1. Introduction to Programming
Lecture 19

2. Random Access Files

3. Files
open ( file_name , mode ) ;
close ( ) ;

4. ifstream myFilePtr ;
myFilePtr.open ( “myFile” , ios :: in ) ;

5. Output File Stream
ios :: app
ios :: trunc
ios :: ate

6. Read/write a character
get ( ) Read a character
put ( ) Write a character

7. Number of characters to read
getline(str,10, ‘\n’) ;
Number of characters to read
Delimiter
Storage Requirements for Text and Binary Modes
Storage Requirements for Text and Binary Modes
for an Integer:
for an Integer:
Page 32
Page 32
The fwrite() Function
The fwrite() Function
size_t fwrite (void *ptr, size_t size, size_t nmemb,
size_t fwrite (void *ptr, size_t size, size_t nmemb,
FILE *fp);
FILE *fp);
fwrite() writes, from the address pointed to by ptr,
fwrite() writes, from the address pointed to by ptr,
up to nmemb elements whose size is specified by
up to nmemb elements whose size is specified by
size, to the file pointed by fp
size, to the file pointed by fp
fwrite() returns the number of elements
fwrite() returns the number of elements
successfully written, which will be less than nmemb
successfully written, which will be less than nmemb
only if a write error is encountered.
only if a write error is encountered.
double earnings[10];
double earnings[10];
fwrite(earnings,sizeof(double),10,fp);
fwrite(earnings,sizeof(double),10,fp); 32 32
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The fread() Function
The fread() Function
size_t fread(void *ptr, size_t size, size_t nmemb,
size_t fread(void *ptr, size_t size, size_t nmemb,
FILE *fp);
FILE *fp);
fread() reads, into the array pointed to by ptr, up to
fread() reads, into the array pointed to by ptr, up to
nmemb elements whose size is specified by size,
nmemb elements whose size is specified by size,
form the file pointed to by fp.
form the file pointed to by fp.
fread() returns the number of elements
fread() returns the number of elements
successfully read, which may be less than nmemb
successfully read, which may be less than nmemb
is a read error or end-of-file is encountered.
is a read error or end-of-file is encountered. 33 33
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Writing file data using block I/O functions
Writing file data using block I/O functions
#include <stdio.h>
#include <stdio.h>
typedef struct {
typedef struct {
char name[20];
char name[20];
int serial_code;
int serial_code;
int amount;
int amount;
float
float
cost;
cost;
} component;
} component;
main(void)
main(void)
{ component comp_data;
{ component comp_data;
FILE
FILE
*fp;
*fp;
char
char
filename[80];
filename[80];
int numofcomp;
int numofcomp;
int i;
int i;
printf("Enter the file name: ");
printf("Enter the file name: ");
gets(filename);
gets(filename); 34 34
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if ((fp = fopen(filename, "wb")) == NULL) {
if ((fp = fopen(filename, "wb")) == NULL) {
printf("can't open file \n");
printf("can't open file \n");
exit(1);
exit(1);
}printf("Enter the number of components: ");
}printf("Enter the number of components: ");
scanf("%d", &numofcomp);
scanf("%d", &numofcomp);
for (i=0; i<numofcomp; i++) {
for (i=0; i<numofcomp; i++) {
printf("Name of the component: ");
printf("Name of the component: ");
gets(comp_data.name);
gets(comp_data.name);
printf("Serial code of the component: ");
printf("Serial code of the component: ");
scanf("%d", &comp_data.serial_code);
scanf("%d", &comp_data.serial_code);
printf("Amount of the component: ");
printf("Amount of the component: ");
scanf("%d", &comp_data.amount);
scanf("%d", &comp_data.amount);
printf("Cost of each component: ");
printf("Cost of each component: ");
scanf("%f", &comp_data.cost);
scanf("%f", &comp_data.cost);
fwrite(&comp_data, sizeof(comp_data), 1, fp);
fwrite(&comp_data, sizeof(comp_data), 1, fp);
}fclose(fp);
}fclose(fp);
return 0;
return 0; } } 35 35
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12.5 Random Access
12.5 Random Access
Random Access: fseek() and ftell()
Random Access: fseek() and ftell()
fseek, ftell: All the previous I/O functions do reading
fseek, ftell: All the previous I/O functions do reading
and writing sequentially, i.e. read the 1st datum, 2rd
and writing sequentially, i.e. read the 1st datum, 2rd
datum, 3rd datum ..., etc.
datum, 3rd datum ..., etc.
fseek
fseek
and
and
ftell
ftell
help doing
help doing
I/O in non-sequential manner, i.e. read the 10th
I/O in non-sequential manner, i.e. read the 10th
datum, then go back and read the 2nd datum.
datum, then go back and read the 2nd datum. 36 36
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File position pointer
File position pointer
The system keeps a file position pointer for
The system keeps a file position pointer for
each open file.
each open file.
It indicates the location in the file at
It indicates the location in the file at
which data will be read or written.
which data will be read or written.
file position marker
file position marker
......
......
Disk
Disk 37 37
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Page 38
The fseek() Function
The fseek() Function
The function prototype of
The function prototype of
fseek()
fseek() is is
int fseek(FILE *fp, long int offset, int mode);
int fseek(FILE *fp, long int offset, int mode); * *
fseek()
fseek()
returns
returns
if OK, and
if OK, and -1 -1
if there is an error.
if there is an error. * *
offset
offset
tells how far to move from the starting point
tells how far to move from the starting point
(depending on the mode). It can be +ve (move
(depending on the mode). It can be +ve (move
forward) or -ve (move backward) or 0.
forward) or -ve (move backward) or 0. * *
mode
mode
identifies the starting point. In stdio.h, the
identifies the starting point. In stdio.h, the
following constants can be assigned to mode:
following constants can be assigned to mode:
Mode
Mode
Measure offset from
Measure offset from
SEEK_SET
SEEK_SET
Beginning of file
Beginning of file
SEEK_CUR
SEEK_CUR
Current position
Current position
SEEK_END
SEEK_END
End of file
End of file 38 38
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Page 39
The ftell() Function
The ftell() Function
The function prototype of
The function prototype of
ftell()
ftell() is is
long int ftell(FILE *fp);
long int ftell(FILE *fp);
returns the current file location.
returns the current file location.
The rewind() Function
The rewind() Function
The function prototype of
The function prototype of
rewind()
rewind() is is
void rewind(FILE *fp);
void rewind(FILE *fp);
Resets the file position marker to the beginning of
Resets the file position marker to the beginning of
the file. This is equivalent to
the file. This is equivalent to
fseek(fp, 0L, SEEK_SET);
fseek(fp, 0L, SEEK_SET); 39 39
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/* reverse.c - displays a file in reverse order */
/* reverse.c - displays a file in reverse order */
#include <stdio.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdlib.h>
#define CNTL_Z '\032'
#define CNTL_Z '\032'
/* eof marker in DOS textfiles */
/* eof marker in DOS textfiles */
{ int main(int argc, char *argv[])
{ int main(int argc, char *argv[])
char ch;
char ch;
FILE *fp;
FILE *fp;
long count, last;
long count, last;
if ((fp=fopen(argv[1],"rb"))== NULL) {
if ((fp=fopen(argv[1],"rb"))== NULL) {
printf("reverse can't open %s\n", argv[1]);
printf("reverse can't open %s\n", argv[1]);
exit(1);
exit(1); } }
fseek(fp,0L, SEEK_END);
fseek(fp,0L, SEEK_END);
/* goto eof */
/* goto eof */
last=ftell(fp);
last=ftell(fp);
for (count=1L; count<=last; count++) {
for (count=1L; count<=last; count++) {
fseek(fp,-count,SEEK_END);
fseek(fp,-count,SEEK_END);
ch=getc(fp);
ch=getc(fp);
if (ch!=CNTL_Z && ch!= '\r')
if (ch!=CNTL_Z && ch!= '\r')
putchar(ch);
putchar(ch);
}fclose(fp);
}fclose(fp);
return(0);
return(0); 40 40 } }

