- You have not seen object oriented programming yet (at least,
some of you)...
- C++ iostreams used object oriented programming style
which looks pretty weird for non-object oriented programmers...
- To avoid confusion, I decided to teach you accessing files using the C library
- C provides a standard library to access text or "formatted" data
- The library uses an internal data structure
(hidden from the programmer)
that is accessible through a "file pointer
- Defining a file pointer variable:
#include <stdio.h> FILE *fp;
#include <stdio.h>
FILE *fp;
fp = fopen("filename", "r");
better:
if ( (fp = fopen("filename", "r") ) == NULL )
{
cout << "Error: can't open file" << endl;
exit(1);
}
|
- Commonly used format strings:
- %d integer
- %f float
- %lf double precision float (long float)
- %s word (characters)
- Example Program:
(Demo sequential reading of formatted data) ---
click here
fscanf(fp, "format string", &variable); |
#include <stdio.h>
FILE *fp;
fp = fopen("filename", "w"); // Overwrites (existing) file
fp = fopen("filename", "a"); // Appends to (existing) file
better:
if ( (fp = fopen("filename", "w") ) == NULL )
{
cout << "Error: can't create file" << endl;
exit(1);
}
|
- After you finish writing a file, you must close
the file to avoid possible data loss due to system loss of your data
fclose(fp);
- Example Program:
(Demo sequential writing of formatted data) ---
click here
fprintf(fp, "format string", &variable); |
- "Sequential file" access means that in order to read
the Nth byte in a file,
you must read the N-1th byte first
(By induction, that means that you must read all the things preceding something first)
- "Random file" access means that you can read
the Nth byte in a file
without
having to read the
N-1th byte first.
- To understand "random file" access, you need to realize that the C-library data structure used to access file contains "file position" information that the programmer can manipulate
- offset
the new file position.
(This exact location of the new position depends on
whence)
- whence
can be one of the following values:
SEEK_SET
The new position is equal to offset bytes from the beginning of the file SEEK_CUR
The new position is equal to offset bytes from the current read position in the file SEEK_END
The new position is equal to offset bytes from the end of the file - fseek() returns -1 when an error occurs; otherwise, it returns 0.
- Example Program:
(Demo random reading of formatted data) ---
click here
#include <stdio.h> fseek(fp, offset, whence); |
- When you read a data file, you have to know what data is stored
in the file...
- A common mistake in reading from a file is not knowing what
is in the file....
- For example:
fscanf(fp, "%d", &i);
will read something unexpected if the next item is NOT an integer...
- Example Program:
(Demo reading the wrong data) ---
click here