C programming session 11


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C programming session 11

  1. 1. Programming in CObjectives In this session, you will learn to: Read and write contents in a file Use random access in files Ver. 1.0 Slide 1 of 37
  2. 2. Programming in CReading and Writing Contents in a File File inputs-outputs is similar to input from/to the terminal. Files are treated as streams of characters. Function are available for single character as well as multiple character input-output from/to files. Ver. 1.0 Slide 2 of 37
  3. 3. Programming in COpening Files • A file needs to be opened to read or to write contents in it. • The fopen() function is used to open a file. • The fopen() function returns a pointer of the FILE type data. • The fopen() function opens a file in a specific access mode. • The various modes in which a file can be opened are: r - Read-Only Mode w - Write-Only Mode a - Append Mode r+ - Read + Write Mode w+ - Write + Read Mode a+ - Read + Append Mode Ver. 1.0 Slide 3 of 37
  4. 4. Programming in CFILE Type Pointers • The FILE type pointer is: – Returned when a file is opened by using the fopen() function. – Used to manipulate a file. – Used to check whether a file has opened successfully. • The stdin, stdout, and stderr FILE pointers refer to the standard input device (keyboard) and standard output and error device (VDU). Ver. 1.0 Slide 4 of 37
  5. 5. Programming in CThe exit() Function • The exit() Function: Is used to terminate a program execution. Is used as shown in the following code snippet: if (argc ! = 3) { print (“invalid arguments n”); exit (); } Ver. 1.0 Slide 5 of 37
  6. 6. Programming in CCharacter Input-Output with Files The functions used for character input-output with files are: – fgetc(): Reads one character at a time from a file, assigns it to a character variable, and moves the file pointer to the next character. It returns an integer type of value. – fputc(): Writes one character at a time in a file. Ver. 1.0 Slide 6 of 37
  7. 7. Programming in CClosing Files • The fclose() function is used to close files. • Closing the file release the resources. • The syntax of the fclose() function is: fclose (ptr1); Where ptr1 is a FILE pointer. Ver. 1.0 Slide 7 of 37
  8. 8. Programming in CPractice: 6.1 1. What does the following code do? while((c = fgetc (fp)) != EOF) { if ((c >= ‘a’) && (c <= ‘z’)) c -= 32; fputc(c, stdout); } 2. Write a program called append, which appends the contents of the first file to the second file specified on the command line. The program should also terminate in the following situations: a. 2 arguments are not specified on the command line. In this case, the following message must be displayed: Usage: append file1 file2 b. In case the file to be read cannot be opened, the following message may be displayed: Cannot open input file Ver. 1.0 Slide 8 of 37
  9. 9. Programming in CPractice: 6.1 (Contd.) Solution: Microsoft Word Document Ver. 1.0 Slide 9 of 37
  10. 10. Programming in CPractice: 6.2 1. Point out the errors in the following code: a. /* this program creates the file emp.dat */ main() { FILE point; fopen(“w”, “emp.dat”); : fclose(emp.dat); } b. /* this program reads the file emp.dat */ main() { #include<stdio.h> file*ptr; ptr = fopen(emp.dat); : ptr= fclose(); } Ver. 1.0 Slide 10 of 37
  11. 11. Programming in CPractice: 6.2 (Contd.) 2. Given the following statements of a C program: fopen(“man.txt”, “r”); fclose(fileptr); What will the FILE declaration statement of this program be? 3. Point out the error(s) in the following code: #include<stdio.h> main() { char emp; FILE *pointer1; pointer1= fopen(“man1.txt”,”w”); while((inp = fgetc(pointer1)) != eof) { printf(“?%c”, inp); } } Ver. 1.0 Slide 11 of 37
  12. 12. Programming in CPractice: 6.2 (Contd.) 4. The copy command of DOS copies the contents of the first file named on the command line to the second file. Make appropriate changes to the file-copy program so that it works identical to the copy command. Ver. 1.0 Slide 12 of 37
  13. 13. Programming in CPractice: 6.2 (Contd.) Solution: Microsoft Word Document Ver. 1.0 Slide 13 of 37
  14. 14. Programming in CLine Input and Output with Files The functions used for line input and output with files are: – fgets(): Is used to read number of specified characters from a stream. Reads number of characters specified – 1 characters. Has the following syntax: fgets(str, 181, ptr1); Str – Character array for storing the string 181 – Length of the string to be read ptr1- FILE pointer – fputs(): Is used to output number of specified characters to a stream. Has the following syntax: fputs(str, ptr1); Str – Character array to be written ptr1- FILE pointer Ver. 1.0 Slide 14 of 37
