In VBA (Visual Basic for Applications), the Seek statement is used with files opened in Binary, Random, or Input/Output (I/O) modes to move the file pointer to a specified position. This is particularly useful when you want to read from or write to a specific location in a file.
Here’s how to use the Seek statement:
Seek [#]filenumber, position
- filenumber: This is the file number used in the Open statement.
- position: This is the position in the file where you want to move the file pointer. The position is specified as a byte number for Binary files and as a record number for Random files.
Example Usage in Binary Mode
To move the file pointer in a file opened in Binary mode:
Open "example.dat" For Binary As #1
Seek #1, 10 ' Move the file pointer to the 10th byte
' Read or write data at the 10th byte
Example Usage in Random Mode
To move the file pointer in a file opened in Random mode:
Open "example.dat" For Random As #1 Len = 128
Seek #1, 5 ' Move the file pointer to the 5th record
' Read or write data at the 5th record
Using Seek with Input and Output
The Seek statement can also be used with files opened for Input or Output, but its behavior is different. For Input, Seek moves to a specified byte position. For Output, Seek is less commonly used since it always writes data to the end of the file.
- If you attempt to seek beyond the end of a file in Binary or Random modes, no error occurs, and the file length is extended.
- If you Seek to a position before the beginning of the file, a runtime error occurs.
- The first byte in a Binary file and the first record in a Random file are both considered to be at position 1 (not 0).
- In Random mode, the Len clause in the Open statement determines the size of each record.
- Seek is particularly useful for manipulating large files where you don’t want to read or write the entire file into memory.
Seek is often used in applications that require direct file manipulation, such as creating and accessing binary files, custom data files, or modifying specific records in a file.
Using Seek effectively allows for efficient and targeted file operations, which can be essential for handling large files or for applications that require random access file I/O.