Largest prime factor and number of prime factors for a number

**Jas11** · 03-29-2005

Hi guys... i need some help getting the following code to display how many prime factors there are for a number (in this case the number is represented by a) and also the largest prime factor of that same number. At the moment, it displays all of the prime factors. Any help would be great.

Code:

    for(p1 = 1; p1 <= a; p1++)
     {
           for(p2 = (a / 2); p2 > 0; p2--) 
            {
                  if (p1 * p2 == a)
                  {
                         cout << p2 << endl;
                            }
                    }
                    }

**The Brain** · 03-29-2005

Here is a little something to get you started. You should be able to easily edit this function that will return a list of all prime factors for a given number.

Once you have a list of all prime factors, it should be easy to get the total number of factors.. and then use a sorting algorithm to determine the largest prime factor.

**MadCow257** · 03-29-2005

Code:

#include <iostream>

using namespace std;

int main()
{
	int a = 100, numfactors = 0;
	for (int p1 = 1; p1 <= a; p1++)
	{
		for (int p2 = (a / 2); p2 > 0; p2--) 
		{
			if (p1 * p2 == a)
			{
				numfactors++;
				if (numfactors == 1)
					cout << "The greatest factor of " << a << " is " << p2;
			}
		}
	}
	cout << "\n" << a << " Has " << numfactors << " factors.\n";
	return 0;
}

BTW...
you should use a different algorithm for finding factors IMO

**Jas11** · 03-29-2005

Thanks alot guys... that has been a huge help

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Largest prime factor and number of prime factors for a number