file handling in c
TRANSCRIPT
File Handling
• Console oriented I/O functions use keyboard asinput device and monitor as output device.
• The I/O functions like printf(), scanf(), getchar(),putchar(), gets(), puts()
• Problem:1. Entire data is lost when either the program is
terminated or the computer is turned off.
2. When the volume of data to be entered is large, ittakes a lot of time to enter the data.
3. If user makes a mistake while entering data, wholedata has to be re-entered.
• Solution: File
Background
2
• File
is a place on the disk (not memory) where a
group of related data is stored. Also called
data files.
• The Data File
allows us to store information permanently
and to access and alter that information
whenever necessary.
Concept of file
3
Classification of disk/file I/O functions
Disk I/O Functions
High-level Low-level
Text Binary
Formatted Unformatted UnformattedFormatted
4
Function name Operation
fopen() •Creates a new file for use•Opens an existing file for use
fclose() •Closes a file which was opened for use
fgetc() •Reads a character from a file
fputc() •Writes a character to a file
fprintf() •Writes a set of data values to a file
fscanf() •Reads a set of data values from a file
fseek() •Sets the position to a desired point in the file
ftell() •Gives the current position in the file (in terms of bytes from the start)
rewind() •Sets the position to the beginning of the file
Some high-level I/O functions
5
• The general format for declaring and opening a file is:
FILE *fp;
fp=fopen(“filename”, “mode”);
• Here, the first statement declares the variable fp as a“pointer to the data type FILE”.
• The second statement opens the file named filenamewith the purpose mode and the beginning address of thebuffer area allocated for the file is stored by file pointerfp.
• Note: Any no. of files can be opened and used at a time.
Defining and Opening a file
6
• There are mainly six modes:
1. “r” (i.e. read mode)
2. “w” (i.e. write mode)
3. “a” (i.e. append mode)
4. “r+” (i.e. read and write mode)
5. “w+” (i.e. write and read mode)
6. “a+” (i.e. append and read mode)
File Opening Modes
7
• The closing a file ensures that all outstandinginformation associated with the file is flushed out fromthe buffers and all links to the file are broken.
• In cases where there is a limit to the no. of files that canbe kept open simultaneously, closing of unwanted fileshelp in opening the required ones.
• Another instance where we have to close a file is whenwe want to reopen the same file in different mode.
• The file is closed using library function fclose() as:
fclose(fp);
Closing a file
8
• Once a file is opened, reading out of or writing
to it is accomplished using the standard I/O
functions.
Library Functions for Reading/Writing
from/to a File: File I/O Functions
9
• Using string I/O functions fgets() and fputs(),
data can be read from a file or written to a file
in the form of array of characters.
i. fgets(): is used to read string from file.
Syntax: fgets(string, int_value, fp);
Here, int_value denotes the no. of characters in the string.
ii. fputs(): is used to write string to file.
Syntax: fputs(string, fp);
String Input/Output Functions
10
#include<conio.h>
#include<stdio.h>
#include<stdlib.h>
void main()
{
FILE *fp;
clrscr();
fp=fopen("D:\\test.txt", "w");
if(fp==NULL)
{
printf("\n Cannot create file.");
exit(0);
}
else
{
printf("\n File is created.");
}
fputs("I study CSIT", fp);
fclose(fp);
getch();
}
11
#include<conio.h>
#include<stdio.h>
#include<stdlib.h>
void main()
{
FILE *fp;
char s[100];
clrscr();
fp=fopen("D:test.txt", "r");
if(fp==NULL)
{
printf("\n Cannot open file.");
exit(0);
}
else
{
printf("\nFile is opened.");
}
fgets(s,19,fp);
printf("\n Text from file is:%s", s);
fclose(fp);
getch();
}
12
#include<conio.h>
#include<stdio.h>
#include<stdlib.h>
void main()
{
FILE *fp;
clrscr();
fp=fopen("D:\\test.txt", "a");
if(fp==NULL)
{
printf("\n Cannot open file.");
exit(1);
}
else
{
printf("\n File is opened.");
}
fputs(“in CAB", fp);
fclose(fp);
getch();
}
13
void main()
{
FILE *fp;
char filename[20];
clrscr();
printf("Enter filename:\t");
gets(filename);
fp=fopen(filename, "w");
if(fp==NULL)
{
printf("\n Cannot create file.");
exit(1);
}
else
{
printf("\n File is created.");
}
getch();
}
// If only filename is given, file is created in C:\TC\BIN otherwise file is created in the given path.
