Transcript Programming In C++

Programming In C++
Spring Semester 2013
Lecture 10
Programming In C++, Lecture 10 By Umer Rana
Files
• Disk I/O operations are performed on entities
called files.
• A files is a collection of bytes that is given a name.
• In most microcomputers systems a files are used
as a unit if storage on secondary storage.
Programming In C++, Lecture 10 By Umer Rana
Standard & System I/O
Standard I/O
As the name, is the most common way of performing
I/O in C programs. It has a wider range of commands,
and in many respects is easier to use than system I/O.
a.
b.
c.
d.
Character I/O
String I/O
Formatted I/O
Record I/O
System I/O
Provides fewer ways to handle data than standard I/O.
The technique it employs are very much like those
used by the operating system. Thus, in some ways,
system I/O is harder to program than standard I/O.
Programming In C++, Lecture 10 By Umer Rana
Categories of Disk I/O
Character I/O
String I/O
getc( )
putc( )
Formatted I/O
fgets( )
fputs( )
Record I/O
fscanf( )
fprintf( )
Programming In C++, Lecture 10 By Umer Rana
fread( )
fwrite( )
Syntax
The fopen ( ) function opens a file whose name is pointed to by
‘filename’ and returns the stream that is associated with it. The
type of operation that will be allowed on the file are defined by
the vale of mode.
Declaration:
FILE *fptr;
fptr=fopen(“filename”, mode);
OR
FILE * fptr=fopen (“filename”, mode);
Programming In C++, Lecture 10 By Umer Rana
Possibilities File Modes
File Type Meaning
“r”
Open an existing file for reading only
“w”
Open a new file for writing only. If a file with the specified
file-name currently exists, it will be destroyed and a new
file created in its place.
“a”
Open an existing file for appending (i.e., for adding new
information at the end of the file). If a file with the
specified file-name currently does not exist, a new file will
be created.
“r+”
Open an existing file for both reading and writing.
“w+”
Open a new file for both reading and writing. If a file with
the specified file-name currently exists, it will be destroyed
and a new file created in its place.
“a+”
Open an existing file for both reading and appending. If a
file with the specified file-name currently does not exist, a
new file will be created.
Programming In C++, Lecture 10 By Umer Rana
Open a File
• Before we can write a file to a disk, or read it, we must open it.
• Opening a file establishes an understanding between our
program and the operating system about which file we’re going
to access and how we’re going to do it.
• The fopen() function returns a pointer to the FILE structure for
our file, which we store in a variable fptr.
Programming In C++, Lecture 10 By Umer Rana
Write to a File
• Once we’ve established a connection with a particular file
by opening it, we can write to it, using the statement.
Putc(ch,fptr)
• The putc() function is similar to the putch() and putchar()
function. While putc() writes to a file.
• The file whose FILE structure is pointed to by the variable
fptr, which we obtained then we open the file.
• This pointer has become our key to the file; we no longer
refer to the file by name, but by the address stored in fptr.
• The writing process continues in the while loop; each time
the putc() function is executed another character is written
to the file.
Programming In C++, Lecture 10 By Umer Rana
Close the File
When we’ve finished writing to the file we need to close it, this is
carried out with the statement fclose(fptr);
Reading from a File
If we can write to a file, we should be able to read from one using
getch() function.
The getc() function reads one character from the file till EOF (end of
file).
End-of-File
It is not a character. It is actually an integer value, sent to the program
by the operating system and defined in the stdio.h file to have a value
of -1.
Programming In C++, Lecture 10 By Umer Rana
Write File On Disk By Character
#include <stdio.h>
#include<conio.h>
int main()
{
char ch;
FILE *fptr;
fptr=fopen("out.txt","w");
while((ch=getche())!='\r')
{
putc(ch,fptr);
}
fclose(fptr);
}
Programming In C++, Lecture 10 By Umer Rana
Read File On Disk By Character
#include <stdio.h>
#include<conio.h>
int main()
{
char ch;
FILE *fptr;
fptr=fopen("out.txt","r");
while((ch=getc(fptr))!=EOF)
{
printf("%c",ch);
}
fclose(fptr);
getch();
}
Programming In C++, Lecture 10 By Umer Rana
String (Line) Input
#include<string.h>
int main()
{
char string[81];
FILE *fptr;
int a;
fptr=fopen("out.txt","w");
while(strlen (gets(string) ) >0)
{
fputs(string,fptr);
fputs("\n",fptr);
}
fclose(fptr);
}
Programming In C++, Lecture 10 By Umer Rana
String (Line) Output
int main()
{
char string[81];
FILE *fptr;
int a;
fptr= fopen("out.txt","r");
fgets(String stored, Length of String, Point to the File)
while(fgets(string,80,fptr)!=NULL)
{
printf("%s",string);
}
fclose(fptr);
getch();
}
Programming In C++, Lecture 10 By Umer Rana
Formatted Input/output
fscanf() & fprintf()
similar to scanf() and printf()
in addition provide file-pointer is include as the first argument.
