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To: cs551@merlot.usc.edu
Subject: Parsing commandline arguments...
Date: Wed, 17 Sep 2008 14:07:39 -0700
From: Bill Cheng <william@bourbon.usc.edu>

Hi everyone,

If you are having trouble parsing commandline arguments,
especially with the optional ones, here is some sample code
on how to deal with them.

Let's take the encrypt command as an example:

    mm2 -lambda 0.25 -mu 0.5 -n 8 -d exp

Argc will be 9 and argv will be:

    argv[0] = "/...something.../mm2"
    argv[1] = "-lambda"
    argv[2] = "0.25"
    argv[3] = "-mu"
    argv[4] = "0.5"
    argv[7] = "-n"
    argv[8] = "8"
    argv[9] = "-d"
    argv[10] = "exp"

Let's say that you pass argc and argv to ProcessOptions().
Here's what ProcessOptions() can look like:

    int ProcessOptiona(int argc, char *argv[])
    {
        /* initialize variables according to the spec */
        lambda = (double)0.5;
        mu = (double)0.5;
        n = 20;
        distr = DISTR_EXP;

        /* initially, *argv is argv[0] */
        for (argc--, argv++; argc > 0; argc--, argv++) {
            /* a commandline argument can either begin with '-' or not */
            if (*argv == '-') {
                /* a commandline argument that begins with '-' */
                if (strcmp(*argv, "-lambda") == 0) {
                    argc--; argv++;
                    if (argc == 0) {
                        Usage("Lambda value missing.");
                        return (-1);
                    }
                    lambda = strtod(*argv, NULL);
                } else if (strcmp(*argv, "-mu") == 0) {
                    argc--; argv++;
                    if (argc == 0) {
                        Usage("Mu value missing.");
                        return (-1);
                    }
                    mu = strtod(*argv, NULL);
                } else if (strcmp(*argv, "-n") == 0) {
                    argc--; argv++;
                    if (argc == 0) {
                        Usage("N value missing.");
                        return (-1);
                    }
                    n = atoi(*argv, NULL);
                } else if (strcmp(*argv, "-d") == 0) {
                    argc--; argv++;
                    if (argc == 0) {
                        Usage("Distribution specification missing.");
                        return (-1);
                    }
                    if (strcmp(*argv, "exp") == 0) {
                        distr = DISTR_EXP;
                    } else if (strcmp(*argv, "det") == 0) {
                        distr = DISTR_DET;
                    } else {
                        fprintf(stderr, "Invalid distribution: %s\n", *argv);
                        Usage(NULL);
                        return (-1);
                    }
                } else {
                    fprintf(stderr, "Invalid commandline option: %s\n", *argv);
                    Usage("");
                    return (-1);
                }
            } else {
                Usage("Malformed command.");
                return (-1);
            }
        }
        return 0; /* 0 means okay */
    }

This code is not complete since it's not doing error checking.
But you should get the idea of how to do it.  Please try to
understand the above code and *adapt* it for your use for all
the assignments!

If you have questions, please feel free to ask me!
--
Bill Cheng // bill.cheng@usc.edu <URL:http://merlot.usc.edu/william/usc/>