14
Naming a file
• Using character I/O functions fgetc() andfputc(), data can be read from file or writtenonto file one character at a time.
i. fgetc(): is used to read a character from a file.
Syntax:
char_variable=fgetc(fp);
ii. fputc(): is used to write a character to a file.
Syntax:
fputc(‘character’or character_variable, fp);
Character I/O Functions
15
void main()
{
FILE *fp;
char filename[20];
char c;
clrscr();
printf("Enter filename:\t");
gets(filename);
fp=fopen(filename,"w");
if(fp==NULL)
{
printf("\n Cannot create file.");
exit();
}
else
printf("\n File is created.");
printf("\n Enter your text until Enter key:\n");
while((c=getchar())!='\n')
fputc(c,fp);
fclose(fp);
getch();
} 16
void main()
{
FILE *fp;
char filename[20];
char c;
clrscr();
printf("Enter filename:\t");
gets(filename);
fp=fopen(filename, "r");
if(fp==NULL)
{
printf("\n Cannot open file.");
exit();
}
printf("\n The content of file is:\n");
while((c=fgetc(fp))!=EOF)
putchar(c);
fclose(fp);
getch();
}
17
• EOF is a special character (an integer with ASCIIvalue 26) that indicates that the end-of-file hasbeen reached. This character can be generatedfrom the keyboard by typing Ctrl+Z.
• Defined in <stdio.h>
• When we are creating a file, the special characterEOF, is inserted after the last character of the fileby the Operating System.
• Caution: An attempt to read after EOF mighteither cause the program to terminate with anerror or result in an infinite loop situation.
18
End-Of-File (EOF)
void main()
{
FILE *fp;
char filename[20];
char c;
clrscr();
printf("Enter filename:\t");
gets(filename);
fp=fopen(filename,"a");
if(fp==NULL)
{
printf("\n Cannot create or open file.");
exit();
}
printf("\nEnter text to append to file %s:\n", filename);
while((c=getchar())!='\n')
fputc(c,fp);
fclose(fp);
getch();
}
19
void main()
{
FILE *sfp,*dfp;
char sfilename[20],dfilename[20];
char c;
clrscr();
printf("Enter source filename:\t");
gets(sfilename);
printf("\n Enter destination filename:\t");
gets(dfilename);
sfp=fopen(sfilename,"r");
if(sfp==NULL)
{
printf("\nSource file can't be opened.");
exit();
}
dfp=fopen(dfilename, "w");
if(dfp==NULL)
{
printf("\n Destination file cannot be created or opened.");
exit();
}
while((c=fgetc(sfp))!=EOF)
fputc(c, dfp);
printf("\n Copied........");
fclose(dfp);
fclose(sfp);
getch();
}
20
• Given a text file, create another text file
deleting all the vowels (a, e, i, o, u).
Question
21
void main()
{
FILE *fp,*fpp;
char c;
fp=fopen("C:\\test.txt","r+");
clrscr();
if(fp==NULL)
{
printf("Cannot open file");
exit();
}
fpp=fopen("C:\\hello.txt","w");
if(fpp==NULL)
{
printf("Cannot create file");
exit();
}
while((c=fgetc(fp))!=EOF)
{
if((c!='a')&&(c!='e')&&(c!='i')&&(
c!='o')&&(c!='u'))
fputc(c, fpp);
}
fclose(fp);
fclose(fpp);
getch();
}
22
• Using formatted I/O functions, fprintf() andfscanf(), numbers, characters or string can be readfrom file or written onto file according to ourrequirement format.i. fprintf(): is formatted output function which is used to
write integer, float, char or string value to a file.Syntax:
fprintf(fp, “control_string”, list_of_variables);
ii. fscanf(): is formatted input function which is used toread integer, float, char or string value from a file.Syntax:
fscanf(fp, “control_string”, &list_of_variables);
23
Formatted I/O Functions
void main()
{
FILE *fp;
char name[20];
int roll;
char address[20];
float marks;
clrscr();
fp=fopen("C:\\student.txt", "w");
if(fp==NULL)
{
printf("\n File cannot be created or opened.");
exit();
}
24
printf("\n Enter name of student:\t");
gets(name);
printf("\n Enter roll number of %s:\t", name);
scanf("%d", &roll);
fflush(stdin);
printf("\n Enter address of %s:\t", name);
gets(address);
printf("\n Enter marks of %s:\t", name);
scanf("%f", &marks);
printf("\n Now writing data to file...");
fprintf(fp, "Name=%s\n Roll=%d\n Address=%s\n Marks=%.2f", name, roll, address, marks);
printf("\n Completed");
fclose(fp);
getch();
}
25
• Here, after providing name of student, we would hitenter key……No Problem……and then we provide rollof student……and hit the enter keyagain…...Problem…...