Example:
fprintf(f1, “%d %f\n”, i, f);
fprintf(stdout, “%f \n”, f);
fscanf(f2, “%d %f”, &i, &f);
fscanf returns EOF when end-of-file reached
Programming In C++, Lecture 10 By Umer Rana
Formatted Input
#include<string.h>
int main()
{
char name[40];
int code;
float height;
FILE *fptr;
fptr=fopen("out.txt","w");
do{
printf("Type Name, code number & height:");
scanf("%s %d %f",name,&code,&height);
fprintf(fptr,"%s %d %f \n",name,code,height);
}while(strlen(name)>2);
fclose(fptr);
}
Programming In C++, Lecture 10 By Umer Rana
Formatted Output
int main()
{
char name[40];
int code;
float height;
FILE *fptr;
fptr=fopen("out.txt","r");
while( fscanf( fptr,"%s %d %f",name,&code,&height)!=EOF)
printf("%s %d %f \n",name,code,height);
fclose(fptr);
getch();
}
Programming In C++, Lecture 10 By Umer Rana
Record Input / Output
• Record some time called block I/O, Record write numbers to the
disk files in binary.
• Records I/O also permits writing any amount of data at once.
• Process is not limited to a single character or string or to the few
values that can be placed in a fprintf() or fscanf() functions.
• Arrays, structures, structures of arrays, and other data
constructions can be written with a single statement.
Record Input / Output functions are
• fwrite()
• fread()
Programming In C++, Lecture 10 By Umer Rana
int main()
{
struct
{
Record Input (fwirte())
char name[40];
int agnumb;
double height;
}agent;
char numstr[40];
FILE *fptr;
if((fptr=fopen("agent.rec","wb"))==NULL)
printf("Cannot open file agents.rec");
do
{
printf("\n Enter Name:");
scanf("%s",&agent.name);
printf("\n Enter Number:");
scanf("%d",&agent.agnumb);
printf("\n Enter Height:");
scanf("%lf",&agent.height);
fwrite(&agent,sizeof(agent),1,fptr);
printf("Add another Agent (y/n)");
}while(getche()=='y');
fclose(fptr);
}
Programming In C++, Lecture 10 By Umer Rana
int main()
{
struct
{
Record Output (fread())
char name[40];
int agnumb;
double height;
}agent;
char numstr[40];
FILE *fptr;
if((fptr=fopen("agent.rec","rb"))==NULL)
{
printf("Cannot open file agents.rec");
}
while (fread(&agent,sizeof(agent),1,fptr)==1)
{
printf("\nName: %s",agent.name);
printf("\nAge: %d",agent.agnumb);
printf("\nHeight: %.2lf",agent.height);
}
fclose(fptr);
getch();
}
Programming In C++, Lecture 10 By Umer Rana
Random Access
• Randomly Access means directly accessing a particular
data items, even thought it may be in the middle of the
file.
Programming In C++, Lecture 10 By Umer Rana
Random Access
int main()
{
struct
{
char name[40];
int agnumb;
double height;
}agent;
FILE *fptr;
int recno;
long int offset;
printf("Enter Record number: ");
scanf("%d",&recno);
offset=(recno-1) * sizeof(agent);
printf("\n%d",offset);
getch();
if(fseek(fptr,offset,0)!=0)
printf("Cannot Move Pointer there.");
if((fptr=fopen("agent.rec","r"))==NULL)
{
printf("Cannot open file agents.rec");
getch();
exit(1);
}
}
Programming In C++, Lecture 10 By Umer Rana
fread(&agent,sizeof(agent),1,fptr);
printf("\nName: %s",agent.name);
printf("\nAge: %d",agent.agnumb);
printf("\nHeight:%.2lf",agent.height);
fclose(fptr);
getch();
File Pointer
• A file pointer is a pointer to a particular byte in a file.
fseek()
fseek(Pointer to File, Position, Mode)
• Give control to move the pointer over the file.
Offset
• This tell the number of bytes from a particulate place to
start reading
Mode
• This determine where the offset will be measured from.
Mode
Offset is Measured from
0
Beginning of file
1
Current position of file pointer
2
End of file.
Programming In C++, Lecture 10 By Umer Rana
Errors that occur during I/O
• Typical errors that occur
– trying to read beyond end-of-file
– trying to use a file that has not been opened
– perform operation on file not permitted by ‘fopen’ mode
– open file with invalid filename
– write to write-protected file
Programming In C++, Lecture 10 By Umer Rana
Error Condition
ferror()
• This function takes one argument the file pointer.
• If it return a value of 0 (False) it mean there is no
error.
• If it return a value of non Zero (True) it mean there
is an error.
if(ferror(fp) !=0)
printf(“An error has occurred\n”);
perror()
• It takes a string supplied by the program as an
argument this string is usually an error message
indicating where in the program the error occurred.
Example See on Page 573
Programming In C++, Lecture 10 By Umer Rana