recursive func to find prime factors

Question

i made a recursive function to find the prime factors of a number but it has a bug which makes turbo c quit. please help

#include<stdio.h>
#include<conio.h>
int prime(int num);
int primefactor(int num,int i);
void main(void)
{
    int num;
    printf("Enter a number whose prime factors are to be calculated:");
    scanf("%d",&num);
    primefactor(num,i);
     i=num 
    getch();
}
int primefactor(int num,int i)
{
    if(i==2)
    return 1;
    if(num%i==0)
    {
        if(prime(num))
        {
            printf(",%d",num);
            num=num/i;
            i++;
        }


    }
    i--;
    primefactor(num,i);
    return 0;
}
int prime(int num)
{
    int i,flag;
    for(i=2;i<num;i++)
    {
        if(num%i==0)
    flag=0;
    }

return flag; }

Answer 1

void main(void)
{
    int num,i=num; // (*)
    printf("Enter a number whose prime factors are to be calculated:");
    scanf("%d",&num);
    primefactor(num,i);
    getch();
}

What value do you think i will have in (*)?

Not sure what you want i to start out as, but I'm pretty sure you don't want it to be something random. If you want it to start with the value of num, you need to assign num to it after you read it:

void main(void)
{
    int num,i; 
    printf("Enter a number whose prime factors are to be calculated:");
    scanf("%d",&num);
    i = num; // assignment goes here.
    primefactor(num,i);
    getch();
}

Answer 2

Agree with IVlad - also, what happens in the case when num is prime? How many times will the recursive function be called for e.g. num = 7?

Answer 3

(little too sleepy to write good code.. so am sorry in advance for any bugs :p )

a simpler non recursive version

printPrimeFactors(int num) {

  for (i = 2; i < sqrt(num); i=getNextPrime()) {
     if (num %i)
        printf("%d", i);
  } 

}

if you have to use recursion

printPrimeFactors(int num) {

 if(isPrime(num)) {
    printf ("%d ", num);
 } else {
    for(i=2; i < sqrt(num); i++) {
        if(num%i ==0) {
              printPrimeFactors(i);
              printPrimeFactors(num/i);   
        }
    }
 }

}