1. 310201 Fundamental Programming Command Line Parameters and File I/O.

2. Topics for this Week Command Line Parameters, Passing Functions and File Objects to other functions, Sequential File I/O, Random File I/O.

3. Command Line Arguments In command line environments such as UNIX or DOS you can send parameters to the main program function when it is run. This can be a very handy feature. Command line parameters provide a means to pass input to a program without having to be prompted for it. For example, we could run a program called my_prog at the DOS command prompt and provide 2 command line parameters (separated by spaces) as follows : my_prog in.dat out.dat

4. Command Line Arguments - What use are they ? Command line arguments are very useful because you can pass input / values directly to your program. This means that your program doesn’t need to prompt you for the values it needs when you run it - it can just get on and do the job. Once the parameters or command line are setup, the command line can be stored in a shell script (UNIX) or batch file (DOS) and later used to run the program at any time in the future. This is often a very handy thing to do – especially when you need to run a program without any user intervention.

5. DOS Examples Many DOS commands and programs use command line arguments, for example : DOS Command Effect / Description dir /sList all files in the current directory sorted in ascending order by their file name. copy *.* a:\Copy all files in current directory to the root directory on the A: drive. del *.bakDelete all files in current dir with a ".bak" file extension.

6. Windows Examples : ProgramEffect / Description win /s Starts Windows 95 or 98 in "Safe Mode" notepad mike.txt Runs the Windows notepad and automatically loads the file "Mike.txt" Screen Savers All Windows screen savers go into Preview, Setup, or Run mode according to a command line parameter passed to them. Help files Windows Help files can be made context sensitive (i.e. jump straight to the required help item) via a command line parameter. Virus Checkers Most popular virus checkers almost always have command line parameters, so that you can specify which files and directories to scan when Windows starts up.

7. Command Line Arguments - General Syntax To accept command line parameters, we need to change the main so that it accepts special parameters. For a program to accept command line parameters, the following syntax is required in the main function : void main (int argc, char* argv[ ]) { // … body of main function … } where : –argc is the number of command line arguments. –argv[ ] is an array of character strings which are the actual arguments passed to the program. –argv[0] has a special value. It is the name of the program that is being executed. e.g. my_prog.

8. Sample Program #include // For cin, cout, etc. int main (int argc, char* argv[]) { cout << "\nThere are " << argc << " strings on the command line:\n"; for (int i = 0; i < argc; i++) { cout << "- argv[" << i << "] contains: " << argv[i] << endl; } return 0; }

9. Sample Program – Run #1 Now, if we compile this project, we can then run the program from the DOS prompt (assuming we are in the same drive and directory as our program) with the command line : my_prog Because we have run the program, and supplied no command line parameters, then the output of the program would be : There are 1 strings on the command line: - argv[0] contains: c:\cpp\my_prog.exe

10. Sample Program – Run #2 However, if we run the program from the DOS prompt and pass 3 parameters to the program with the command line : my_prog one two three Then the program's output would now be : There are 4 strings on the command line: - argv[0] contains: c:\cpp\my_prog.exe - argv[1] contains: one - argv[1] contains: two - argv[1] contains: three

11. Command Line Arguments and Numeric Data / Values All command line values are passed to your main program as strings, and you need to specially convert any numeric ones to float or int. Built in functions in stdlib.h are useful for this (see week 3), such as : –atoi string to int conversion –atof string to float conversion –atol string to long conversion –strtod string to double conversion –etc

12. Passing Functions as Arguments to Other Functions Functions can be passed to other functions ! When a function is passed as a parameter in a function call only the function name is given.

13. double sum_square (double SomeFunction(double a), int m, int n) { double sum = 0.0; for (int k = m; k < n; k++) sum += SomeFunction (k) * SomeFunction (k); return sum; } double function1 (double n) { return 1.0 / n; } double function2 (double n) { return 1.0 / (n * n); }

14. Passing Functions as Arguments to Other Functions (cont …) void main() { // Tell sum_square to invoke function1 cout << sum_square (function1, 1, 100) << endl; // Tell sum_square to invoke function2 cout << sum_square (function2, 1, 100) << endl; } N.B. When a function is passed as a parameter in a function call only the function name is given – none of the function parameters are supplied.

15. C++ File Operations A file is a sequential / serial data structure usually consisting of data components of the same type. Files permanently store data. e.g. on magnetic disk. Files could be thought of as a permanent one- dimensional array, with no set / specific dimension. Usually file access is sequential, but C++ also provides direct / random access mechanism, which can be used to jump to particular locations in files. Files can be plain ASCII text (such as files created with Windows Notepad) or binary. e.g. executable machine code.

