Can't identify what is wrong with my code.

[prhray]

I wrote this code to check whether the user-entered integer is prime. If not, the program is to display the integer's factorization as a product of primes in ascending order.

For example, if you enter 17, the program should print
"17 is a prime number." ;
if you enter 24, the program should print
"24 is not a prime number.
24=2x2x2x3"

Right now, if you enter a prime number, my program will correctly identify it as such. If you enter a non-prime number, the program will correctly identify it as non-prime, but will not print the correct prime factorization.
e.g., if you enter 25, the program will print:
"25 is not a prime number.
25=5x7x0x0x0x0x1"

if you enter 24, you'll get this:
"24 is not a prime number.
24=2x5x0x2x3"

I can't identify what is wrong with my code. I think the way I worked the array is wrong. But I have no clue. Please help.

#include <stdio.h>
#include <math.h>
int countprime, primefactorstorage[];
void getprimefactors(int);

void getprimefactors (int x) /*x will be replaced with the value entered by the 
                              user (n)*/
{
    countprime=0; /*the number of prime factors found thus far is 0*/

    while(x%2==0) /*check if x is divisible by 2, the smallest prime number and 
                    the only even prime number*/
    {
        primefactorstorage[countprime]=2; /*store 2 in primefactorstorage*/
        countprime++;
        x=x/2;           /*reduce n further by 2 so in the end n will have no 
                           divisor that is a multiple of 2*/
    }

    /*At this point, x is no longer even, so we start dividing x with odd 
      numbers*/
    int i=3;
    for (i=3; i<=sqrt(x); i=i+2) /*use i+2 because only want to sample odd 
                                   numbers to divide x stop until i>sqart(x) 
                                   because non-prime numbers have no prime 
                                   factor greater than its square root*/
    {
        while(x%i==0)      /*the loop continues as long as x is divisible by i*/
        {
            primefactorstorage[countprime]=i; /*store i in primefoactorstorage*/
            countprime++;
            x=x/i;         /*devide n further by i so in the end n will have no 
                             divisor that is a multiple of i*/
        }
    }

    /*now what remains as x if it's greater than 2 is an odd prime factor*/
    if (x>2);
    {
        primefactorstorage[countprime]=x;
        countprime++;
    }
    return;
}

int main()
{   int n, j;
    printf("Enter a positive integer that is greater than 1\n");
    scanf("%d", &n);
    if(n==2)
        printf("2 is a prime number.");
    else
    {
        getprimefactors(n);
        if (countprime==1)
            printf("%d is a prime number.\n", n);
        else
        {
            printf("%d is not a prime number.\n %d=", n, n);
            for (j=0; j<countprime-1; j++)
                printf("%dx", primefactorstorage[j]);
            printf("%d", primefactorstorage[countprime-1]);
        }

    }
    return 0;
}

[Peter87]

You have not specified the size of primefactorstorage which you have to do if you want the program to compile.

After fixing the problem, it seems to work.

25 is not a prime number.
 25=5x5x1

24 is not a prime number.
 24=2x2x2x3

[prhray]

But how did you get the size of the array before getting the number of prime factors to be store in there?

[Peter87]

I used a fixed size of 500 when testing the program.

It's possible to use a fixed size for the array but then you will have a limit on how many prime factors you can store. It can also waste a lot of memory if you pick a large size. Maybe you can calculate some upper limit on how big size is needed and dynamically allocate the array of that size.

If you used C++ I would have recommended using something like a vector because it's very easy to resize. In C you could do something similar by dynamically allocating the array, and if the array is too small you allocate a new array that is bigger and then copy the elements over to the new array.

http://en.wikipedia.org/wiki/Dynamic_array

Using a linked list, or some other data structure is also possible.

http://en.wikipedia.org/wiki/Linked_list

C doesn't have this functionality "built-in" so you would have to implement it yourself (or use a library).

It is also possible to write the program without using arrays. You could have a function similar to what you have but instead of adding the element to the array it could print the number to the screen.

[prhray]

Thanks for the help.

After I fix the problem with the size of the array, the program works now if the user enters an even non-prime number, but if you enter an odd non-prime number, the program will print its prime factors and a "1" which shouldn't be there.

For example: you enter 25, you get 25=5x5x1
you enter 27, you get 27=3x3x3x1

I wrote the code as such that only 2 and values of i which are 3 or greater are stored in the array primefactorstorage , and the program should print the values stored in the array. How exactly does 1 ended up there? I'm baffled.

[Peter87]

You have a semicolon at the end of line 38.

[prhray]

Thanks. Problem solved.