• At this time the enter key which is in the keyboardbuffer is read by the gets()/scanf() function for address(as enter key is a character, \n), so that we are able tofill only the marks.
• To avoid this problem, we use the function fflush().
• It is designed to remove or flush out any data remainingin the buffer.
26
Use of fflush()
• Define a structure for Vehicle Owner having
data members name, address, telephone
number, vehicle number and license
number. Take the data for ten owners, write
them in file “Own.txt”. Read the data from
the file and display them.
Question
27
struct vehicle_owner
{
char name[20];
char address[20];
long int phone_no;
int vehicle_no;
int license_no;
};
void main()
{
FILE *fp;
struct vehicle_owner vehicle[10], v[10];
int i;
clrscr();
fp=fopen("C:\\Own.txt","w");
if(fp==NULL)
{
printf("\nCannot create file.");
exit();
}
28
for(i=0;i<SIZE;i++)
{
printf("\n Enter information about vehicle owner %d",i+1);
printf("\n Enter name :\t");
gets(vehicle[i].name);
printf("\n Enter address:\t");
gets(vehicle[i].address);
printf("\n Enter telephone no:\t");
scanf("%ld", &vehicle[i].phone_no);
printf("\n Enter vehicle no:\t");
scanf("%d", &vehicle[i].vehicle_no);
printf("\n Enter license no:\t");
scanf("%d", &vehicle[i].license_no);
fprintf(fp,"%s\t%s\t%ld\t%d\t%d\n", vehicle[i].name, vehicle[i].address, vehicle[i].phone_no, vehicle[i].vehicle_no, vehicle[i].license_no);
fflush(stdin);
}
29
fclose(fp);
fp=fopen("C:\\Own.txt","r");
for(i=0;i<10;i++)
{
fscanf(fp,"%s %s %ld %d
%d",&v[i].name,&v[i].address,&v[i].phone_no,&v[i].vehicle_no,&v[i].license_
no);
printf("%s\t%s\t%ld\t%d\t%d\n",v[i].name,v[i].address,v[i].phone_no,v[i].vehicl
e_no,v[i].license_no);
}
fclose(fp);
getch();
}
• Given a text file, create another text file
deleting the following words “three”, “bad”,
and “time”.
Problem
31
#include <stdio.h>
void main()
{
FILE *fp,*fpp;
char c[10];
fp=fopen("C:\\test.txt",“r");
clrscr();
if(fp==NULL)
{
printf("Cannot open file");
exit();
}
fpp=fopen("C:\\hello.txt","w");
if(fpp==NULL)
{
printf("Cannot create file");
exit();
}
while(fscanf(fp,"%s",c)!=EOF)
{
if((strcmp(c,"three")!=0)&&(strcmp(c,"bad")!=0)&&(strcmp(c,"time")!=0))
{
fprintf(fpp,"%s ",c);
}
}
fclose(fp);
fclose(fpp);
getch();
}
32
• Some text file is given, create another text
file replacing the following words “Ram” to
“Hari”, “Sita” to “Gita”, and “Govinda” to
“Shiva”.
