4 – File Processing

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Department of Computer Engineering Faculty of Engineering, Prince of Songkla University

4 – File Processing4 – File Processing

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Opening a File

Declare a file pointer:FILE *fp;

Assign to a the file pointer:fp = fopen("clients.dat", "w");

Function prototype:FILE *fopen(char *name, char *mode);

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fopen() Modes

Mode Meaning

"r" open text file for reading

"w" open text file for writing. If the file already exists, discard the contents.

"a" open text file for appending

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Reading and Writing

of text files

reading and writing will be sequential

Writing clients.dat

Reading

55

Writing

/* write sequentially to clients.dat */

#include <stdio.h>int main(){ int acc; char name[10]; float bal; FILE *cfptr;

if ((cfptr = fopen("clients.dat","w")) == NULL) printf("File could not be opened\n"); else {

:

continued

66

printf("Enter acc, name, & bal.\n? "); scanf("%d%s%f", &acc, name, &bal);

while (!feof(stdin)) { fprintf(cfptr, "%d %s %.2f\n",

acc, name, bal); printf("? "); scanf("%d%s%f", &acc, name, &bal); } fclose(cfptr);}return 0;

}

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clients.dat

1 Davison 23.672 Paun 0.03356 Mason 89.0145 Jackson 11.00

:

88

Reading

/* read sequentially from clients.dat */#include <stdio.h>

int main(){ int acc; char name[10]; float bal; FILE *cfptr;

if((cfptr = fopen("clients.dat", "r")) == NULL) printf("File could not be opened\n"); else {

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continued

99

printf("Acc Name Bal\n"); fscanf(cfptr, "%d %s %f",

&acc, name, &bal);

while(!feof(cfptr)) { printf("%d %s %f\n", acc,name,bal); fscanf(cfptr, "%d %s %f",

&acc, name, &bal); } fclose(cfptr); } return 0;}

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Example: Double Space a File

Call:

$ dbl_space file1 file2

dbl_space.c

double_space()

prn_info()

A Graceful fopen()

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dbl_space.c

#include <stdio.h>#include <stdlib.h>

void double_space(FILE *, FILE *);void prn_info(char *);

int main(int argc, char *argv[]){ FILE *ifp, *ofp;

if (argc != 3){ prn_info(argv[0]); exit(1); }

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continued

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ifp = fopen(argv[1], "r"); /* read */ ofp = fopen(argv[2], "w"); /* write */

double_space(ifp, ofp);

fclose(ifp); fclose(ofp);

return 0;}

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double_space()

void double_space(FILE *ifp, FILE *ofp){ int c;

while ((c = getc(ifp)) != EOF) { putc(c, ofp); if (c == ' ') putc(' ', ofp); /* found space -- double it */ }}

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prn_info()

void prn_info(char *pgn_name)/* version 1 */{ printf("\nUsage: %s infile outfile\n", pgn_name); }

void prn_info(char *pgn_name)/* version 2 */{ fprintf(stderr, "\nUsage: %s inf outf\n",

pgn_name);}

1515

A Graceful fopen()

FILE *gfopen(char *file_name, char *mode){ FILE *fp;

if ((fp = fopen(file_name, mode)) == NULL) { fprintf(stderr, "\nCannot open %s\n", file_name); exit(1); } return fp;}

1616

Random Access

Insertion Problem

How to do Random Access

Binary Filesfwrite()

Creation Programfseek()

Manipulation Programfread()

Printing Program

1717

Insertion Problem

clients.dat:

0 White 12.5630 Davison 2345.7832 Deitel 0.00

:

1002 Paun 0.0323 White 12.5135 Brown 1.45

insertion using sequential read/write is very expensive

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How to do Random Access

Creation Program

Create a binary file called creditaccount.dat using fwrite() where each entry is a fixed length.

::

::

::

::

1

2

3

100

Davison Andrew 12.27

White Jim 3.24

Brown Mary 23.67

Wai Loke Seng 4.45

acctnum lastname firstname bal

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Manipulation Program

Use fseek() with acctnum to go to an entry immediately.

2020

Binary Files

A binary file stores the system's internal representation of C data structures.

fopen() modes must include a "b": "rb", "wb", "ab", etc.

