C - Print list of Prime Numbers (recursion) - c

Question

I'm having some problems with this C programming assignment that I have in school. I'm supposed to return the prime numbers from within a given range, and it has to be done using recursion.
The code I've got so far is this:
#include <stdio.h>
#include <stdlib.h>
int primeNumberList(int n, int m, int z);
int main() {
int n1 = 0,
n2 = 10,
d = 2;
printf("n1 = %d | n2 = %d | d = %d\n\n", n1, n2, d);
printf("Prime Numbers between %d and %d are: \n", n1, n2);
primeNumberList(n1, n2, d);
printf("\n\n");
return 0;
}
int primeNumberList(int n, int m, int z) {
int notPrime = 0;
if (n <= 1) {
primeNumberList(n + 1, m, z);
} else
if (n < m) {
if (z <= n / 2) {
if (n % z == 0) {
notPrime = 1;
z = 2;
} else {
primeNumberList(n, m, z + 1);
}
}
if (notPrime == 0) {
printf("%d ", n);
}
primeNumberList(n + 1, m, z);
}
}
What happens when I run this, is that after it's gone through all the numbers up to the limit (in the function it's m (n2 in main)) it won't break the recursion, but somehow manage to subtract numbers from n, and starts printing some other numbers that are not prime numbers.
When I run it in debug, it seems to be looping at the end, but there's nothing there to loop... I've tried adding a return 0; or even a printf with some text, but it ignores it completely.
Can anyone see what I've done wrong here? Why doesn't it stop when n < m?

Answer 1

I found your problem. You have the potential to make two recursive calls for each time you call primeNumberList.
After you return from primeNumberList(n, m, z+1); (under the innermost else) you still can go on to print a prime and do a call to primeNumberList(n+1, m, z);. This is not the behavior you want, you want to return directly after this inner else call.
So simply add a return before each of your calls to primeNumberList (primeNumberList(x); becomes return primeNumberList(x);) and also a return 0 at the end of this function (this last return is just to make the compiler happy).

Answer 2

Try something cleaner: defining a separate isPrime function and calling it.
http://www.cquestions.com/2011/08/prime-number-program-in-c-using.html
int isPrime(int num,int i){
if(i==1){
return 1;
}else{
if(num%i==0)
return 0;
else
isPrime(num,i-1);
}
}

Answer 3

Your recursive function exit criteria is not right.
When you you call the recursive function, it winds up, then down. When I run your code through step by step, As it is winding up, I get the complete list of primes, as following:
Then as it continues and the following digits, are printed:
Your question: Why doesn't it stop when n < m?
Because as you start unwinding, the values stored in the value n also start unwinding down through the iterations of the recursions that have been called, allowing execution flow to stay in the loop.
Further more, if unwind brings execution flow to a point that passes the printf("%d ", n); statement, the value for n, whatever it is at that unwind iteration, will be printed out.
One way way to leave without printing anything beyond n == 10 is to create a bypass variable, and use it as a criteria for printing:
static done = 0;
Then set done to 1 when you do not want to print any more values as it is unwinding.
Here is a modified function that will do this:
int primeNumberList(int n, int m, int z) {
int notPrime = 0;
static done = 0;//add a bypass variable, init to zero
if (n <= 1) {
primeNumberList(n + 1, m, z);
}
else
{
if(n == 10)
{
done = 1; //at this point all primes (except 1) are printed
//so set done to 1
}
if (n < m)
{
if (z <= n / 2)
{
if (n % z == 0)
{
notPrime = 1;
z = 2;
}
else
{
primeNumberList(n, m, z + 1);
}
}
if ((notPrime == 0) && (!done)) //test done before printing
{
printf("%d ", n);
}
primeNumberList(n + 1, m, z);
}
}
return 0;//add this return statement
}