16. Streams or Data Channels Just as there are input / output data streams for keyboard entry and screen output, there is also a C++ file stream for input / output to devices such as disks, printers, etc. Input / Output (I/O) for keyboard and screen is provide by the iostream class and its derived classes ostream and istream. File / peripheral device I/O is provided the fstream (Input and Output), ifstream (Input / Read), and ofstream (Output / Write) classes.

17. Some File Member Functions FunctionDescription. eof()Checks for end of file. If you are not at end-of-file, then this function returns zero. fail()Checks for file operation fail. e.g. Open failure. If some error or failure occurs, then this function returns a non-zero value. close() Close file getline() Gets a line of text with spaces. get() Get a single character. cin / cout Your normal cin / cout member and formatting functions can also be used, but the same limitations apply to files as with keyboard input. open() Opens a file.

18. Steps for Handling Files 1.Define a C++ file object(s) that may be used for input, output or both. 2.Open the disk file(s) in the mode required (Read, Write, or both). 3.Read / Write data To / From File as required. 4.Close file(s) objects.

19. Steps for Handling Files (cont…) Just with any form of input, programs that process files also need to check for errors. With file processing, the main areas that need to be checked for errors are when : –opening the file. e.g. files doesn’t exist, disk error, file locked by another user, etc. –reading from or writing to a file. e.g. out of disk space, file corrupted / truncated, disk errors, invalid data, etc.

20. Defining a C++ File Object Before we can begin working with files we need to include the fstream header in our project, as follows : #include // for file handling Now, within our program, we can open files for input, output, or both, as follows : ifstream Input;// File object to be read (input). ofstream Output;// File object to be written (output). fstream InputOutput;// Both read & write file object

21. Defining a C++ File Object (cont …) In these examples, the file object names used are used to indicate our purpose for the file. As with all C++ variable and function names, meaningful names should be used wherever possible. For example, if we were reading an input file of student results, then we might declare the following file object : ifstream Stud_Results_Input_File;

22. File Opening Modes The general syntax for opening a file is :.open (,, ); The file open mode and file protection mode may be optional parameters. Example : ifstream Input;// File object to be read (input). Input.open ("a:\sample.dat"); // Default is read Here we have declared the file object to be of type ifstream, which can only be used for input, so there is no need to specify any file open mode.

23. File Opening Modes (cont …) Another example : ofstream Output;// File to be written (output). Output.open ("C:\ass1\ass1.txt"); // Default is write Here, we have declared the file object to be of type ofstream, which can only be used for output, so once again there is no need to specify any file open mode.

24. File Opening Modes (cont …) However, if we declare the file object to be of type fstream, then we must tell C++ how whether we want to read (input) the file, write (output) the file, or both, or append more data onto the end of the file. fstream InputOutput;// Declare file object Now, we can open the file for reading (input) : InputOutput ("myfile.dat", ios::in); or, open the file for writing (output) : InputOutput ("myfile.dat", ios:: out); or, open the file to append data (write) : InputOutput ("myfile.dat", ios::app); // Append data (write) or, open the file for both read and write : InputOutput ("myfile.dat", ios::in | ios:: out);

25. Checking for Open Failure There are two main methods : Input.open ("a:\sample.dat"); if (Input == 0) // Check Input = 0 or Null on fail { cerr << "File Open Fail!"; exit(1); } or Input.open ("a:\sample.dat"); if (Input.fail() != 0) // Check fail member function { cerr << "File Open Fail!"; exit(1); } Use whichever of method you like best.

26. Instead of : if (Input == 0) // Check Input = 0 or Null on fail you can also use the slightly more obscure method that many C++ programmers use : if (!Input) // Check Input = 0 or Null on fail

27. Reading and Writing a File const int MAX = 81; void main() { char Line [MAX]; ifstream Input; // Input file object ofstream Output; // **** Write data to the file **** Output.open ("c:\sample.txt"); // Open for default read as ASCII file if (Output == 0)// Check if open failed { cerr << "File Open Failed !"; exit(1); }

28. Reading and Writing a File (cont …) strcpy (Line, "Line 1 : This will be the first line in the file"); Output << Line << endl; strcpy (Line, "Line 2 : This will be the second line in the file"); Output << Line << endl; strcpy (Line, "Line 3 : And we could keep adding lines"); Output << Line << endl; strcpy (Line, "Line 4 : for as long as we want."); Output << Line << endl; Output.close(); // Close the input file // **** Read in and Display the file **** Input.open ("c:\sample.txt"); // Open for default read as ASCII file

29. Reading and Writing a File (cont …) if (Input.fail() != 0)// Check if open failed { cerr << "File Open Failed !"; exit(1); } while (Input.eof() == 0)// Repeat until end of file { Input.getline (Line, MAX); // Read a line from the file cout << Line << endl;// Display the line to the screen. } Input.close(); // Close the input file }

30. Assignment 2 – Start Work Now ! At this point, we have now covered enough material for you to complete Assignment 2. Nothing else we do this term is relevant to Assignment 2. Follow the Development Hints and Tips. Think about things carefully as you go. If you are finding it difficult, stop work, and do some reading and tutorial questions.

