Introduction to Programming

1
Introduction to Programming

• Lecture 19

2

• Random Access Files

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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

• 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
00110101
00110101
00111001
00111001
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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 earnings10
double earnings10
fwrite(earnings,sizeof(double),10,fp)
fwrite(earnings,sizeof(double),10,fp)
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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.
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Writing file data using block I/O functions
Writing file data using block I/O functions
include ltstdio.hgt
include ltstdio.hgt
typedef struct
typedef struct
char name20
char name20
int serial_code
int serial_code
int amount
int amount
float
cost
float
cost
component
component
main(void)
main(void)
component comp_data
component comp_data
FILE
fp
FILE
fp
char
filename80
char
filename80
int numofcomp
int numofcomp
int i
int i
printf("Enter the file name ")
printf("Enter the file name ")
gets(filename)
gets(filename)
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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 (i0 iltnumofcomp i)
for (i0 iltnumofcomp 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


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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.
fseek
and
ftell
help doing
datum, 3rd datum ..., etc.
fseek
and
ftell
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.
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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
0 10 20 30 40 50 60 70 80 90
0 10 20 30 40 50 60 70 80 90
......
......
Disk
Disk
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Page 38
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The fseek() Function
The fseek() Function
The function prototype of
fseek()
is
The function prototype of
fseek()
is
int fseek(FILE fp, long int offset, int mode)
int fseek(FILE fp, long int offset, int mode)

fseek()
returns
0
if OK, and
-1
if there is an error.

fseek()
returns
0
if OK, and
-1
if there is an error.

offset
tells how far to move from the starting point

offset
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
identifies the starting point. In stdio.h, the

mode
identifies the starting point. In stdio.h, the
following constants can be assigned to mode
following constants can be assigned to mode
Measure offset from
Mode
Measure offset from
Mode
Beginning of file
SEEK_SET
Beginning of file
SEEK_SET
Current position
SEEK_CUR
Current position
SEEK_CUR
End of file
SEEK_END
End of file
SEEK_END
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Page 39
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The ftell() Function
The ftell() Function
The function prototype of
ftell()
is
The function prototype of
ftell()
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
rewind()
is
The function prototype of
rewind()
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
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fseek(fp, 0L, SEEK_SET)
fseek(fp, 0L, SEEK_SET)
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/ reverse.c - displays a file in reverse order
/
/ reverse.c - displays a file in reverse order
/
include ltstdio.hgt
include ltstdio.hgt
include ltstdlib.hgt
include ltstdlib.hgt
define CNTL_Z '\032'
/ eof marker in DOS textfiles /
define CNTL_Z '\032'
/ 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 ((fpfopen(argv1,"rb")) NULL)
if ((fpfopen(argv1,"rb")) NULL)
printf("reverse can't open s\n", argv1)
printf("reverse can't open s\n", argv1)
exit(1)

exit(1)

fseek(fp,0L, SEEK_END)
/ goto eof /
fseek(fp,0L, SEEK_END)
/ goto eof /
lastftell(fp)
lastftell(fp)
for (count1L countltlast count)
for (count1L countltlast count)
fseek(fp,-count,SEEK_END)
fseek(fp,-count,SEEK_END)
chgetc(fp)
chgetc(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)
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8

• File Positions

9

• File Position Pointer

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tellg ( ) Function

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

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tellp ( ) Function

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

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For Positioning in the file

• seekg ( )
• seekp ( )

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seekg ( )
Number of characters to move to
Starting point

• filePtr.seekg ( long Num , ios origin )

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seekg ( )

• seekg ( 10L , ios beg )
• seekg (10L , ios cur )
• seekg ( 10L , ios end )

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Example 1

• includeltfstream.hgt
• main ( )
• int length
• ifstream inFile ( myFile.txt )
• inFile.seekg ( 0L , ios end )
• length inFile.tellg ( )

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File

• Name city Date-of-Birth
• Jamil Ahmed Sukkur 10-10-1982

Rawalpindi
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Merge Method
Original file
Empty file
This is a text data And needs
To be replaced
NOT
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seekg ( )

• seekg ( 2201L , ios beg )

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fstream

• fstream myFile ( Sample.txt , ios in ios
  out )

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OR Function

• A B Output
• 0 0 0
• 0 1 1
• 1 0 1
• 1 1 1

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Example 2

• This is an Apple
• This is a Sample

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get ( ) and put ( ) character in a file

• myInputFile.get ( c )
• myOutputFile.put ( c )

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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
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Example 3

• char str 10000
• myInputFile.read ( str , 10000 )
• myOuputFile.write ( str , 10000 )

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seekg ( )

• seekg ( 0L , ios end )

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seekg ( )

• seekg ( -1L , ios end )

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seekg ( )

• seekg ( -2L , ios cur )

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Address of the integer i
Number of bytes to be written

• myOutputFile.write ( i , 4 )

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• sizeof ( )

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Address of the integer i
Size of integer

• myOutputFile.write ( i , sizeof ( i ) )

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• for ( i 0 i lt 100 i )
• myOutputFile.write ( i , sizeof ( i ) )
• myOutputFile.close ( )