Getting Factors of a Number

Question

I\'m trying to refactor this algorithm to make it faster. What would be the first refactoring here for speed?

public int GetHowManyFactors(int numberToCheck         


        

Answer 1

Well if you are going to use this function a lot you can use modified algorithm of Eratosthenes http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes and store answars for a interval 1 to Max in array. It will run IntializeArray() once and after it will return answers in 0(1).

const int Max =1000000;
int arr [] = new int [Max+1];

public void InitializeArray()
{
    for(int i=1;i<=Max;++i)
        arr[i]=1;//1 is factor for everyone

    for(int i=2;i<=Max;++i)
        for(int j=i;i<=Max;i+=j)
           ++arr[j];
}
public int GetHowManyFactors(int numberToCheck)
{
   return arr[numberToCheck];
}

But if you are not going to use this function a lot I think best solution is to check unitll square root.

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Note: I have corrected my code!