File handling in C

trinath · #1 09-Jan-2006, 12:04

Hi all,

I have a problem in handling files through C.

here is the problem im having:

i will query the database (for instance consider employees table )
for empno,ename,job,salary fields.The query returns me some 100 of rows. now i need to place them in a file in row wise pattern as they exist in the database.

now , if any row is updated in the database i need to update the same row in the file . also if any row is deleted i need to remove the same row in the file , and if any new row is inserted into the data base i need to insert the row in the file.

can any one suggest me any method to do this .
The performance is and main criteria(i mean time to do file operations. )

please help me in this regard,

thanks in advance,

Trinath Somanchi,
Hyderabad.

davekw7x · #2 09-Jan-2006, 13:19

Quote:

Originally Posted by trinath

if any row is updated in the database i need to update the same row in the file . also if any row is deleted i need to remove the same row in the file , and if any new row is inserted into the data base i need to insert the row in the file.

A common method for editing files in general goes something like this:

Code:

1. Open the original (old) file for reading.

2. Open a new file for writing. Give it a temporary name: whatever you want.

3. Repeat this loop until the end of the old file is reached:

     If this is a place to insert a new line, then
        Write the new line to the new file and continue the loop.

     Read a line from the old file into memory.

     If this line is supposed to be edited then
         Make changes to the line in memory.

     If this line is not supposed to be deleted then
         Write the line from memory to the new file.

4. Close the old file and close the new file.

5. Delete the old file (or rename it to something else so that becomes
   the backup file for the original).

6. Rename the new file so that its name is the same as the original (old) file.

Quote:

Originally Posted by trinath

The performance is and main criteria(i mean time to do file operations. )

I don't know what this means. Reading and writing files is not something over which you have much control (disk buffering, etc. are functions of hardware and operating system software). Making the data base on disk have some kind of indexed structure where all records have exactly the same length might give some possibilities for optimizing access time, but I can't really know what might be appropriate without a lot more information.

Regards,

Dave

WaltP · #3 09-Jan-2006, 13:25

After you make all your changes in the database, rewrite the files. To try to change the file at the same time as the database defeats the purpose of the database itself. In other words, the reason for the database is so you don't have to manipulate flat files.

It's the same concept as purchasing a family car for the efficiency of transporting the family. Then when you have to go somewhere, half the family takes the car (your database), the other half takes their bicycles (your files). You're downgrading your efficiency and power for the sake of the old technology.

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File handling in C

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