Problem
33
#include <stdio.h>
void main()
{
FILE *fp,*fpp;
char c[10];
fp=fopen("C:\\test.txt","r");
clrscr();
if(fp==NULL)
{
printf("Cannot open file");
exit();
}
fpp=fopen("C:\\hello.txt","w");
if(fpp==NULL)
{
printf("Cannot create file");
exit();
}
while(fscanf(fp,"%s",c)!=EOF)
{
if(strcmp(c, "Ram")==0)
fprintf(fpp, "Hari ",c);
else if(strcmp(c, "Sita")==0)
fprintf(fpp,"Gita",c);
else if(strcmp(c, "Govinda")==0)
fprintf(fpp, "Shiva",c);
else
fprintf(fpp,"%s ",c);
}
fclose(fp);
fclose(fpp);
getch();
}
34
• Create a program to create a data file and
write the integers from 1 to 20 to this file
and then read the numbers from the file to
display the squares of the stored numbers.
35
Question
#include <stdio.h>
void main()
{
FILE *fp;
register unsigned int i;
unsigned filedata;
clrscr();
fp=fopen("C:\\data.txt","w");
if(fp==NULL)
{
printf("\nCannot create data file.");
exit();
}
for(i=1;i<21;i++)
{
fprintf(fp,"%u\t",i);
}
fclose(fp);
fp=fopen("C:\\data.txt","r");
printf("\nThe squares of the stored numbers are:\t");
for(i=1;i<21;i++)
{
fscanf(fp,"%u",&filedata);
filedata=filedata*filedata;
printf("%u\t", filedata);
}
getch();
}
36
• A file named DATA contains a series of
integer numbers. Code a program to read
these numbers and then write all odd
numbers to a file to be called ODD and all
even numbers to a file to be called EVEN.
37
Question
#include <stdio.h>
void main()
{
FILE *fpdata;
FILE *fpodd;
FILE *fpeven;
int i,n;
int num;
clrscr();
printf("\nHow many integers you want in data file?:\t");
scanf("%d",&n);
printf("\nEnter %d integers:\t",n);
fpdata=fopen("C:\\DATA.txt","w");
for(i=0;i<n;i++)
{
scanf("%d",&num);
fprintf(fpdata,"%d\n",num);
}
fclose(fpdata);
fpdata=fopen("C:\\DATA.txt","r");
fpodd=fopen("C:\\ODD.txt","w");
fpeven=fopen("C:\\EVEN.txt","w");
38
for(i=0;i<n;i++)
{
fscanf(fpdata,"%d", &num);
if(num%2==0)
fprintf(fpeven,"%d\t", num);
else
fprintf(fpodd,"%d\t", num);
}
fclose(fpdata);
fclose(fpodd);
fclose(fpeven);
getch();
}
39
• Trying to read beyond the end-of-file mark.
• Trying to use a file that has not been opened.
• Trying to perform an operation on a file, when
the file is opened for another type of operation.
• Opening a file with an invalid filename.
40
Error situations during I/O operations
• I/O errors can be detected using two status-inquirylibrary functions: feof() and ferror().
• feof(): It is used to test for an end-of-file condition. Ittakes a FILE pointer as its only argument and returns anonzero integer value if all of the data from thespecified file has been read, and returns zero otherwise.If fp is a pointer to a file that has just been opened forreading, then the statement
if(feof(fp))
printf(“End of data”);
would display the message “End of data” on reachingthe end-of-file condition.
41
Error handling functions
• ferror(): This function reports the status of the file
indicated. It takes a FILE pointer as its argument
and returns a nonzero integer if an error has been
detected up to that point, during processing. It
returns zero otherwise. So the statement
if(ferror(fp)!=0)
printf(“An error has occurred”);
would print the error message, if the reading is
not successful.
42
Error handling functions…
void main()
{
FILE *fp1;
char *filename;
int i, num;
clrscr();
fp1=fopen("C:\\test.txt", "w");
for(i=10;i<=100;i += 10)
{
fprintf(fp1,"%d\t", i);
}
fclose(fp1);
printf("\n Enter filename:\t"); //Type C:\test.txt
open_file:
scanf("%s", filename);
43
if((fp1=fopen(filename,"r"))==NULL)
{
printf("\nAn error occured while opening the file.");
printf("\nType filename again:\t");
goto open_file;
}
else
for(i=1;i<=20;i++)
{
fscanf(fp1,"%d", &num);
if(feof(fp1))
{
printf("\nRan out of data.");
break;
}
else
printf("%d\n", num);
}
fclose(fp1);
getch();
}
44
void main()
{
FILE *fp;
int num;
clrscr();
fp = fopen("DUMMY.FIL", "w");
/* force an error condition by attempting to read */
fscanf(fp,"%d", &num);
if (ferror(fp)!=0)
{
printf("Error reading from DUMMY.FIL\n");
}
fclose(fp);
getch();
}
45
• The binary files organize data into blockscontaining contiguous bytes of information.
