CODEWITHSUNDEEP
cfseek
system
system

Reputation: 11

fseek() doesn't work

I have opened a file using a and r+ but when I use fseek and ftell the file pointer is always 0. My file looks like this:

 1 -3     
 2 -8

And I want to add another line between the two but it is added in the end after the last line. Someone in another forum said that when you open the file in append the pointer is always zero and you have to open it in r+ and if that doesn't work "you have to read the complete data and then insert the data in the variables and write it back." but I don't understand what they mean by that.

Can anyone help with inserting numbers in the middle of a file?

Thanks!

Would something like this work? To transfer the data?

    rewind(fp);
    fscanf(fp,"%d",&ch);
    fprintf(fp1,"%d",ch);
    fseek(fp,1,0);
    fscanf(fp,"%d",&ch);
    fprintf(fp1,"%d",ch);

Upvotes: 1

Views: 2102

Answers (3)

Luiz Rodrigo
Luiz Rodrigo

Reputation: 936

Like others already said, there's no easy way to insert data in the middle of a file. If you really want to do this, you can implement the following steps:

  1. Create a second file
  2. Copy all data before the place you want to insert to the second file
  3. Insert the line you want to the second file
  4. Copy the remaining data to the second file
  5. Delete the original file
  6. Rename the second file

Other approach is using binary files instead of text files. Although binary files are a bit harder to learn, once you understand how they work you'll see that working with them is much like working with arrays. To perform this task, for example, you'd not even need to use an auxiliary file.

Upvotes: 2

andrewdski
andrewdski

Reputation: 5505

There isn't an easy way to insert data in the middle of the file. A file is basically an array of characters. To add a character in the middle, you need to copy everything following your insertion point down one location. With a file you need to read the data that follows and write it after your addition.

Generally, when you want to do something like this you create a new file. You copy the old file into it up to the point where you want to insert, then you write the data you want to insert, then you copy the rest of the old file. Finally, you rename the new file to the old file.

Upvotes: 0

Mat
Mat

Reputation: 206689

There is no open mode that will allow you to "insert" data into a file at a random point. The only place you can add data without overwriting existing data is the end of the file (what you get opening with mode "a").

If you want to insert at a random position, you need to do it yourself.

One of the easier ways is to re-write the file completely (transfer the start of the old file to a new file, add your data to the new file, transfer the rest of the old file, and rename/overwrite at the end).

The hard way: you need to "shift" all the data from your insertion point to the end-of-file manually. That's not trivial to get right.

Upvotes: 1

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