31. Random File Access Functions : So far, we have looked at sequential processing of files. However, as indicated earlier, C++ also provides direct / random access mechanism, which can be used to jump to particular locations in files.

32. Some Random File Access Functions : FunctionClass (include file) Purpose tellg ()istreamReturns current position in input file tellp ()ostreamReturns current position in output file seekg ()istreamSets position in input file seekp ()ostreamSets position in output file

33. Example Random File Access : To demonstrate some of these functions, we will now look at an example which : –prompts you to enter the name of a file to process –prompts you to enter a line of text in this file to change –searches for and replaces this line of text with a string of *'s –prompts you for the replacement line of text –adds the new line of text to the end of the file

34. Example Random File Access (cont …) For example, if the input file contained the following : –abcdefg –QWERTY –Mike –Hi There and we wanted to change the line "Mike" to "Zebra", then we would end up with a file that looked like : –abcdefg –QWERTY –**** –Hi There –Zebra

35. Example Random File Access (cont …) const int SIZE = 81;// MAX LINE SIZE void main() { char OldLine [SIZE] = {'\0'}; // Current Line from File. char NewLine [SIZE] = {'\0'}; // The New Line to add to the file. char Entry [SIZE] = {'\0'}; // What line the User wants to change. char FileName [SIZE]= {'\0'}; // File to Open for I/O. long WindowPos = -2; // Current Window Position int Match_Found = 0; // Flag to indicate if desired line was found. cout << "Enter name of file to open : "; cin >> FileName;// Ask user for the File to Process. fstream IO_File;// Declare an I/O file object.

36. Example Random File Access (cont …) IO_File.open (FileName, ios :: in | ios :: out); // Open for Input and Output if (IO_File.fail() != 0)// Check for failure... { cerr << "\nThis file could NOT be opened or created."; exit(1); } // Ask the user to enter which line in the file they wish to change. cout << "\nWhich line do you wish to change ? : " << endl; cin >> Entry; IO_File >> OldLine;// Read a line from the file.

37. Example Random File Access (cont …) // Read all lines in the file and look for a match. while ((IO_File.eof() == 0) && (Match_Found == 0)) { cout << OldLine << endl; // See if the current line matches the one the user wants to change. if (strcmp (OldLine, Entry) == 0) { Match_Found = 1;// We found a match ! // Move to start of the matching line in the file. IO_File.seekg(WindowPos + 2, ios :: beg); for (int i = 1; i <= strlen(OldLine); ++i) IO_File << '*'; // Replace Old Line in file with *'s. // And add a CR/LF. IO_File << endl;

38. Example Random File Access (cont …) // Get the replacement line from the User. cout << "Line found ! What do you wish to change it to ? " << endl; cin >> NewLine; // Move to the end of the file. IO_File.seekg(0,ios :: end); // Add this new line to the end of the file IO_File << NewLine << endl; } else { // We haven't found a match yet - so read another line. // Save the Current Windows Position. WindowPos = IO_File.tellg(); IO_File >> OldLine;// Read a line from the file. } }// END WHILE

39. Example Random File Access (cont …) if (Match_Found == 0) cout << "\nLine not found." << endl; else cout << "\nLine now changed to *'s, and new line appended to bottom of file !" << endl; // Close the I/O File. IO_File.close(); }

40. Passing File References to Functions We will now look at an example which passes a file object to a function for further processing. N.B. This is a pretty silly example - normally a function would be in charge of opening, processing and closing the file. But, this example serves to illustrate the method of passing File References to functions.

41. Passing File References to Functions (cont …) const int SIZE = 81;// MAX File Name size void Display_Open_File_To_Screen (ifstream &File_In); void main() { char FileName [SIZE]= {'\0'}; // File to Open for I/O. ifstream Input; // Read file object cout << "Enter name of file to display : "; cin >> FileName; Input.open (FileName); // Open for default read as ASCII file if (Input.fail() != 0)// Check fail member function { cerr << "File Open Fail !"; exit(1); } Display_Open_File_To_Screen (Input); }

42. Passing File References to Functions (cont …) void Display_Open_File_To_Screen (ifstream &File_In) // Function to read a file that has already been opened for INPUT // and display it line by line to the screen, and then close it. { char Line[81]; while (!File_In.eof()) // Check for end of file { File_In.getline (Line,81,'\n'); // Read file line cout << Line << endl; // Display } File_In.close(); // Close file }