• In binary file, the opening mode of text file isappended by a character b i.e.
i. “r” is replaced by “rb”
ii. “w” is replaced by “wb”
iii. “a” is replaced by “ab”
iv. “r+” is replaced by “r+b”
v. “w+” is replaced by “w+b”
vi. “a+” is replaced by “a+b”
46
Binary Data Files
Note: For text mode we can write“rt” in place of “r”, “wt” in place of“w”and so on. However, it isunnessary because default mode istext mode
void main()
{
FILE *fp;
char c;
clrscr();
fp=fopen("C:\\test.txt","w+b");
if(fp==NULL)
{
printf("\nCannot create file.");
exit();
}
fputs("I study B.Sc. CSIT", fp);
fclose(fp);
getch();
}
47
• Analyze with 3 factors:
I. How newlines (\n) are stored?
II. How end-of-file is indicated?
III. How numbers are stored in the file?
48
So, what’s the difference between textmode and binary mode and which mode touse???
/*Count no. of characters, spaces, and newlines in a file*/
void main()
{
FILE *fp;
char text[100];
char c;
int noc=0,nos=0,nol=0;
fp=fopen("C:\\poem.txt", "r");
if(fp==NULL)
{
printf("\nCannot create or open file.");
exit();
}
while(1)
{
c=fgetc(fp);
if(c==EOF)
break;
noc++;
if(c==' ')
nos++;
if(c=='\n')
nol++;
}
fclose(fp);
printf("\n No. of characters:%d", noc);
printf("\n No. of spaces:%d", nos);
printf("\n No. of lines:%d", nol);
getch();
}
49
Johnny Johnny
Yes Papa
Eating Sugar
No Papa
Telling Lies
No Papa
Open Your Mouth
hahaha
50
The poem.txt file contains:
So output is:
No. of characters=87
No. of spaces=8
No. of lines=7
which is correct.
Now, go to DOS shell and use the
DIR command in C-drive to view the
no. of characters (bytes) that the file
poem.txt occupies which is 94.
• In text mode, a newline character is converted into thecarriage return-linefeed combination before beingwritten to disk.
• Likewise, the carriage return-linefeed combination onthe disk is converted back into a newline when the fileis read by a C program.
• However, if a file is opened in binary mode, as opposedto text mode, these conversions do not take place.
• In binary mode, each end of line is signified by acarriage return-linefeed combination and is counted astwo characters in binary mode (similar to DIRcommand in DOS).
51
First factor
52
/*Count no. of characters, spaces,
and newlines in a file*/
void main()
{
FILE *fp;
char text[100];
char c;
int noc=0,nos=0,nol=0;
fp=fopen("C:\\poem.txt", "rb");
if(fp==NULL)
{
printf("\nCannot create or open
file.");
exit();
}
while(1)
{
c=fgetc(fp);
if(c==EOF)
break;
noc++;
if(c==' ')
nos++;
if(c=='\n')
nol++;
}
fclose(fp);
printf("\n No. of characters:%d",
noc);
printf("\n No. of spaces:%d", nos);
printf("\n No. of lines:%d", nol);
getch();
}
• In text mode, a special character EOF whoseASCII value is 26 is inserted after the lastcharacter in the file to mark the end of file.
• However, there is no such special characterpresent in the binary mode files to mark theend of file.
• The binary mode files keep track of the end offile from the number of characters present inthe directory entry of the file.
53
Second Factor
• In text mode, the text and numbers are stored as string ofcharacters such that the number 12345 will occupy 5 bytes(1 byte/character).
• Similarly 1234.56 occupies 7 bytes on disk in text mode.
• However, in binary mode the numbers are stored in thesame way as they are stored in RAM so that the number12345 occupies only 2 bytes and 1234.56 occupies only 4bytes on disk in binary mode.
• Therefore, when large amount of numerical data is to bestored onto disk, binary mode is suitable by using functionsfread() and fwrite() instead of fprintf() and fscanf().
