1. Chapter 4 File Processing
Aree Teeraparbseree, Ph.D

2. Data Hierarchy Bit - Smallest unit of data : 0 or 1
Byte - 8 bits (used to store a character)
Field - group of characters conveying meaning
Record - group of related fields
Represented by a struct or a class
File - group of related records
Database - group of related files

3. What is file ?
File : a series of bytes of data ends with the end-of-file marker (EOF)
Stream : (created when a file is opened ) provide communication channel between files and programs
Opening a file returns a pointer to a FILE structure
Program
File
Stream

4. File Processing To create, read, write and update
Sequential-access file processing
Random-access file processing

5. Defining and Opening File
Defining : must specify
filename (e.g. student.txt, input.dat)
purpose /mode (e.g. reading, writing, appending)
Opening : using the fopen() function, which returns a file pointer.
Prototype:
FILE *fopen(char *filename, char *mode);
FILE *fp; /* variable fp is pointer to type FILE */
fp = fopen("filename", "mode");
/*opens file with name filename , assigns identifier to fp */

6. File modes Files are open in a certain mode.
MODE USED FOR FILE CREATED? EXISTING FILE?
"a" Appending Yes Appended to
"a+" Reading/appending Yes Appended to
"r" Read only No Yes
"r+" Reading/writing No Yes
"w" Write only Yes Destroyed
"w+" Reading/writing Yes Destroyed

7. Example: fopen()
Every call to fopen() will typically be followed with a test, like this:
File *fp;
fp = fopen("data.txt", "r");
if(fp == NULL){
printf("File could not be opened\n"); } or
if((fp = fopen("data.txt", "r")) == NULL){

8. fclose()
The fclose() function closes the stream. All buffers are flushed.
Prototype:
int fclose(FILE *stream);
Example:
FILE *fp;
fp = fopen("file.txt", "w");
… // do something
fclose(fp);

9. Read/Write a Sequential-Access File
fscanf() / fprintf()
File processing equivalents of scanf and printf
fgetc()
Reads one character from a file
Takes a FILE * as an argument
fputc()
Writes one character to a file
Takes a FILE * and a character to write as an argument
fgets()
Reads a line from a file
fputs()
Writes a line to a file

10. fprintf() / fscanf() Prototypes (as in stdio.h):
int fprintf(FILE *stream, const char *format, ...);
int fscanf(FILE *stream, const char *format, ...);
fprintf(stdout,"Hello");  printf("Hello");
fscanf(stdin,"%d",&i);  scanf("%d",&i);
Return numbers of arguments which are successfully matched with the specified formats

11. Example: fprintf() #include <stdio.h> int main(){ int i;
FILE *ifp;
char name[30];
if((ifp=fopen("name.txt", "w"))!= NULL){
for(i=0; i<10; i++) {
printf("Enter name: ");
gets(name);
fprintf(ifp, "%s\n",name); }
fclose(ifp);
return 0;

12. Example: fscanf() #include <stdio.h> int main(){ char name[30];
FILE *ofp;
ofp = fopen("name.txt", "r");
while(fscanf(ofp, "%s", &name) != EOF)
printf("%s\n", name);
fclose(ofp);
return 0; }

13. fputc() / fgetc() Prototype: int fgetc(FILE *stream);
return a read character which is promoted to int
Prototype: int fputc(int char,FILE *stream);
return a character “int char” on success
file pointer moves by one character position after every fgetc() and fputc()
fgetc() returns end-of-file marker (EOF) when file end reached

14. Example: fputc() #include<stdio.h> int main() { FILE *fp;
int ch;
fp = fopen("alphabet.txt","w+");
for(ch='a';ch<='z';ch++) {
fputc(ch,fp); }
fclose(fp);
return(0);

15. Example: fgetc() #include <stdio.h> int main() { FILE *fp;
int c, n=0;
fp= fopen("phabet.txt", "r");
if(fp == NULL) {
printf("Error in opening file"); return -1; }
do { c = fgetc(fp);
printf("%c",c);
}while(c != EOF);
fclose(fp);
return 0;

16. fputs()/fgets() Prototype: char *fgets(char *str,int n,FILE *stream);
int fputs(const char *str,FILE *stream);
(return non-negative value/ EOF)
char *fgets(char *str,int n,FILE *stream);
(return the same as *str)
fgets() stops when either (n-1) characters are read,
the newline character is read,
or the end-of-file is reached, whichever comes first.

