File in C language

File operations in C- language in details

1. C Files Module VIII

2. What is a File ? A file is a collection of related data that a computers treats as a single unit. Computers store files to secondary storage so that the contents of files remain intact when a computer shuts down. When a computer reads a file, it copies the file from the storage device to memory; when it writes to a file, it transfers data from memory to the storage device. C uses a structure called FILE (defined in stdio.h ) to store the attributes of a file.

3. Console oriented Input / Output Console oriented – use terminal (keyboard/screen) scanf (“% d”,&i ) – read data from keyboard printf (“% d”,i ) – print data to monitor Suitable for small volumes of data Data lost when program terminated

4. Real-life applications Large data volumes E.g. physical experiments (CERN collider), human genome, population records etc. Need for flexible approach to store/retrieve data Concept of files

5. Files File – place on disc where group of related data is stored E.g. your C programs, executables High-level programming languages support file operations Naming Opening Reading Writing Closing

6. Defining and opening file To store data file in secondary memory (disc) must specify to OS Filename (e.g. sort.c , input.data ) Data structure (e.g. FILE) Purpose (e.g. reading, writing, appending)

7. Filename String of characters that make up a valid filename for OS May contain two parts Primary Optional period with extension Examples: a.out , prog.c , temp, text.out

8. General format for opening file FILE * fp ; /*variable fp is pointer to type FILE*/ fp = fopen (“filename”, “mode”); /*opens file with name filename , assigns identifier to fp */ fp contains all information about file Communication link between system and program m ode can be r open file for reading only w open file for writing only a open file for appending (adding) data

9. Different modes Writing mode if file already exists then contents are deleted else new file with specified name created Appending mode if file already exists then file opened with contents safe else new file created Reading mode if file already exists then opened with contents safe else error occurs. FILE *p1, *p2; p1 = fopen (“ data”,”r ”); p2= fopen (“results”, w”);

10. Steps in Processing a File Create the stream via a pointer variable using the FILE structure: FILE *p; Open the file, associating the stream name with the file name. Read or write the data. Close the file.

11. The basic file operations are fopen - open a file- specify how its opened (read/write) and type (binary/text) fclose - close an opened file fread - read from a file fwrite - write to a file fseek / fsetpos - move a file pointer to somewhere in a file ftell / fgetpos - tell you where the file pointer is located

12. String of characters that make up a valid filename for OS May contain two parts Primary Optional period with extension Examples : a.out , prog.c , temp, text.out Filename

13. File Open Modes

14. More on File Open Modes

15. Additionally r + - open for reading and writing, start at beginning w + - open for reading and writing (overwrite file) a + - open for reading and writing (append if file exists )

16. File Open The file open function ( fopen ) serves two purposes: It makes the connection between the physical file and the stream. It creates “a program file structure to store the information” C needs to process the file. Syntax : filepointer = fopen (“filename”, “mode”);

17. More On fopen The file mode tells C how the program will use the file. The filename indicates the system name and location for the file. We assign the return value of fopen to our pointer variable: spData = fopen (“MYFILE.TXT”, “w”); spData = fopen (“A:\\MYFILE.TXT”, “w”);

18. More On fopen

19. Closing a File When we finish with a mode, we need to close the file before ending the program or beginning another mode with that same file . To close a file, we use fclose and the pointer variable: fclose ( spData );

20. fprintf () Syntax: fprintf ( fp , "string", variables ); Example: int i = 12; float x = 2.356; char ch = 's'; FILE * fp ; fp = fopen (“ out.txt”,”w ”); fprintf ( fp , "%d %f %c", i, x, ch );

21. fscanf () Syntax : fscanf ( fp ," string",identifiers ); Example: FILE * fp ; Fp = fopen (“ input.txt”,”r ”); int i; fscanf ( fp ,“% d",i );

22. getc () Syntax: identifier = getc (file pointer); Example: FILE * fp ; fp = fopen (“ input.txt”,”r ”); char ch ; ch = getc ( fp );

23. putc () Write a single character to the output file, pointed to by fp. Example : FILE * fp ; char ch ; putc ( ch,fp );

24. End of File There are a number of ways to test for the end-of-file condition. Another way is to use the value returned by the fscanf function: FILE *fptr1; int istatus ; istatus = fscanf (fptr1, "%d", & var ) ; if ( istatus == feof (fptr1) ) { printf ("End-of-file encountered.\n”) ; }

25. Reading and Writing Files #include < stdio.h > int main ( ) { FILE * outfile , * infile ; int b = 5, f ; float a = 13.72, c = 6.68, e, g ; outfile = fopen (" testdata ", "w") ; fprintf ( outfile , “ %f %d %f ", a, b, c) ; fclose ( outfile ) ; infile = fopen (" testdata ", "r") ; fscanf ( infile ,"%f %d %f", &e, &f, &g) ; printf (“ %f %d %f \n ", a, b, c) ; printf (“ %f %d %f \n ", e, f, g) ; }

26. Example #include < stdio.h > #include< conio.h > void main() { char ch ; FILE * fp ; fp = fopen (" out.txt","r "); while(! feof ( fp )) { ch = getc ( fp ); printf ("\ n%c ", ch ); } getch (); }

