Chapter 11 data files and file processing
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Chapter 11: Data Files and File Processing. Files and streams Creating a sequential access file Reading data from a sequential access file Using fgetc() and fputc() Using fread() and fwrite(). Files and Streams. C views a file as a sequential stream of bytes.

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Chapter 11: Data Files and File Processing

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Chapter 11 data files and file processing

Chapter 11: Data Files and File Processing

  • Files and streams

  • Creating a sequential access file

  • Reading data from a sequential access file

  • Using fgetc() and fputc()

  • Using fread() and fwrite()


Files and streams

Files and Streams

  • C views a file as a sequential stream of bytes.

  • A file ends either with an EOF (end-of-file) marker or at a specified byte number specified by the system.

  • When a file is opened, a stream is associated with a file.

  • Streams provide communication channels between files and the programs.


Chapter 11 data files and file processing

  • In addition to providing access to a file, a stream can also be used to access devices.

  • For example, when a program (any program) is executed, 3 streams are automatically opened:

    • standard input (stdin)

      • enable the program to read data from keyboard

    • standard output (stdout)

      • enable the program to print data on the screen

    • standard error (stderr)

      • enable program to print errors on the screen

  • They are all manipulated using file pointers.


Creating a sequential access file

Creating a Sequential Access File

#include <stdio.h>

void main (void) {

int account;

char name[30];

float balance;

FILE *f;

if ((f = fopen(“clients.txt”, “w”)) == NULL)

printf(“Error: the file cannot be opened\n”);

else {

printf (“Enter the account, name and balance or EOF to end input:\n”);

scanf (“%d%s%f”, &account, name, &balance);

while (!feof(stdin)) {

fprintf(f, “%d %s %.2f”, account, name, balance);

scanf (“%d%s%f”, &account, name, &balance);

}

}

fclose (f);

}


Chapter 11 data files and file processing

  • FILE *f;

    • States that f is a pointer to a file structure

    • If there is more than one file, each file needs to have its own FILE pointer.

  • if ((f = fopen(“clients.txt”, “w”)) == NULL)

    • The fopen() function takes 2 arguments: the file name and the file mode.

    • This statement will try to open a file named “clients.txt”.

    • If the file is to be placed in a different directory than the program directory, the full path need to be specified. For example: “A:\clients.txt”

    • The function returns a pointer to the successfully opened file. But if the file cannot be opened, a NULL is returned.

    • The option “w” is the mode of the file to be opened.


Chapter 11 data files and file processing

  • while (!feof(stdin))

    • Determines whether the input from the keyboard is an EOF character or not. If it is an EOF character, the while loop will terminate.

    • An EOF character is not the word “EOF” !!!


Chapter 11 data files and file processing

  • fprintf (f, “%d %s %.2f\n”, account, name, balance);

    • Writes data to the file clients.dat

    • Equivalent to printf() except that it receives as an argument a file pointer for the file to which the data will be written.

  • fclose(f);

    • Closes the file “clients.txt”.

    • Will actually write the data to the file.

    • If fclose() is not called explicitly, the operating system will close the file when the program terminates.


Chapter 11 data files and file processing

  • If an error occurs while opening a file (in which case fopen() returns a NULL), it could be due to any of these errors:

    • Opening a non-existing file for reading

    • Opening a file for reading or writing without having granted the appropriate access to the file by the operating system.

    • Opening a file for writing when no disk space is available.

  • Always remember that the mode “w” will overwrite the current data in the file. When we want to update a file, always use the update mode “r+”.


Reading data from a sequential access file

Reading Data from a Sequential Access File

void main (void) {

int account;

char name[30];

float balance;

FILE *fptr;

if ((fptr = fopen(“clients.txt”, “r”)) == NULL)

printf(“Error: the file cannot be opened\n”);

else {

printf (“%-10s%-13s%s\n”, “Account”, “Name”, “Balance”);

fscanf (fptr, “%d%s%f”, &account, name, &balance);

while (!feof(fptr)) {

printf (“%-10s%-13s%s\n”, “Account”, “Name”, “Balance”);

fscanf (fptr, “%d%s%f”, &account, name, &balance);

}

}

fclose (fptr);

}


Chapter 11 data files and file processing

  • if ((fptr = fopen(“clients.txt”, “r”)) == NULL) ;

    • Attempts to open the file clients.txt for reading and determines whether the file is opened successfully. Notice that the mode used is “r”.

  • fscanf (fptr, “%d%s%f”, &account, name, &balance);

    • Reads data from the file clients.txt

    • Equivalent to function scanf() except that it receives as an argument a file pointer for the file from which the data is read.


Using fgetc and fputc

Using fgetc() and fputc()

  • Syntax:

    • int fgetc(FILE *f)

    • fputc(int c, FILE *f)

  • fgetc() reads a character from the file referred to by the file pointer f.

  • fputc() puts the character c into the file referred to by the file pointer f.

  • If the file has reached the end, fgetc() will return the EOF character.


Using fread and fwrite

Using fread() and fwrite()

  • Syntax:

    • size_t fread (void *d, size_t s, size_t n, FILE *f)

    • size_t fwrite (const void *d, size_t s, size_t n, FILE *f)

  • fread() reads n items of size s into the memory pointed to by d. The function returns the total size of data that has been read.

  • fwrite writes n items of size s from the memory pointed to by d. The function returns the total size of data that has been successfully written.


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