Misc. C++ Programming Information

The following C++ programming details will helpful in completing the initial homeworks for the course.

Command Line Arguments

Most of the programs we write will expect additional information from the user when the program is launched. We will do this by providing this data on the command line:

  my_program.exe blue 3.14159 my_input_data_file.txt my_output_data_file.txt

In order for your program to receive arguments from the command line, you will make use of the optional arguments to the main function. Here is the prototype:

  int main(int argc, char* argv[])

The parameter argc contains the number of strings on the command line, including the executable name. For the example above, argc = 5. The array argv stores those C-style strings. You can access the executable name with argv[0] and the arguments with argv[1], argv[2], etc.

Important Note: To compare command line argument strings using the == operator, you must cast this data to a STL C++-style string:

  if (std::string(argv[1]) == std::string("blue")) {
     std::cout << "my favorite color is blue too!" << std::endl;
  }

Command Line Arguments in Visual Studio

You can also specify command line arguments when running the program inside of the Visual Studio development environment. Go to "Project" → "Properties" → "Configuration Properties" → "Debugging" and in "Command arguments" enter:

  blue 3.14159 my_input_data_file.txt my_output_data_file.txt

Note, that the default directory for files is the working directory.

Converting a C/C++ String to an Integer or Floating Point Value

This is done using the atoi (char-to-integer) and atof (char-to-float) functions. First, be sure to #include <cstdlib>. Then, to convert a C++-style STL std::string variable to an int:

  std::string my_stl_string = "17";
  int x = atoi(my_stl_string.c_str());

Similarly, to convert C-style char array (C-style string) to a float:

  char* my_char_string = argv[2];
  float y = atof(my_char_string);

Reading From & Writing To Files

The STL streams std::cin & std::cout are used to read data from and write data to the "console". Often, we would rather read data from a file and/or write the output to a file. We can do this using the STL file stream library:

  #include <fstream>

Here's an example fragment of code that attempts to open an input file stream for a file name specified on the command line above:

  std::ifstream in_str(argv[3]);

It is good coding practice to verify that the input stream was successfully opened:

  if (!in_str) {
    std::cerr << "Can't open " << argv[3] << " to read.\n";
    exit(1);
  }

Likewise here's how to open a stream for output:

  std::ofstream out_str(argv[4]);
  if (!out_str) {
    std::cerr << "Can't open " << argv[4] << " to write.\n";
    exit(1);
  }

Once the streams are created, you can use in_str & out_str just like you use std::cin & std::cout.

In general, for this course, we encourage you to use the stream operator>> for all input parsing, rather than using getline, eof, getc, etc. We will not deduct points for using the other methods of parsing, but we have designed the assignment input format specifications for easy parsing with >>.

Note that the following code has a bug. If the input file ends with one or more extra newlines, the inner loop will process the last successfully read element twice. It is important to check the return value of each >> expression to be sure the read was successful.

  while (!in_str.eof()) {
    in_str >> my_variable;
    // do something with my_variable
  }

A simpler and more robust way to write the same code is:

  while (in_str >> my_variable) {
    // do something with my_variable
  }

Comparing Two Text Files

To check the correctness of your program, you can compare your output text file to the provided sample output text file using the UNIX utility diff (available on Cygwin & MacOSX):

  diff my_output.txt sample_output.txt

Any lines in the two files that are not identical (including whitespace), will be printed to the console. WinDiff is another option for Windows users. Please see a TA or the instructor in office hours if you have a question about these programs.

Redirecting Input & Output

What if you have an interactive program that uses std::cin & std::cout to read from and write to the "console", but you'd like to take the input from a file and you'd rather not rewrite the program to use the input & output streams described above? This following trick is handy for repeated testing and debugging an interactive program (you'd rather not have to manually type in the same input test data many times). Asking the executable to read from a file instead of the console and/or write to a file instead of the console is called I/O redirection.

• First, create the input.txt file that contains input which you would otherwise type at the console during program's execution.

• Then on the Cygwin/Linux/FreeBSD/UNIX command prompt simply type:

     program.exe < input.txt > output.txt
  

• When the program has finished, look in the newly created file output.txt.

You can do the same thing in Visual Studio. (See section on "Command Line Arguments in Visual Studio" above).

Warning: There is a I/O re-direction bug in some versions of Visual Studio .NET. The program does not change to the working directory before command line I/O takes place (< input.txt > output.txt). One workaround is to specify the full path of these files on your computer, e.g., c:\Documents and Settings\username\My Documents\ds\hw\hw1\input.txt. However, you can't have spaces in your file name. So you should put the files in a place with no spaces, like c:\input.txt.

< c:\input.txt > c:\output.txt