54
Third Factor
• The character I/O and string I/O functions allow
reading/writing of character data only, while the
formatted I/O functions allow reading/writing of
character data and numeric data both.
• Problem: Numbers are always stored as a
sequence of characters using these I/O functions
(irrespective of whether text mode or binary mode
is being used), so that they occupy a lot of disk
space.
55
Record I/O……Background Problems
• Another Problem: There is no direct way to read and writecomplex data types such as arrays and structures. Arraysand structures are handled by writing/reading one elementat a time or using loops, but this approach is inefficient.
• Example:
56
#include <stdio.h>
void main()
{
FILE *fp;
char another='Y';
struct emp
{
char name[40];
int age;
float salary;
};
struct emp e;
fp=fopen("c:\\emp.dat","wb");
if(fp==NULL)
{
puts("Cannot create or open file");
exit();
}
while(another=='Y')
{
printf("\nEnter name, age and basic salary");
scanf("%s %d %f",e.name,&e.age,&e.salary);
fprintf(fp,"%s\t%d\t%f",e.name,e.age,e.salary);
printf("\nAdd another record(Y/N):\t");
fflush(stdin);
another=getche();
}
fclose(fp);
getch();
}
Here, if the no. of fields in the structure
increase (say by adding address, house
rent allowance etc.), writing structures
using fprintf(), or reading them using
fscanf(), becomes quite clumsy.
• Defined in <stdio.h>
• fwrite(): is used for record output.
Syntax:
fwrite(&p, size_of_array_or_structure, no._of_array_or_structure, fp);
• fread(): is used for record input.
Syntax:
fread (&p, size_of_array_or_structure, no._of_array_or_structure, fp);
57
Record Input/Output Functions
#include <stdio.h>
void main()
{
FILE *fp;
char another='Y';
struct emp
{
char name[40];
int age;
float salary;
};
struct emp e;
clrscr();
fp=fopen("c:\\emp.dat","wb");
if(fp==NULL)
{
puts("Cannot create or open file");
exit();
}
while(another=='Y'||another=='y')
{
printf("\n Enter name, age and basic salary:");
scanf("%s %d %f", e.name, &e.age,
&e.salary);
fwrite(&e,sizeof(e),1,fp);
printf("\n Add another record(Y/N):\t");
fflush(stdin);
another=getche();
}
fclose(fp);
fp=fopen("c:\\emp.dat","rb");
if(fp==NULL)
{
puts("Cannot create or open file");
exit();
}
while(fread(&e,sizeof(e),1,fp)==1)
{
printf("\n%s\t%d\t%f", e.name, e.age,
e.salary);
}
fclose(fp);
getch();
}
58
#include <stdio.h>
void main()
{
FILE *fp;
struct emp
{
char name[40];
int age;
float salary;
};
struct emp e[2],ee[2];
int i;
float temp;
clrscr();
fp=fopen("c:\\employee.dat","wb");
if(fp==NULL)
{
puts("Cannot create or open file");
exit();
}
for(i=0;i<2;i++)
{
printf("\nEnter name, age and basic salary:");
scanf("%s %d %f",e[i].name,&e[i].age,&temp);
e[i].salary=temp;
fflush(stdin);
}
fwrite(&e,sizeof(e),2,fp);
fclose(fp);
fp=fopen("c:\\employee.dat","rb");
if(fp==NULL)
{
puts("Cannot open file");
exit();
}
fread(&ee,sizeof(ee),2,fp)
for(i=0;i<2;i++)
printf("\n%s\t%d\t%.2f", ee[i].name, ee[i].age, ee[i].salary);
fclose(fp);
getch();
}
59
• Till now, reading and writing data from/to a
file has been done sequentially.
• But we may need to access a particular data
item placed in any location without starting
from the beginning.
• This is called random access or direct access.
60
Random Access in a File (Direct Access)
• A file pointer is a pointer to a particular byte in a file.
• While opening a file in write mode, the file pointer is at thebeginning of the file, and whenever we write to a file, thefile pointer moves to the end of the data items written sothat writing can continue from that point.
• While opening a file in read mode, the file pointer is at thebeginning of the file, and whenever we read from a file, thefile pointer moves to the beginning of the next data item sothat reading cam continue from that point.
• While opening a file in append mode, the file pointer is atthe end of the existing file, so that new data items can bewritten from there onwards.