17. Example: fputs()
#include <stdio.h> int main () { FILE *fp; fp = fopen("file.txt", "w"); fputs("This is c programming.\n", fp); fputs("This is an example of fputs", fp); fclose(fp); return 0; }

18. Example: fgets()
#include <stdio.h> int main() { FILE *fp; char str[60]; if((fp = fopen("file.txt" , "r"))== NULL){ printf("Error opening file"); return -1; } while( fgets(str, 60, fp)!=NULL ) { puts(str); } fclose(fp); return 0;

19. Random access files Random access files
Access individual records without searching through other records
Instant access to records in a file
Data can be inserted without destroying other data
Data previously stored can be updated or deleted without overwriting
Implemented using fixed length records
Sequential files do not have fixed length records

20. Text Mode vs. Binary Mode
Human Readable
Usually process sequentially
Binary Mode
Data can be in any format
Random access file usually in this mode

21. Open a binary File FILE *fopen("filename", mode); Mode:
rb: read binary file
wb: write binary file
ab: append binary file
+: update binary file
: "rb+", "wb+", "ab+"

22. Main Operations for Random Access Files
fwrite() : write to a file
fread() : read from a file
fseek() : move a file pointer to somewhere in a file

23. fwrite() / fread() Prototype:
size_t => unsigned integer => platform independent
Prototype:
size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream);
(return total number of elements successfully written)
size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);
(return total number of elements successfully read)
ptr: pointer to data structure
size: size of data
nmemb: number of data items

24. Example: fwrite()
#include<stdio.h> struct clientData { int acctNum; char lastName[15]; char firstName[10]; double balance; }; int main() { int i; struct clientData blankClient = {0,"","",0.0}; FILE *cfPtr;

25. Example: fwrite() (conf.)
if((cfPtr=fopen("credit.dat","wb")) == NULL ) printf("File could not be opened.\n"); else { for(i=1; i<=100;i++) fwrite(&blankClient,sizeof(struct clientData) ,1,cfPtr); fclose(cfPtr); } return 0;

26. Example: fread()
#include<stdio.h> struct clientData { int acctNum; char lastName[15]; char firstName[10]; double balance; }; int main() { struct clientData client = {0,"","",0.0}; FILE *cfPtr; if((cfPtr=fopen("credit.dat","rb"))== NULL) printf("File could not be opened.\n");

27. Example: fread() (conf.)
else { printf("%-6s%-16s%-11s%10s\n","Acct","Last Name", "First Name","Balance"); while(!feof(cfPtr)) { //check End-of-file fread(&client,sizeof(struct clientData), 1, cfPtr); if(client.acctNum != 0) printf("%-6d%-16s%-11s%10.2f\n", client.acctNum, client.lastName, client.firstName,client.balance); } fclose(cfPtr); return 0;

28. fseek()
Prototype:
int fseek(FILE *stream, long int offset, int whence);
(return non-zero: error / 0 = success)
offset: number of bytes to seek from the given whence (origin) position.
whence: starting position for a seek:
SEEK_SET beginning of file SEEK_CUR current location in file SEEK_END end of file

29. Example: fseek()
#include<stdio.h> struct clientData { int acctNum; char lastName[15]; char firstName[10]; double balance; }; int main() { struct clientData client = {0,"","",0.0}; FILE *cfPtr; if((cfPtr = fopen("credit.dat","rb+")) == NULL ) printf("File could not be opened.\n"); else {

30. Example: fseek() (conf.)
printf("Enter account number (0 to end): "); scanf("%d",&client.acctNum); while(client.acctNum != 0) { printf("Enter lastname, firstname, balance: "); scanf("%s%s%lf",client.lastName,client.firstName, &client.balance); fseek(cfPtr,(client.acctNum-1)*sizeof(struct clientData),SEEK_SET); fwrite(&client,sizeof(struct clientData),1,cfPtr); printf("Enter account number: "); } fclose(cfPtr); return 0;

31. General Procedure for Access a File
Open a file (new or old)
fopen(): returns a FILE* on success
Store the return value for operation
Do something on the file
Read/write on a FILE*
Close the file
fclose(FILE*)