What is a File

1. What is a File • A file is a package of information with a name attached to it. • Files are used for various purposes: • Files can record data, such as text or numbers. • Some files record ways to perform various processing procedures on data. These are referred to as programs or commands. • Conceptually, a file is a sequence of characters, which resides somewhere on a disk.

2. Access Files • To access a file • Open • Read / Write • Close Read/write A data file Your c code

3. File Pointer • A new data-type in C to communicate with files • Defined in stdio.h • Written as FILE * • E.g. a file pointer called output_file is declared in a statement like • FILE *output_file; • File pointer holds disk location of the disk file

4. Open a File • Must open a file before access it • FILE *fopen(const char *filename, const char *mode); • Connect a file pointer to a specified file • The file now can be accessed via this file pointer, not others • File is accessed sequentially • Position information stored in FILE structure • Like playing audio cassette • Great for dumping and retrieving data

5. Mode • r - open for reading • w - open for writing (file need not exist) • a - open for appending (file need not exist) • 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)

6. Example • In the filename, use \\ rather than \ • FILE *fp; • fp=fopen("c:\\test.txt", "r"); • If file does not exist, fopen returns a null pointer

7. Close a File • Use function fclose • int fclose(FILE *a_file); • fclose returns zero if the file is closed successfully. • Example FILE *fp = fopen("c:\\test.txt", "r"); // codes for accessing the file fclose(fp); • Do not use close();

8. Read from a File – fscanf • Read in data at certain format • int fscanf(FILE *fp, const char *format, ... ); • If success, return the number of successfully matched and assigned input items. • E.g. • n = fscanf(fp, "%d%f%s", &i, &x, name);

9. Read from a File – getc, fgets • gets: read in a single character from a file • char getc(FILE *fp) • If at the end of the file, getc returns EOF – a special value to indicate that the end of file has been reached • Normally -1 is used for EOF • int feof(FILE *fp) test end-of-file indicator • Returns 0 if not end of a file • Non-zero if yes • fgets: read in a string with a specific length from a file • fgets(char *string, int n, FILE * stream); • Reading stops when • A newline character • n-1 characters have been read

10. Write to a File – fprintf • One common way: use function fprintf • int  fprintf (FILE * stream , const char * format [ , argument , ...] ); • Similar to printf • E.g • fprintf(fp, "All numbers are there\n“); • More examples

11. Write to a File – putc, and fputs • Writes a character to a file. • int putc(int c, FILE *fp); • Writes a string • int fputs(const char *s, FILE *fp); • E.g. • fputs(“good”, fp); • no conversion specifications • always ends with new line

12. More I/O: puts char str[] = “Clock strikes 12”; printf(“The message: %s\n”, str); • we can put the length in the conversion specifications • Another way: puts(str); • no conversion specifications, • always ends with new line

13. Example – I • Write a program to count the number of lines and characters in a file.

14. Example – II • Write a program to compare two files specified by the user, displaying a message indicating whether the files are identical or different.

15. Example – III • Write a file copy program • Procedure Open files (f1 and f2) appropriately Read characters from f1 and while not end of file do write character to f2 read next character from f1 Close files