Files in C - Intro to Computer Programming - Lecture Slides, Slides of Computer Engineering and Programming

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File I/O in C

Lecture 7

Files in C

• In C, each file is simply a sequential stream of bytes. C imposes no structure on a file.
• A file must first be opened properly before it can be accessed for reading or writing. When a file is opened, a stream is associated with the file.
• Successfully opening a file returns a pointer to (i.e., the address of) a file structure, which contains a file descriptor and a file control block.

Opening Files

• The statement:

fptr1 = fopen ( "mydata", "r" ) ; would open the file mydata for input (reading).

• The statement:

fptr2 = fopen ("results", "w" ) ; would open the file results for output (writing).

• Once the files are open, they stay open until you close them or end the program (which will close all files.)

Testing for Successful Open

• If the file was not able to be opened, then the value returned by the fopen routine is NULL.
• For example, let's assume that the file mydata does not exist. Then: FILE *fptr1 ; fptr1 = fopen ( "mydata", "r") ; if (fptr1 == NULL) { printf ("File 'mydata' did not open.\n") ; }

End of File

• The end-of-file indicator informs the program when there are no more data (no more bytes) to be processed.
• There are a number of ways to test for the end-of- file condition. One is to use the feof function which returns a true or false condition: fscanf (fptr1, "%d", &var) ; if ( feof (fptr1) ) { printf ("End-of-file encountered.\n”); }

End of File

• There are a number of ways to test for the

end-of-file condition. Another way is to use

the value returned by the fscanf function:

int istatus ;

istatus = fscanf (fptr1, "%d", &var) ;

if ( istatus == EOF )

printf ("End-of-file encountered.\n”) ;

Closing Files

• The statements:

fclose ( fptr1 ) ;

fclose ( fptr2 ) ;

will close the files and release the file

descriptor space and I/O buffer memory.

Reading and Writing Files

#include <stdio.h>

int main ( )

FILE *outfile, *infile ;

int b = 5, f ;

float a = 13.72, c = 6.68, e, g ;

outfile = fopen ("testdata", "w") ;

fprintf (outfile, "%6.2f%2d%5.2f", a, b, c) ;

fclose (outfile) ; Docsity.com

A Little Help for Assignment G

• Some things that make it easier to understand:
  • Copy the file from the common directory to your directory
  • Use more g06.dat to see what is in the file
  • What kind of numbers are the numbers in the file? How many columns and how many rows?
  • Based on your observations, you can declare a couple of variables, example: dat1 and dat
  • You want to sum these columns so you need a couple more variables, example: sum1 and sum

A Little Help for Assignment G

• You want the average of the columns so you need

some additional variables.

• You will be dealing with a couple of files so you

need a couple of file pointers

• Assignment G06 requires us to read several lines

of some information from a disk file and write it out to the screen and to a file. (Sounds repetitive, doesn't it?)

A Little Help for Assignment G

• In order to use the for loop, the counter, k,

must be declared to be an integer at the

beginning of the main function along with all

of the other variables