Question

Let’s say I have a text file that is 100 lines long. I want to only change what is in the 50th line.

One way to do it is open the file for input and open a new file for output. Use a for-loop to read in the first half of the file line by line and write to the second file line by line, then write what I want to change, and then write out the second half using a for-loop again. Finally, I rename the new file to overwrite the original file.

Is there another way besides this? A way to modify the contents in the middle of a file without touching the rest of the file and without writing everything out again?

If there is, then what’s the code to do it?

Answer 1

Open the file, use fseek to jump to the place you need and write the data, then close the file.

from http://www.cplusplus.com/reference/clibrary/cstdio/fseek/…

#include <stdio.h>

int main ()
{
  FILE * pFile;
  pFile = fopen ( "example.txt" , "r+" );
  fputs ( "This is an apple." , pFile );
  fseek ( pFile , 9 , SEEK_SET );
  fputs ( " sam" , pFile );
  fclose ( pFile );
  return 0;
}