8. File Positions

9. File Position Pointer

10. myFile.tellg ( ) ; tellg ( ) Function
Returns a whole number which tell you the position
of the next character to be read from the file

11. tellp ( ) Function myFile.tellp ( ) ;
Returns a whole number which tell you the position of the next character to be written in a file

12. For Positioning in the file
seekg ( ) ;
seekp ( ) ;

13. filePtr.seekg ( long Num , ios :: origin ) ;
Number of characters to move to
Starting point
filePtr.seekg ( long Num , ios :: origin ) ;

14. seekg ( ) seekg ( 10L , ios :: beg ) ; seekg (10L , ios :: cur ) ;
seekg ( 10L , ios :: end ) ;

15. Example 1 #include<fstream.h> main ( ) { int length ;
ifstream inFile ( “myFile.txt” ) ;
inFile.seekg ( 0L , ios :: end ) ;
length = inFile.tellg ( ) ; }

16. File Name city Date-of-Birth : : : Jamil Ahmed Sukkur 10-10-1982 : : :
: : :
Jamil Ahmed Sukkur
: : :
Rawalpindi

17. Merge Method Original file Empty file This is a text data And needs
To be replaced
NOT

18. seekg ( )
seekg ( 2201L , ios :: beg ) ;

19. fstream myFile ( “Sample.txt” , ios :: in | ios :: out ) ;

20. OR Function
A B Output

21. Example 2
This is an Apple
This is a Sample

22. get ( ) and put ( ) character in a file
myInputFile.get ( c ) ;
myOutputFile.put ( c ) ;

23. read ( ) and write ( ) Functions
Area in memory
Number of bytes to be read
read ( char *buff , int count ) ;
write ( char *buff , int count ) ;
Area in memory
Number of bytes to be written

24. Example 3 char str [ 10000 ] ; myInputFile.read ( str , 10000 ) ;
myOuputFile.write ( str , ) ;

25. seekg ( )
seekg ( 0L , ios :: end ) ;

26. seekg ( )
seekg ( -1L , ios :: end ) ;

27. seekg ( )
seekg ( -2L , ios :: cur ) ;

28. myOutputFile.write ( &i , 4 ) ;
Address of the integer ‘i’
Number of bytes to be written
myOutputFile.write ( &i , 4 ) ;

29. sizeof ( ) ;

30. myOutputFile.write ( &i , sizeof ( i ) ) ;
Address of the integer ‘i’
Size of integer
myOutputFile.write ( &i , sizeof ( i ) ) ;

31. for ( i = 0 ; i < 100 ; i ++ ) {
myOutputFile.write ( &i , sizeof ( i ) ) ; }
myOutputFile.close ( ) ;