  15. 15. Programming in CPractice: 6.3 1. State whether True or False: Files created using the fputs() function will always have records of equal length. 3. Consider the following C statement to input a record from a file called number-list: fgets (line, MAXLEN, file_ind); Given that MAXLEN is a #define and that all lines in the file number-list are 25 characters long what will the declaration statements for the parameters of fgets() be? 3. Assume that the file number_list contains the following records: 120 1305 Ver. 1.0 Slide 15 of 37
  16. 16. Programming in CPractice: 6.3 (Contd.) Given that the file has been opened and the first input statement executed is as follows: fgets(line, 3, file_ind); Which of the following will the array called line contain? a. 1 followed by 0. b. 12 followed by 0. c. 120 followed by 0. 3. Match the following functions with the values they can return: a. fgets() 1. NULL b. fgetc() 2. EOF c. fopen() 3. FILE type pointer Ver. 1.0 Slide 16 of 37
  17. 17. Programming in CPractice: 6.3 (Contd.) If a function can return more than one type of these values, state the conditions under which the values are returned. • A utility called hprint has to be written in C, which will allow a user to display, on screen, any number of lines from the beginning of any file. The user has to specify both the number of lines and the file name on the command line in the following format: hprint number file-name The maximum line length is 80 characters. The program should handle possible errors. Ver. 1.0 Slide 17 of 37
  18. 18. Programming in CPractice: 6.3 (Contd.) Solution: – False. fputs() writes the contents of a string onto a file. So even if a string has size 100, but contains only 20 characters before a 0, only 20 characters get written. – The declarations are: #define MAXLEN 26/* macro definition outside main() */ char line[26]; – b. fgets() will read either 3 - 1 characters , i.e. 2 characters, or until it comes across a newline character. Since the newline occurs after the third character, it will read in 2 characters from the first record. Ver. 1.0 Slide 18 of 37
  19. 19. Programming in CPractice: 6.3 (Contd.) 4. a. 1 b. 2 c. 1 and 3. (NULL in case file cannot be opened; FILE type pointer in case of successful open) 5. The answer to this practice will be discussed in class. Work out your solution. Ver. 1.0 Slide 19 of 37
  20. 20. Programming in CFormatted Input and Output with Files The functions for formatted input and output with files are: – fscanf(): • Scans and formats input from a stream. • Is similar to scanf(). • Has the following syntax: int fscanf(FILE *Stream, const char *format[,address,..]); fprintf(): • Sends formatted output to a stream. • Is similar to printf(). • Has the following syntax: int fprintf(FILE *Stream, const char *format[,address,..]); Ver. 1.0 Slide 20 of 37
  21. 21. Programming in CPractice: 6.4 • Rewrite the following printf() statement using the function fprintf(): printf(“The test value is %d”, x); 3. The following statement is written to input 2 fields from the keyboard: scanf(“ %6s%d”, array, &num); It is rewritten as: fscanf(“%6s%d”, array, &num); This statement is erroneous. Give the correct fscanf() statement. Ver. 1.0 Slide 21 of 37
  22. 22. Programming in CPractice: 6.4 (Contd.) • Write the appropriate statements to input fields from a record of a file called alpha-doc, the first field being a float value, and the second field a string of size 10. In case the file does not have he required data, and the end- of-file occurs, the following message should be displayed: End of file encountered. Ver. 1.0 Slide 22 of 37
  23. 23. Programming in CPractice: 6.4 (Contd.) • A utility called format is required to create a formatted report from a file called manufact. This report is also to be stored on disk with suitable report headings. The name of the file to be created should be accepted during program execution. The program should also ask for a report title, which should appear after every 60 record of the file manufact. The file manufact contains the following 3 fields separated by space. Field Size Manufacturer Code 4 Name 20 Address 60 In the output file, the fields should be separated by one tab character. Ver. 1.0 Slide 23 of 37
  24. 24. Programming in CPractice: 6.4 (Contd.) Solution: Microsoft Word Document Ver. 1.0 Slide 24 of 37
