- 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
- The
standard C library
provides a number of
functions
to access
text
or
formatted
data
- The library uses an internal data structure
(hidden from the programmer)
that is accessible through a
file pointer
- In order to
access a file,
you must
define a file pointer variable
- Defining a file pointer variable:
#include <stdio.h> FILE *fp;
- All file operations will need to use a file pointer to locate the file
- You must first
open
the file before
reading data from the file
- C libary function to open a file:
#include <stdio.h> FILE *fopen(char *filename, char *mode);
NOTE:
- (char *)
is the type of a
string
(character array)
- Use mode = "r" to open a file for reading
- Use mode = "w" to open a file for writing
- Use mode = "a" to open a file for appending
- The fopen()
function returns
- a pointer to a file structure if successful
- NULL if UNsuccessful
- (char *)
is the type of a
string
(character array)
- Example:
#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); }
- Text data
are data represented using
ASCII code
- Text data files
can be viewed using an
editor
-
Formatted read/write
operations converts between
ASCII representation and
internal (computer) representation
- The
formatted read fread()
function is used to
read formatted data
- Syntax:
- The
"format string"
specifies the
type
and
number
of
data items to read
fscanf(fp, "format string", &variable1 [, &variable2, ...] );
- The
"format string"
specifies the
type
and
number
of
data items to read
- Commonly used format strings:
- %d integer
- %f float
- %lf double precision float (long float)
- %s string (array of characters)
- Examples:
FILE *fp; char line[100]; int i; float f; double d; fp = fopen("textFile", "r"); fscanf(fp, "%s", line); // line = &line[0] fscanf(fp, "%d", &i); fscanf(fp, "%f", &f); fscanf(fp, "%lf", &d);
- Example Program:
(Demo above code)
- Prog file: click here
- Input file: click here
- C libary function to open a file for writing:
#include <stdio.h> FILE *fopen(char *filename, "w"); // Overwrite file with new data FILE *fopen(char *filename, "a"); // Append new data to file
- Example:
#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); }
- When you
finish writing a file,
you must
close the file
to avoid possible data loss
fclose(fp);
- The
formatted print fprintf()
function is used to
write formatted data
- Syntax:
- The
"format string"
specifies the
type
and
number
of
data items to read
- Examples:
FILE *outputfp; char line[100]; int i; float f; double d; strcpy(line, "Hello"); i = 4; f = 3.1415; d = 2.718281828; outputfp = fopen("outFile", "w"); // Try changing to "a" fprintf(outputfp, "%s %d %f %lf", &line[0], i, f, d); fclose(outputfp);
fprintf(fp, "format string", &variable1 [, &variable2, ...] );
- The
"format string"
specifies the
type
and
number
of
data items to read
- Example Program:
(Demo above code)
- Prog file: click here
- "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 SEEK_SET)
- Prog file: click here
- Input file: 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 above code)
- Prog file: click here
- Input file: click here