27. fread  () Declaration: size_t fread (void * ptr , size_t size, size_t n, FILE *stream); Remarks: fread reads a specified number of equal-sized data items from an input stream into a block. ptr = Points to a block into which data is read size = Length of each item read, in bytes n = Number of items read stream = file pointer

28. Example Example: #include < stdio.h > int main() { FILE *f; char buffer[11]; if (f = fopen ("fred.txt", “r”)) { fread (buffer, 1, 10, f); buffer[10] = 0; fclose (f); printf ("first 10 characters of the file:\ n%s \n", buffer); } return 0; }

29. fwrite () Declaration: size_t fwrite ( const void * ptr , size_t size, size_t n, FILE*stream); Remarks: fwrite appends a specified number of equal-sized data items to an output file. ptr = Pointer to any object; the data written begins at ptr size = Length of each item of data n =Number of data items to be appended stream = file pointer

30. Example #include < stdio.h > int main() { char a[10]={'1','2','3','4','5','6','7','8','9','a'}; FILE *fs; fs= fopen (" Project.txt","w "); fwrite (a,1,10,fs); fclose (fs); return 0; }

31. fseek () This function sets the file position indicator for the stream pointed to by stream or you can say it seeks a specified place within a file and modify it. SEEK_SET Seeks from beginning of file SEEK_CUR Seeks from current position SEEK_END Seeks from end of file Example : #include < stdio.h > int main() {       FILE * f;       f = fopen ("myfile.txt", "w");        fputs ("Hello World", f);        fseek (f, 6, SEEK_SET); SEEK_CUR, SEEK_END        fputs (" India", f);        fclose (f);       return 0; }

32. ftell () offset = ftell ( file pointer ); " ftell " returns the current position for input or output on the file # include < stdio.h > int main(void) { FILE *stream; stream = fopen ("MYFILE.TXT", "w"); fprintf (stream, "This is a test"); printf ("The file pointer is at byte %ld\n", ftell (stream)); fclose (stream); return 0; }

33. Closing a file File must be closed as soon as all operations on it completed Ensures All outstanding information associated with file flushed out from buffers All links to file broken Accidental misuse of file prevented If want to change mode of file, then first close and open again

34. Closing a file Syntax: fclose ( file_pointer ); Example : FILE *p1, *p2; p1 = fopen (“INPUT.txt”, “r”); p2 = fopen (“OUTPUT.txt”, “w”); …….. …….. fclose (p1); fclose (p2 ); P ointer can be reused after closing

35. Input/Output operations on files C provides several different functions for reading/writing getc () – read a character putc () – write a character fprintf () – write set of data values fscanf () – read set of data values getw () – read integer putw () – write integer

36. getc() and putc() handle one character at a time like getchar () and putchar () syntax : putc (c,fp1); c : a character variable fp1 : pointer to file opened with mode w syntax : c = getc (fp2); c : a character variable fp2 : pointer to file opened with mode r file pointer moves by one character position after every getc () and putc () getc () returns end-of-file marker EOF when file end reached

37. Program to read/write using getc/putc #include < stdio.h > main() { FILE *fp1; char c; f1= fopen (“INPUT”, “w”); /* open file for writing */ while((c= getchar ()) != EOF) /* get char from keyboard until CTL-Z*/ putc (c,f1); /* write a character to INPUT */ fclose (f1); /* close INPUT */ f1= fopen (“INPUT”, “r”); /* reopen file */ while((c= getc (f1))!=EOF ) /*read character from file INPUT*/ printf (“%c”, c); /* print character to screen */ fclose (f1); } /* end main */

38. fscanf() and fprintf() Similar to scanf () and printf () in addition provide file-pointer given the following file-pointer f1 (points to file opened in write mode) file-pointer f2 (points to file opened in read mode) integer variable i float variable f Example : fprintf (f1, “%d %f\n”, i, f); fprintf ( stdout , “%f \n”, f); /* note: stdout refers to screen */ fscanf (f2, “%d %f”, &i, &f ); fscanf returns EOF when end-of-file reached

39. getw() and putw() handle one integer at a time syntax: putw (i,fp1); i : an integer variable fp1 : pointer to file ipened with mode w syntax: i = getw (fp2); i : an integer variable fp2 : pointer to file opened with mode r file pointer moves by one integer position, data stored in binary format native to local system getw () returns end-of-file marker EOF when file end reached

40. C program using getw, putw,fscanf, fprintf #include < stdio.h > main() { int i,sum1=0; FILE *f1; /* open files */ f1 = fopen (" int_data.bin","w "); /* write integers to files in binary and text format*/ for(i=10;i<15;i++) putw (i,f1); fclose (f1); f1 = fopen (" int_data.bin","r "); while((i= getw (f1))!=EOF) { sum1+=i; printf ("binary file: i=%d\ n",i ); } /* end while getw */ printf ("binary sum=%d,sum1); fclose (f1); } #include < stdio.h > main() { int i, sum2=0; FILE *f2; /* open files */ f2 = fopen (" int_data.txt","w "); /* write integers to files in binary and text format*/ for(i=10;i<15;i++) printf (f2,"%d\ n",i ); fclose (f2); f2 = fopen (" int_data.txt","r "); while( fscanf (f2,"%d",&i)!=EOF) { sum2+=i; printf ("text file: i= %d\ n",i ); } /*end while fscanf */ printf ("text sum=%d\n",sum2); fclose (f2); }

41. Thank You