Advice please--working but not working program.

[moriarty]

The program compiles and runs, gives the correct answers to the sample data to be inputted. So, it appears as if I am done. However, upon entering a really large non palindrome it comes back as true. (Super large palindromes also come back as true). I guess maybe I could fix this in the function by running a comparison after the main part of the math is done to verify the first and last digits are the same, or even more if I wanted to which would probably get rid of *most false positives. But there is most likely a more efficient way, like mathematically fixing the program. I would also like to add a loop that can keep asking for input rather than restarting program each time I want to test a number, but every where I put it came back with some sort of error, or out of place to work.

Thanks very much,
M

#include <cstdlib>
#include <iostream>
#include <cmath>
     
using namespace std;
bool isNumPalindrome (int num); /* function prototype */
     
    int main()
    {
        bool isNum = false;
        int integer = 0;
        
        
 
              cout << "Which number do you want to see whether it is a palindrome?" 
                 << endl;
             cin >> integer;
     
    
        
     
        isNum = isNumPalindrome (integer);
        
        
     
        if (isNum == true)
     
             cout << "Your number is a palindrome"; /*next two statement display results */
        else            
             cout << "Your number is not a palindrome";
     
             cout << endl;
        
     system("pause"); /* used for editor/compiler purposes to see output */
        
        
       
        
     
    }
     
     
        bool isNumPalindrome(int num) /* function that determines if palindrome:*/
        //This function was prewritten from the text is not program authors property
    {
        double pwr = 0.0;
         
     
        if(num < 5)
            return true;
        else
        {
            while (num / static_cast<double>(pow(10.0, pwr)) >= 10)
            pwr++;
     
            while (num >= 10)
            {
                int tenTopwr = static_cast<int>(pow(10.0, pwr));
     
                if ((num / tenTopwr) != (num % 10))
                    return false;
                else
                {
                    num = num % tenTopwr;
                    num = num / 10;
                    pwr = pwr - 2;
                 
                }
            }
                return true;
        }
    }

[hamsterman]

What is a really large number? You have to realize, that large numbers need a lot of space, and int only goes up to 2³¹-1, unsigned int would allow up to 2³²-1, unsigned long long - 2⁶⁴-1. If you feel that you need more (though you probably don't), you should take a look at http://gmplib.org/ . Another solution would be simply to put user input into a string rather than an integer. You only need the digits anyway..

[keepjumpin]

your really large number is probably too big for an int. try using an unsigned long. Here's a reference page: http://www.cplusplus.com/doc/tutorial/variables/

[keepjumpin]

lol. I guess I should have refreshed the page before posting. Also, here is a slightly different version:

#include<iostream>

using namespace std;

bool IsPalindrome(unsigned long);

int main()
{
    int num;
    
    cout << "Enter a number to check for palindromicity: ";
    cin >> num;
    while (num >= 0)
    {
        if (IsPalindrome(num))
            cout << "The number is a palindrome.\n";
        else
            cout << "The number is not a palindrome.\n";
        cout << "Enter a number to check for palindromicity: ";
        cin >> num;
    }
}

bool IsPalindrome(unsigned long original)
{
  unsigned long reversed = 0;
  unsigned long savedOrig = original;

  while (savedOrig > 0)
  {
    reversed = reversed * 10 + savedOrig % 10;
    savedOrig /= 10;
  }

  return original == reversed;
}