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Lecture 11

Lecture 11. CIS 208 Friday February 18th , 2005. Wednesday - Review. Email me questions and topics No new material is scheduled. Dynamic 2-d arrays. Must first allocate array of pointers Then allocate each sub-array.

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Lecture 11

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  1. Lecture 11 CIS 208 Friday February 18th, 2005

  2. Wednesday - Review • Email me questions and topics • No new material is scheduled.

  3. Dynamic 2-d arrays • Must first allocate array of pointers • Then allocate each sub-array. • When freeing, go in reverse direction, free sub-arrays, then main array.

  4. File I/O • Combines Pointers and console I/O • Most of the same basic functions. • Works on text files or binary files

  5. FILE pointers • point to a stream • FILE *fp; • stdin, stdout are FILE pointers • point to certain place in file.

  6. fopen • Must first open a file FILE *fopen(const char *filename, const char *mode) • mode is how file is to be opened • returns NULL if unsuccessful

  7. mode values r : Open a text file for reading w : Create a text file for writing a : Append to a text file r+ : Open a text file for read/write w+ : Create a text file for read/write etc… Check your book

  8. fopen FILE *fp; fp = fopen(“test.dat”,”w”); • Now we can use the stream pointed to by fp • “w” will create a new file, if one already exists, it overwrites it.

  9. safe fopen • should check to see if open was sucessful. • File might not exist, or protected if ((fp = fopen(“test”,”w”)) == NULL) { printf(“Cannot open file.\n”); exit(1); }

  10. more fopen • Some systems have limit on number of streams • Usually at least 8. Platform dependent • FOPEN_MAX

  11. fclose • Any open file must be closed • int fclose(FILE *fp) readies file for closure • Finishes any leftover writing • Tell OS to formally close file. • unclosed files cause problems • closing == ‘saving’

  12. int fclose(FILE *fp) FILE *fp = fopen(“test”,”w”); … fclose(fp); • fp is now closed and this pointer can be used on something else.

  13. int fclose(FILE *fp) • returns 0 if succesful, • EOF if error • Errors are rare • trying to close an unopened file • closing file on removed storage device.

  14. character writing • Just like console I/O int putc(int ch, FILE *fp); int fputc(int ch, FILE *fp); • ch is a char, even though listed as int. • Only lower byte is sent.

  15. int putc(int ch, FILE *fp) • prints one character to file stream fp • if successful, returns the character • Else, returns EOF

  16. Reading Characters • Just like reading from stdin int getc(FILE *fp) int fgetc(FILE *fp) Both do the same thing. Returns 1 character from stream; Returns EOF if error.

  17. fgetc() char ch; ch = fgetc(fp); do { ch = getc(fp); } while (ch != EOF);

  18. KTOD DTOS • Examples on web page

  19. feof() • in binary files, EOF isn’t a number • int feof(FILE *fp) • given a file pointer, true if it’s reached EOF • false if not • works on text files too.

  20. feof() while (!feof(fp)) { ch = getc(fp); }

  21. FILE and strings • Already used these before int fputs(char *str,FILE *fp) char *fgets(char *str,int length, FILE *fp)

  22. fgets • reads length characters • terminated by newline • writes to pre-allocated character array • must replace newline character with null terminator.

  23. fputs • write entire string to file char *str = “abcd”; fputs(str,fp);

  24. Errors • trying to write to a “r” file • doesn’t destroy file, but isn’t good. • be careful

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