• If we are able to move the file pointer according as ourneed, then any data item can be read from a file or writtenonto a file randomly…………Random Access
61
Use of file pointer for random access…
1. ftell(): This function takes a file pointer asargument and returns a number of type long,that indicates the current position of the filepointer within the file. This function is usefulin saving the current position of a file, whichcan be used later in the program.
Syntax
n = ftell(fp);
Here, n would give the relative offset (in bytes) ofthe current position. This means that n bytes havealready been read (or written).
62
Functions used in random access
2. rewind():This function takes a file pointer asargument and resets the current position of thefile pointer to the start of the file.
Syntax: rewind(fp); What these statements do?: rewind(fp);
n=ftell(fp);
• Here, n would be assigned 0, because fileposition has been set to the start of the file byrewind().
• Note: The first byte in the file is numbered as 0,second as 1, and so on.
63
Functions used in random access
3. fseek(): This function is used to move the file pointerto a desired position within a file.
Syntax
fseek(fp, offset, position);
where fp is a file pointer, offset is a number orvariable data type long, and position is an integernumber
• The offset specifies the number of positions (bytes) tobe moved from the location specified by position.
• The position can have one of the following 3 values:
64
Functions used in random access…
Value Meaning
0 Beginning of file
1 Current position
2 End of file
• The offset may be positive, meaning moveforwards, or negative, meaning move backwards.
• Examples:
65
fseek()…
Statement Meaning
fseek(fp, 0L, 0); Move file pointer to beginning of file. (Same as rewind.)
fseek(fp, 0L, 1); Stay at the current position. (File pointer is not moved.)
fseek(fp, 0L, 2); Move file pointer past the last character of the file. (Go to
the end of file.)
fseek(fp, m, 0); Move file pointer to (m+1)th byte in the file.
fseek(fp, m, 1); Move file pointer forwards by m bytes.
fseek(fp, -m, 1); Move file pointer backwards by m bytes from the current
position.
fseek(fp, -m, 2); Move file pointer backwards by m bytes from the end.
(Positions the file pointer to the mth character from the
end.)
• When the operation is successful, fseek()
returns a 0 (zero).
• If we attempt to move the file pointer beyond
the file boundaries, an error occurs and fseek()
returns -1 (minus one).
• It is good practice to check whether an error
has occurred or not, before proceeding further.
66
fseek()…
67
/* A program that uses the functions ftell() and fseek() */
#include <stdio.h>
void main()
{
FILE *fp;
char c;
long n;
clrscr();
fp=fopen("RANDOM","w");
if(fp==NULL)
{
printf("\nCannot create file.");
exit();
}
while((c=getchar())!=EOF)
fputc(c,fp);
printf("\nNo. of characters entered=%ld",ftell(fp));
fclose(fp);
fp=fopen("RANDOM","r");
if(fp==NULL)
{
printf("\nCannot create file.");
exit();
}
n=0L;
while(feof(fp)==0)
{
fseek(fp,n,0); //Position to (n+1)th character
printf("Position of %c is %ld\n",fgetc(fp),ftell(fp));
n=n+5L;
}
putchar('\n');
fseek(fp,-1L,2); /*Position to the last character*/
do
{
putchar(fgetc(fp));
}while(!fseek(fp,-2L,1));
fclose(fp);
getch();
}
68
Explanation• A file called RANDOM is created with the
following contents:
Stored Character: A B C … Z
File Pointer Position: 0 1 2 … 25
• Then the file is read twice.
• At first, we read the contents of every fifthposition and print its value with its position on thescreen.
• At second, we read the contents of the file fromthe end and print the same on screen.
