:  - Asks the user for a file.
 '    - Writes it all to a the file.
 
 Dim FilePath As String
 
 FilePath = InputBox("Enter the path for a text file", _
 "The Two R's")
 'Asks the user for some input via an input box.
 
 Open FilePath For Output As #1
 'Opens the file given by the user (for Output).
 
 Print #1, Text1.Text
 'Writes the data into the file number #1
 
 Close #1
End Sub

   
Once again, read all the comments. Do you understand what's 
   happening?
   
Hit F5 to run your program!
   
Congratulations! You've just created a simple text editor. The 
   Write button performs a simple 'Save As' whilst the Read button is 
   the equivalent of 'Open'.
   
Didn't I 
   Mention Those?
   
You may have noticed a few things in my code that I haven't told 
   you about. Let's take a peek at a few geeky code words...
Line Input #filenumber, MyVariableName

   
- This is the same as Input but it reads the whole line instead 
   of stopping at a comma (which can be useful - sometimes)
EOF(#filenumber)

   
- This outputs a true or false value, depending on whether it has 
   hit the 'end' of the file. In my code, I used it in the Do…Loop for 
   Read. When EOF=True, the loop ends.
Close #filenumber

   
- This closes the file. It allows other files to open this 
   file.
   
And if you'll be getting real friendly with files in Visual 
   Basic, here are a few other file writing functions you may be 
   interested in:
Write #filenumber, outputlist

   
This is similar to Print but it writes "screen formatted data" 
   instead of raw data to a file. This means that values (numbers) have 
   hashes ("#") put around them and strings have quote marks put around 
   them. This makes the text less human readable but easier to read for 
   your programs.
LOC(#filenumber)

   
The LOC function returns the current read/write position within 
   an open file.
LOF(#filenumber)

   
The LOF function gives you the length of the file 
open.
FreeFile

   
This will give you next available file number.
   
Example:
MyFileNumber = FreeFile
Open "c:\windows\faq.txt" For Input as #MyFileNumber
Input$(number_of_chars_to_return, #filenumber)

   
This is the same as Input and Line Input except it has no limits 
   (except the ones you set in number_of_chars_to_return). By using LOF 
   - this can be used to get the whole file. We could replace all the 
   stuff in the Do…Loop in the Read button, with:
Text1.Text = Input$(LOF(1),#1)

   
Why not have a play around and see what these do? Go on, have a 
   go!
   
That's about it for the Input, Output and Append modes.