A c++ app for converting CM to inches. Helping a friend out by getting it partially working for his homework. :)

cmtoinches.cpp

#include <iostream>

using namespace std;

int main( int argc, char *argv[] ) {
    string firstArg = "";
    char * secondArg;


    // Warning if no arguments are found
    if ( argc == 0 ) {
        cout << "Warning! You didn't enter any arguments.\n";
        return 0; // Get out
    }


    // No need for a for loop since you know you're only accepting two arguments.
    // Just check if we have the first argument.
    if ( argv[1] != NULL ) {
        string firstArg = argv[1];

        // I would give the warning about the valid arguments here before you do anything else
        if ( firstArg != "help" && firstArg != "i" && firstArg != "c") {
            cout << "Warning! You didn't use a valid argument. Please use 'help', 'i', or 'c' as the first argument.\n";
            return 0; // Get out
        }


        // Show the help message and then exit because the rest of the program isn't necessary
        // if the user entered this.
        if ( firstArg == "help" ) {
            cout << "Usage: Converting in to cm or cm to in\n";
            cout << "i for in to cm conversion\n";
            cout << "c for cm to in conversion\n";
            cout << "Example: `i 10` or `c 20`\n";
            return 0;
        }
    }


    // Now check for the second argument
    if ( argv[2] != NULL ) {
        secondArg = argv[2];

        // @todo Now you can modify everything below and remove the user input here. We no longer need it since we're getting the unit and value directly from the command line (`i 10`). Use second argument (argv[2]) to calculate the value below instead. I left some pseudo code below to outline what you may do:

        // If the first argument was i, use the second argument and calculate inches to centemeters
        // If the first argument was c, use the second argument and calculate centimeters to inches
        // Return the value

        const double cm2inch = 2.54;
        int val;
        char unit;

        cout << "Please type in a unit and its value.\n";
        cout << "Type in 'help' for man page.\n";

        while (cin >> val >> unit) {
             if (unit == 'i')
                 cout << val << "in == " << val * cm2inch << "cm\n";
             else if (unit == 'c')
                 cout << val << "cm == " << val / cm2inch << "in\n";
             else
                 return 0;
        }
    }
    else {
        // Warn them if they didn't enter a second argument
        cout << "Warning! You didn't enter a numeric value to convert.\n";
        return 0; // Get out
    }
}