Command-Line Processing

File-processing programs often use command-line arguments to identify files. Command-line arguments are arguments that appear on the command line when you type a command. For example, to count the number of words in some files on a Unix or Linux system, you would type this command at the command-line prompt:

wc report1 report2 report3

Here wc is the program name, and report1, report2, and report3 are filenames passed to the program as command-line arguments.

C++ has a mechanism for letting a program running in a command-line environment access the command-line arguments. You can use the following alternative function heading for main():

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

The argc argument represents the number of arguments on the command line. The count includes the command name itself. The argv variable is a pointer to a pointer to a char. This sounds a bit abstract, but you can treat argv as if it were an array of pointers to the command-line arguments, with argv[0] being a pointer to the first character of a string holding the command name, argv[1] being a pointer to the first character of a string holding the first command-line argument, and so on. That is, argv[0] is the first string from the command line, and so on. For example, suppose you have the following command line:

wc report1 report2 report3

In this case, argc would be 4, argv[0] would be wc, argv[1] would be report1, and so on. The following loop would print each command-line argument on a separate line:

for (int i = 1; i < argc; i++)
       cout << argv[i] << endl;

Starting with i = 1 just prints the command-line arguments; starting with i = 0 would print the command name as well.

Command-line arguments, of course, go hand-in-hand with command-line operating systems such as the Windows command prompt mode, Unix, and Linux. Other setups may still allow you to use command-line arguments:

• Many Windows IDEs (Integrated Development Environments) have an option for providing command-line arguments. Typically, you have to navigate through a series of menu choices that lead to a box into which you can type the command-line arguments. The exact set of steps varies from vendor to vendor and from upgrade to upgrade, so check your documentation.

• Many Windows IDEs can produce executable files that run under in the Windows command prompt mode.

Listing 17.17 combines the command-line technique with file stream techniques to count characters in files listed on the command line.

Listing 17.17. count.cpp

// count.cpp -- counting characters in a list of files
#include <iostream>
#include <fstream>
#include <cstdlib>   // for exit()
int main(int argc, char * argv[])
{
    using namespace std;
    if (argc == 1)          // quit if no arguments
    {
        cerr << "Usage: " << argv[0] << " filename[s] ";
        exit(EXIT_FAILURE);
    }

    ifstream fin;              // open stream
    long count;
    long total = 0;
    char ch;

    for (int file = 1; file < argc; file++)
    {
        fin.open(argv[file]);  // connect stream to argv[file]
        if (!fin.is_open())
        {
            cerr << "Could not open " << argv[file] << endl;
            fin.clear();
            continue;
        }
        count = 0;
        while (fin.get(ch))
            count++;
        cout << count << " characters in " << argv[file] << endl;
        total += count;
        fin.clear();           // needed for some implementations
        fin.close();           // disconnect file
    }
    cout << total << " characters in all files ";

    return 0;
}

On a Linux system, for example, you could compile Listing 17.17 to an executable file called a.out. Then sample runs could look like this:

$ a.out
Usage: a.out filename[s]
$ a.out paris rome
3580 characters in paris
4886 characters in rome
8466 characters in all files
$

Note that the program uses cerr for the error message. A minor point is that the message uses argv[0] instead of a.out:

cerr << "Usage: " << argv[0] << " filename[s] ";

This way, if you change the name of the executable file, the program will automatically use the new name.

The program uses the is_open() method to verify that it was able to open the requested file. Let’s examine that matter further.