Algorithm:

1. Start with an integer n.
2. If n is even, divide by 2.
3. If n is odd, multiply by 3 and add 1.
4. Repeat the process with the new value of n, terminating when n = 1.

//3n + 1 problem
#include <stdio.h>

int main(){
	int num;			//first number
	int ans;			//answer
	int ctr;			//counter

	//the first number in the series

	printf("enter integer n: ");
	scanf("%i", &num);
	ans = num;

	if(ans > 0){
		printf("\t%i\n", ans);
		ctr = 1;

		while(ans > 1){
			if(ans % 2 == 0)
				ans = ans / 2;
			else
				ans = ans * 3 + 1;
			printf("\t%i\n", ans);
			ctr ++;
		}
		printf("cycle length: %i", ctr);
	}
	else
		printf("positive integer only");

	return 0;
}