2121

fwrite()

Informally:fwrite( <pointer to data being inserted>,

<size of the data type>,<number of data items being inserted>,<file pointer>)

Example:fwrite( &blankclient,

sizeof(struct clientdata),1, cfptr);

2222

Creation Program

/* create 100 empty clients in clientaccount.dat */

#include <stdio.h>

struct clientdata { int acctnum; char lastname[15]; /* size specified */ char firstname[10]; /* size specified */ float balance;};

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continued

2323

int main(){ int i; struct clientdata blankclient =

{0, "", "", 0.0}; FILE *cfptr;

if ((cfptr =fopen("creditaccount.dat", "wb"))==NULL) printf("File could not be opened.\n"); else {

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continued

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for (i = 1; i <= 100; i++) fwrite( &blankclient,

sizeof(struct clientdata), 1, cfptr);

fclose (cfptr); } return 0;}

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fseek()

Function prototype:int fseek(FILE *fp, long int offset,

int whence);

offset is the position of the entry.

In this case, it is:(acctnum of entry - 1) * size of an entry

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whence is a symbolic constant indicating the starting position for a seek:

SEEK_SET beginning of file

SEEK_CUR current location in file

SEEK_END end of file

2727

Manipulation Program

/* insert client info for a specified accnum into creditaccount.dat */

#include <stdio.h>#include <string.h>

struct clientdata { int acctnum; char last[15]; char first[10]; float bal;};

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continued

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int main(){ FILE *cfptr; struct clientdata client;

if ((cfptr =fopen("creditaccount.dat", "rb+")) == NULL)

printf("File could not be opened.\n"); else {

:

continued

2929

scanf("%d%s%s%f",&client.acctnum, client.last, client.first, &client.bal);

fseek(cfptr, (client.acctnum - 1) * sizeof(struct clientdata), SEEK_SET);

fwrite(&client, sizeof(struct clientdata), 1, cfptr);

} fclose(cfptr); return 0;}

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fread()

Informally:fread( <pointer to data structure for

holding extracted data>,<size of the data type>,<number of data items>,<file pointer>)

Example:fread( &client,

sizeof(struct clientdata),1, cfptr);

3131

Printing Program

/* print out creditaccount.dat */

#include <stdio.h>#include <string.h>

struct clientdata { int acctnum; char last[15]; char first[10]; float bal;};

:

continued

3232

int main(){ FILE *cfptr; struct clientdata client;

if ((cfptr = fopen("creditaccount.dat", "rb")) == NULL)

printf("File could not be opened.\n"); else {

:

continued

3333

printf("Acct Lastname Firstname Balance\n"); while (!feof(cfptr)) { fread(&client,

sizeof(struct clientdata), 1, cfptr);

if (strcmp(client.last, "") != 0) printf("%d %s %s %f\n",

client.acctnum, client.last, client.first, client.bal);

} } fclose(cfptr); return 0;}

3434

Text Files and Binary Files Compared

Example Data Structures

Writing to a Text File

Writing to a Binary File

When to Use a Binary File

When to Use a Text File

3535

Example Data Structures

#define PNUM 50 /* number of planets */#define NAMELEN 20

struct planet { char name[NAMELEN]; /* known size */ double diameter; double dist_sun;};struct planet earth;struct planet planets[PNUM];

/* initialise earth and planets */

3636

Writing to a Text File

Assume a text file is being accessed by the file pointer tp:

fprintf(tp, "%s %lf %lf", earth.name, earth.diameter, earth.dist_sun);

for (i = 0; i < PNUM; ++i)fprintf(tp, "%s %lf %lf",

planets[i].name, planets[i].diameter, planets[i].dist_sun);

3737

Writing to a Binary File

Assume a binary file is being accessed by the file pointer bp:

fwrite(&earth, sizeof(planet), 1, bp);

fwrite(planets, sizeof(planet)*PNUM, PNUM, bp);

3838

When to Use a Binary File

If the file will contain complicated data structures.

If the file is to be manipulated using byte size-related functions (fwrite(), fseek(), etc).

3939

If speed of access is important(use fseek() instead of a sequential search).

If storage space is important (the data structure size can be defined).

4040

When to Use a Text File

If the file will contain text.

If the file is to be moved between systems (text is portable).