lecture11 files

7/28/2019 Lecture11 Files

1/22

PROGRAMMING II

Files


2/22

Files Used for permanent storage of data

Organization

Sequential

Random

Types

Binary

Text


3/22

Angy Green 234

John Brown 789

Nick Case 345

John Brown 654

Harry Potter 169

File

Record

J o h n Field

01001010 Byte ( ACII character J)

1 bit

Data Hierarchy


4/22

Files and Streams Provide communication channel

between files and programs

C views files as a sequential stream ofbytes

When a file is opened, a stream isassociated with the file


5/22

Files and Streams When a program is executed, three files

and their associated streams are

opened Standard input

Standard output

Standard error

File ends with an EOF marker or at aspecific byte number


6/22

Files and Streams Read C How to Program

Chapter 11 File Processing

Relationship between FILE pointers,FILE structures and File Control Blocks(FCBs)


7/22

File Pointer General Format

FILE * pointer-name

Eg. FILE *fPtr

Keyword FILE establishes a buffer area

Pointer indicates beginning of area


8/22

Opening / Creating a File fPtr = fopen(file_name, file_mode);

Example

fPtr = fopen(client.dat, r);

fPtr = fopen(client.dat, w); fPtr = fopen(client.dat, a);


9/22

File Modesr Open for readingw Create for writing. If file already

exists, discard contentsa Append. Open or create for

writing at end of file

r+ Open for reading & writing

w+ Create for update. If file alreadyexists, discard contents

a+ Append. Open or create for

update at end of file


10/22

Opening / Creating a File If (fPtr == NULL)//results in error

Check to determine that file wasopened successfully

if(fopen(file.txt,a+)==NULL)

printf(Error opening file)


11/22

Binary vs text FileA text file stores line endings with the

\n character though some Windows use

a combination of \r and \n characters.End of file may also be treated with theCtrl-Z character.


12/22

Binary vs Text File Binary files

Most straightforward

Assumes nothing about the OS

To declare file mode to be binary include abin the mode string e.g. wb,


13/22

Sequential Access Files Records typically stored in order by the

record key field

Data stored sequentially one after theother


14/22

Sequential Access File Example of Program

while (!feof(stdin))

fscanf(writePtr,"%d%s%f",&acct_no,name,&balance);

fprintf(writePtr,"%d\t%s\t%.2f\n",acct_

no,name,balance); rewind(writePtr);

fclose(writePtr);
http://localhost/var/www/apps/conversion/tmp/scratch_2/lec11_seq_file.chttp://localhost/var/www/apps/conversion/tmp/scratch_2/lec11_seq_file.c


15/22

Updating Records Risk exists if data in a sequential file is to be modified Example

1 Harry 3456.88

2 John 78901.34Harry to be changed to1 Harrington 3456.88

New record has more characters than previous

Characters beyond the dot would overwrite beginning of next sequentialrecord

Problem: formatted input/output using fprintf/fscanf suggests that recordscan vary in size

Entire file is usually re-written (records before the one requiring update

copied to a new file, new record written, records after one requiring updatecopied to new file


16/22

Random Access Files

Individual records normally fixed in length

May be accessed directly without searching

through other records

Exact location of record can be calculatedrelative to beginning of file

Data can be inserted without destroying otherdata


17/22

Random Access Files

Uses fwrite and fread instead of fprintfand fscanf

Often write records rather thanindividual fields to files (hence uses structures)

See sample program
http://localhost/var/www/apps/conversion/tmp/scratch_2/lec11_rand_file.chttp://localhost/var/www/apps/conversion/tmp/scratch_2/lec11_rand_file.c


18/22

Writing to Random AccessFiles

fwrite( void *pOutArea,

int elementSize,

int count,

FILE* sp)e.g. fwrite(&client1,sizeof(Acct),1,writePtr);

pOutArea area in memory where data is currently

elementSize how many bytes to be written to the file

Count how many of elementSize to write

*sp pointer to the file where the data is to be written


19/22

Reading from Random AccessFiles

fread( void *pIntArea,

int elementSize,

int count,

FILE* sp)fread(&client1, sizeof(Acct),1,writePtr);

pInArea pointer to the input area in memory usually a structure

elementSize is muliplied by count to determine how many data isto be transferred to memory

*sp pointer to the file where the data is


20/22

Random Access FilesPositioning Functions

Rewind File: rewind

void rewind(FILE* stream)

Positions the file pointer to the beginning ofthe file.

Set Positions: fseek int fseek(FILE *stream, long offset, int

wherefrom);

e.g. fseek(writePtr, (client1.acct_no - 1) * sizeof(Acct), SEEK_SET);


21/22

FSEEK

1st parameter is a pointer to an opened file

2nd parameter specifies the number of bytes

the position indicator should move (skip)relatively or absolutely (3rd parameter)

3rd parameter specifies the starting point ofthe seek can either be

SEEK_SET (beginning of file)

SEEK_CUR (current position in file)

SEEK_END (from the end of the file)


22/22

Files

Read C How to Program

Chapter 11 File Processing

Full menu-based (add, update, deleteetc.) transaction processing program

lecture11 files

Documents