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Unlike random file access, binary file access has variable length records.
There is no wasted space in a binary accessed file. If you retrieve the
data at file location 112 as an integer, bytes 112 and 113 are retrieved
to make up an integer value, because an integer data type requires two
bytes.
It does not matter that these two bytes may be part of 4 bytes previously
stored as Long. It is up to your application to keep track of the contents
of the file and make sure that such actions are correct. The following is
an example data type for binary file access:
Type Person
LName as String
FName as String
Age as Integer
End Type
Note that LName and FName are strings, which is a variable-length data
type. Age is an integer, which is a 2-byte data type.
For more information about the bytes required by data types, see the
online Help topic, "Data Type Summary."
Advantages of Binary Access Files
- You can conserve disk space by building variable-length records.
- You can read and write to a file opened for binary access like you
can a file opened with random access.
Disadvantage of Binary Access Files
- You must know precisely how the data is written to the file to
manipulate it successfully.
Writing to Files Opened for Binary Access
Because records with binary access can be of variable length, it is
necessary to actually store information about the size of each field and
record so that it can be read successfully. A good way to accomplish this
is to store an integer with each string to indicate the length of the
string. The following is an example of creating such a file:
Type Person
LName as String
FName as String
Age as Integer
End Type
Sub WriteOneRecord(PRecord as Person)
Dim StrSize as Integer
' Write the LName field and indicate the length of LName
' because it is a variable-length string.
StrSize = Len(PRecord.LName)
Put #1,,StrSize
Put #1,,PRecord.LName
' Write the FName field and indicate the length of FName
' because it is a variable-length string.
StrSize = Len(PRecord.FName)
Put #1,,StrSize
Put #1,,PRecord.FName
' Write the Age field - this is type integer so it is not
' necessary to indicate a length.
Put #1,,PRecord.Age
End Sub
Sub WriteBinary()
Dim P as Person
' Create a new file and open it for Binary access.
Open "BINARY.TXT" For Binary As #1
' Create and write the first record.
P.LName = "Doe"
P.FName = "Jane"
P.Age = 9
WriteOneRecord P
' Create and write the second record.
P.LName = "Thompson"
P.FName = "Richard"
P.Age = 4
WriteOneRecord P
' Close the file.
Close #1
End Sub
When the WriteBinary macro is run, it will create a file called
BINARY.TXT.
The two records in this example take up 34 bytes (as opposed to the 44
bytes required by the same data with random access). Keep in mind that
when opening this file in a text editor, such as Notepad, the file
will not be readable. It is a binary file, not a text file.
A trade-off in using variable-length field and binary access instead of
fixed-length fields and random access is that the entire record could be
written with a single function call using random-access. While binary
access provides greater flexibility, it also requires more code to handle
I/O operations.
Reading Files Opened for Binary Access
The Get statement reads a number of bytes equal to the bytes required for
the variable that is used. When you use Get with a variable-length string,
the number of bytes read from the file equals the current length of the
string. To temporarily set the length of a variable-length string, you can
use the STRING$ function to set the variable equal to a specific number of
blanks, or spaces.
The following example reads a file like the one created with the
WriteBinary macro:
Type Person
LName as String
FName as String
Age as Integer
End Type
Sub ReadOneRecord(PRecord as Person)
Dim StrSize As Integer
' Determine the size of the LName field and read it.
Get #1, , StrSize
PRecord.LName = String(StrSize," ")
Get #1, , PRecord.LName
' Determine the size of the FName field and read it.
Get #1, , StrSize
PRecord.FName = String(StrSize," ")
Get #1, , PRecord.FName
' Read the Age field.
Get #1, , PRecord.Age
End Sub
Sub ReadBinary()
Dim P as Person
' Open the file for Binary access.
Open "BINARY.TXT" For Binary As #1
' Read each record in the file and display it in the Debug
' window.
Do Until EOF(1)
ReadOneRecord P
Debug.Print P.LName, P.FName, P.Age
Loop
' Close the file.
Close #1
End Sub