• A book record consists of its title, author, pagesand price. Write a program to performfollowing operations:
– Read the records of 13 books
– Create at least one structure pointer to display therecords of 13 books
– Store records of all 13 books in the file“booklist.dat”
– Read only the information of 9 books from“booklist.dat” skipping 2 books from first and 2books from last and display in terminal
69
Problem
#define SIZE 13
void main()
{
struct book
{
char title[40];
char author[20];
int pages;
float price;
};
struct book b[SIZE];
int i;
float temp;
struct book *bp;
FILE *fp;
struct book bb[SIZE];
clrscr();
for(i=0;i<SIZE;i++)
{
printf("\nEnter record of book%d",i+1);
printf("\nEnter title:\t");
scanf("%s",b[i].title);
fflush(stdin);
printf("\nEnter author:\t");
scanf("%s",b[i].author);
printf("\nEnter no. of pages:\t");
scanf("%d",&b[i].pages);
printf("\nEnter price:\t");
scanf("%f",&temp);
b[i].price=temp;
}
70
bp=b; //bp=&b[0];
for(i=0;i<SIZE;i++)
{
printf("\nRecord of Book%d",i+1);
printf("\nTitle:%s\tAuthor:%s",(bp+i)->title,(bp+i)->author);
printf("\nNo. of pages:%d\tPrice:%.2f\n",(bp+i)->pages,(bp+i)->price);
}
fp=fopen("booklist.dat","w+b");
if(fp==NULL)
{
puts("Cannot create file");
exit();
}
for(i=0;i<SIZE;i++)
fwrite(&b,sizeof(b),1,fp);
rewind(fp);
fseek(fp,sizeof(b)*2,0);
i=2;
printf("\nReading from file:");
while(fread(&bb,sizeof(bb),1,fp)==1)
{
while(i<SIZE-2)
{
printf("\nTitle:%s\tAuthor:%s", bb[i].title, bb[i].author);
printf("\nNo. of pages:%d\tPrice:%f\n", bb[i].pages, bb[i].price);
i++;
}
}
fclose(fp);
getch();
}
71
• A car record consists of its model,manufacture_year and price. Write a program toperform following operations:
– Read the records of 13 cars.
– Create at least one structure pointer to displaythe records of 13 cars.
– Store records of all 13 cars in the file “c.mpg”.
– Read only the information of 5 cars from“c.mpg”, skipping 8 cars from first and displayin standard output device.
72
PROBLEM
• Create a structure named employee having
empname, age and salary as its members. Read
these information for a number of employees (till
user wants) and write these information to a file
named employee.txt in C-drive. Finally, the
program should be able to search the information
of a particular employee by its empname from the
file.
73
PROBLEM
void main()
{
struct employee
{
char empname[20];
int age;
float salary;
};
struct employee e;
FILE *fp;
char name[20];
char ch='y';
int search=0;
fp=fopen("C:\\employee.txt","w+b");
clrscr();
do
{
printf("\nEnter name, age and salary of employee:");
scanf("%s %d %f", e.empname, &e.age, &e.salary);
fwrite(&e,sizeof(e),1,fp);
fflush(stdin);
printf("\nDo you want to information for another employee(y for yes):");
scanf("%c", &ch);
}while(ch=='y');
rewind(fp);
printf(“\n\tEnter employee to be searched:\t");
fflush(stdin);
gets(name);
while(fread(&e,sizeof(e),1,fp)==1)
{
if(strcmp(name, e.empname)==0)
{
search=1;
printf("\nName:%s", e.empname);
printf("\nAge:%d", e.age);
printf("\nSalary:%.2f", e.salary);
}
}
if(search==0)
printf("\nThere is no employee with name %s", name);
fclose(fp);
getch();
}
74
• Write a program to open a file employee.txt
created in above program and edit/modify
the details of a particular employee.
75
PROBLEM
void main()
{
struct employee
{
char empname[20];
int age;
float salary;
};
struct employee e;
FILE *fp;
char name[20];
int search=0;
int record_count=0;
fp=fopen("C:\\employee.txt","rb+");
clrscr();
if(fp==NULL)
{
printf("Cannot open file");
exit();
}
printf("\tEnter employee name to be modified:\t");
gets(name);
while(fread(&e,sizeof(e),1,fp)==1)
{
if(strcmp(name, e.empname)==0)
{
search=1;
printf("\n Old record is:");
printf("\n Name:%s",e.empname);
printf("\n Age:%d",e.age);
printf("\n Salary:%.2f",e.salary);
printf("\n Enter new record(name,age and
salary):");
scanf("%s %d %f", e.empname, &e.age,
&e.salary);
fseek(fp,sizeof(e)*record_count,0);
if(fwrite(&e,sizeof(e),1,fp)==1)
printf("\nRecord modified!!!");
}
record_count++;
}
if(search==0)
printf("\n There is no employee with name %s",
name);
fclose(fp);
getch();
}
76