Working with Sequential Access Files (151262)

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Microsoft provides programming examples for illustration only, without warranty either expressed or implied. This includes, but is not limited to, the implied warranties of merchantability or fitness for a particular purpose. This article assumes that you are familiar with the programming language that is being demonstrated and with the tools that are used to create and to debug procedures. Microsoft support engineers can help explain the functionality of a particular procedure, but they will not modify these examples to provide added functionality or construct procedures to meet your specific requirements. The advantages of binary access files are as follows:

• Files of this type can be easily created or manipulated by text editors.
• Just about all applications can read/write sequential access files.
• Files of this type are easy to process in Visual Basic for Applications.

The disadvantages of binary access files are as follows:

• Files of this type are not well suited for storing large amounts of data because all values are stored as a character string.
• Files of this type usually require more storage space than the other access types.
• You cannot read and write to a file that is opened for sequential access simultaneously. You can only do one or the other at one time.

Opening Files for Sequential Access

When you open a file for sequential access, you open it to perform one of the following operations:

• Input characters to the file (Input)
• Output characters to the file (Output)
• Append characters to the file (Append)

To open a file for any one of these operations, use the Open statement, as follows: When you use sequential access to open a file for Input, the file must already exist; otherwise, Visual Basic for Applications generates a trappable error. When you try to open a nonexistent file for Output or for Append, the Open statement actually creates the file first, and then opens it. Each time you open the file, you must use the Close statement to close the file before reopening the file for another type of operation.

Reading Files Opened for Sequential Access

To retrieve the contents of a text file, first open the file for sequential input. Then, use Line Input # or Input # to copy the file into variables. Use Line Input # when you need to read a file, one line at a time. With delimited files (such as CSV), use Input # to read each line of the file into a list of variables.

Reading a File Line-by-Line

Use Line Input # with a file opened for sequential access if the data is stored in the file one line at a time. The Line Input # statement reads from a file one character at a time until it encounters a carriage return (Chr(13)) or a carriage return-linefeed sequence. Carriage return-linefeed sequences are skipped rather than appended to the character string.

The following sample code uses the Line Input # statement to read data in from a sample text file called Textfile.txt, one line at a time.

NOTE: You will have to create the text file called Textfile.txt, if one does not already exist.

  Sub ReadStraightTextFile()
      Dim LineofText As String

      ' Open the file for Input.
      Open "TEXTFILE.TXT" For Input As #1

      ' Read each line of the text file into a single string
      ' variable.
      Do While Not EOF(1)
         Line Input #1, LineofText
         MsgBox LineofText
      Loop

      ' Close the file.
      Close #1

   End Sub
				

Reading a Delimited Text File

As mentioned before, use Input # to read delimited files. When read, standard string or numeric data is assigned to variables as they appear in the text file. Delimiting commas or blank lines within the file are returned as Empty. Double quotation marks ("") that surrounds each field in the input data is ignored and fields surround with #s (pound signs) can be interpreted as dates. When using Input #, data items in a file must appear in the same order as the variables in the variable list and be matched with variables of the same data type. If the actual data doesn't match the variable type, you may encounter run-time errors.

If you have a text file named Delimit.txt and enter the following data, the macro below will read in the data, one line at a time. The following sample code uses the Input # statement to read data from the sample text file into variables:

    Sub ReadDelimitedTextFile()

      Dim LName As String, FName As String, Addr As String, City As String
      Dim state As String
      Dim age As Integer

      ' Open the file for Input.
      Open "DELIMIT.TXT" For Input As #1

      ' Read each line of the text file into the list of variables
      ' until the end of the file is reached.
      Do While Not (EOF(1))
         Input #1, LName, FName, age, Addr, City, state, zip
         MsgBox LName & ", " & FName & ", " & age & ", " & Addr & ", " _
            & City & ", " & state & ", " & zip
      Loop

      ' Close the file.
      Close 1

   End Sub
				

Writing to Files Opened for Sequential Access

To store the contents of variables in a sequential text file, open it for sequential access, and then use either the Print # or Write # statement to write the data to the file.

The Write # statement is used to write raw data to the text file as comma- delimited and has the following syntax:

   Write #filenumber[,outputlist]
				

When you write to the file using Write #, string fields are surrounded with double quotation marks and date fields are surrounded with #s (pound signs). In this respect, the Write # is a companion to Input #. The following macro demonstrates how you can write to a delimited text file:

   Sub WriteFile()
      Dim LName As String
      Dim BDay As Date
      Dim age As Integer

      ' Create a new text file called Test.txt.
      Open "TEST.TXT" For Output As #1

      ' Create and then write the first "record."
      LName = "Doe"
      BDay = #1/1/95#
      age = 1
      Write #1, LName, BDay, age

      ' Create and then write the second "record."
      LName = "Smith"
      BDay = #4/29/56#
      age = 39
      Write #1, LName, BDay, age

      ' Create and then write the third "record."
      LName = "Jones"
      BDay = #5/1/80#
      age = 15
      Write #1, LName, BDay, age

      ' Close the file.
      Close #1

   End Sub
				

After the WriteFile macro is finished, you will have a comma-delimited text file named Test.txt that looks as follows: Unlike Write #, the Print # statement writes display-formatted data (or space-formatted data) to a sequential file. The Print # statement has the following syntax:

   Print #filenumber,[outputlist]
				

In the output variable list for Print #, you can specify a number of spaces to separate fields. For more information, search for "Print # Statement," using the Microsoft Excel Help Index.

If you change Write # to Print # in the previous WriteFile macro, the Test.txt file would look something like this instead:

Modifying Data in a File Opened for Sequential Access

Note that when you Open a file for Output, if the file already exists, you are essentially replacing the contents of the file when you write to it. The new data that is written is not appended to the file. In order to append data to the end of a file, you must Open the file for Append. All data that is written to a file opened for Append is added to the end of the file. You cannot modify the "records" in a file that is opened for sequential access. What you have to do is as follows:

1. Open the file for Input and read all of the data into variables.
2. Close the file.
3. Make the necessary modifications to the data contained in the variables.
4. Open the file for Output and write all of the modified data to the file.
5. Close the file.

Because of all the steps required to make modifications to a file opened for sequential access, sequential access may not be the most efficient method for modifying data in a text file, especially if the text file is large.