  25. 25. Programming in CUsing Random Access in Files A file can be accessed using sequential access or random access. In sequential access, the file is always accessed from the beginning. In random access the file can be accessed arbitrarily from any position. Ver. 1.0 Slide 25 of 37
  26. 26. Programming in CThe fseek () Function • The fseek() function: – Is used for repositioning the current position on a file opened by the fopen() function. – Has the following syntax: rtn = fseek (file-pointer, offset, from-where); Here: int rtn is the value returned by fseek()(0 if successful and 1 if unsuccessful). file-pointer is the pointer to the file. offset is the number of bytes that the current position will shift on a file. from-where is the position on the file from where the offset would be effective. Ver. 1.0 Slide 26 of 37
  27. 27. Programming in CThe rewind () Function • The rewind() function: Is used to reposition the current position to the beginning of a file. Is useful for reinitializing the current position on a file. Has the following syntax: rewind(file-pointer); Here: file-pointer is the pointer returned by the function fopen(). After rewind() is executed, current position is always 1, i.e. beginning of file. Ver. 1.0 Slide 27 of 37
  28. 28. Programming in CPractice: 6.5 • Write the equivalent of the function rewind() using fseek(). • Assume the following representation of the first 30 bytes of a file. Ver. 1.0 Slide 28 of 37
  29. 29. Programming in CPractice: 6.5 (Contd.) What will the current position on the file be after the following instructions are performed in sequence? a. fp = fopen ("FOR DEMO.DAT", “r”); b. fseek(fp, 29L, 1); c. rewind(fp); d. fgets(buffer, 20L, fp); e. fseek(fp, 4L, 1); Ver. 1.0 Slide 29 of 37
  30. 30. Programming in CPractice: 6.5 (Contd.) Solution: 1. fseek(fp, 0L, 0); 2. The following current positions are relative to the beginning of the file: a. 1 b. 30 c. 1 d. 20 e. 24 Ver. 1.0 Slide 30 of 37
  31. 31. Programming in CPractice: 6.6 1. Write a function to update the field balance in the file SAVINGS.DAT based on the following information. If balance is Increment balance by < Rs 2000.00 Rs 150.50 Between Rs. 2000.00 Rs 200.00 and Rs 5000.00 <Rs 5000.00 Rs 300.40 The structure of the file SAVINGS.DAT is as follows. Account number Account holders name Balance (5 bytes) (20 bytes) (5 bytes) Ver. 1.0 Slide 31 of 37
  32. 32. Programming in CPractice: 6.6 (Contd.) Solution: Microsoft Word Document Ver. 1.0 Slide 32 of 37
  33. 33. Programming in CPractice: 6.7 • Go through the following program called inpcopy.c and its error listing on compilation and then correct the program: 1 #include <stdio.h> 2 main() 3 { 4 file fp; 5 char c; 6 7 fp = fopen(“file”, w); 8 9 while (( c = fgetc(stdin)) != EOF) 10 fputc(c,fp); 11 12 fclose(fp); 13 } Ver. 1.0 Slide 33 of 37
  34. 34. Programming in CPractice: 6.7 (Contd.) Error listing: "inpcopy/.c", line 4: file undefined "inpcopy/.c". line 4: syntax error "inpcopy/.c", line 7: fp undefined "inpcopy/.c", line 7: w undefined "inpcopy/.c", line 7: learning: illegal pointer/integer combination, op = "inpcopy/.c", line 9: c undefined Ver. 1.0 Slide 34 of 37
  35. 35. Programming in CPractice: 6.7 (Contd.) Solution: 1. Work out for your answer. The solution will be discussed in the classroom session. Ver. 1.0 Slide 35 of 37
  36. 36. Programming in CSummary In this session, you learned that: – C treats file input-output in much the same way as input-output from/to the terminal. – A file needs to be opened to read or to write contents in it. – The fopen() function is used to open a file. – C allows a number of modes in which a file can be opened. – When a file is opened by using the fopen() function, it returns a pointer that has to be stored in a FILE type pointer. – This FILE type pointer is used to manipulate a file. – The exit() function is used to terminate program execution. – The fgetc() and fputc() functions are used for character input-output in files. – After completing the I/O operations on the file, it should be closed to releases the resources. Ver. 1.0 Slide 36 of 37
  37. 37. Programming in CSummary (Contd.) – The fclose() function is used to close a file. – The fgets() and fputs() functions are used for string input-output in files. – The fscanf() and fprintf() functions are used for formatted input-output in files. – In sequential access, the file is always accessed from the beginning. – In random access the file can be accessed arbitrarily from any position. – C provides the fseek() function for random access. – The function rewind() is used to reposition the current position to the beginning of a file. Ver. 1.0 Slide 37 of 37