OOP344's Basic Math

This is my solution to OOP344's "basicMath" problem. User is suppose to call the program from 
command line with a simple mathematical expression (i.e  bm  a + b). 
**Fixed some errors regarding validation**


/*
   Program: basicMath.cpp
   Date: September 25, 2013
   Author: Benson Wong
   Purpose: To practice coding for command line arguments
*/

#include <iostream>
#include <cstdlib>
#include <cstring>
using namespace std;
bool NaN(char* x);

int main(int argc, char** argv)
{
    double a;
    double b;
    a = b = 0;
    bool error = false;

    if (argc == 4) {
         if (NaN(argv[1]) || NaN(argv[3])) {
             error = true;
         } else {
             a = atof(argv[1]);
             b = atof(argv[3]);
             if (!strcmp(argv[2], "+")) {
                  cout << a << " + " << b << " = " << a + b << endl;
             } else if (!strcmp(argv[2], "-")) {
                  cout << a << " - " << b << " = " << a - b << endl;
             } else if (!strcmp(argv[2], "x")) {
                  cout << a << " x " << b << " = " << a * b << endl;
             } else if (!strcmp(argv[2], "/") && argv[3] != 0) {
                  cout << a << " / " << b << " = " << a / b << endl;
             } else
                  error = true;
         }
    } else
         error = true;

    if (error) {
         cout << "bm <number> <+-x/> <number><ENTER>" << endl;
    }

    return 0;
}

bool NaN(char *x)
{
    int i;
    int d;
    bool ec = false;
    i = d = 0;

    if (x[0] == '-') {
         i++;
    }
    for (; x[i] != '\0' && ec == false; i++) {
         if (x[i] == '.') {
             d++;
             if (d > 1) {
                  ec = true;
             }
         } else if (!isdigit(x[i])) {
             ec = true;
         }
    }

    return ec;
}