I've got infinite looping

Question

i am practicing my java by counting possible ways to reach n with a dice. and when input n value to smaller number, it works. but when i input n value to 100, it got infinite looping.

can u guys help me ?

here is my code :

public static void main(String[] args) 
{
    testCode test = new testCode();

    System.out.println(test.countWays(100));
}

 private static int countWays(int n)
    {
        if(n<=1)
        {
            return 1;
        }
        else
        {
            System.out.println("counting ....");
            return countWays(n-6) + countWays(n-5) + countWays(n-4) + countWays(n-3) + countWays(n-2) + countWays(n-1);
        }
    }

.... but it doesn't look infinite actually, just very large. — Hovercraft Full Of Eels, Feb 05 '17 at 15:14
@HovercraftFullOfEels Actually it's not infinite recursion, but before it reaches the base case it most likely hits stack overflow. — Tim Biegeleisen, Feb 05 '17 at 15:15
Consider: Every single time you call `countWays` with `n > 1`, you trigger six further calls to it. Each of those triggers six further calls (e.g., 36 more; 43 so far if you're counting); each of those 36 triggers six further calls (259 so far), and so on, and so on. — T.J. Crowder, Feb 05 '17 at 15:16
@T.J.Crowder So then something like `6^100` function calls. Yes, this should overflow the call stack. — Tim Biegeleisen, Feb 05 '17 at 15:17
@HovercraftFullOfEels okay i see, that's why ! but what if i want the large number to works? i'm sorry i can't tag all of you — rinaldy31, Feb 05 '17 at 15:19
@user2871904, try changing your implementation similarly to how it is suggested in [this question](http://stackoverflow.com/questions/10073471/java-how-to-avoid-stackoverflowexception) — M. Prokhorov, Feb 05 '17 at 15:25
Then use a non-recursive solution such as by using a stack. Edit: and now I see, this is exactly as @MikhailProkhorov recommends as well. — Hovercraft Full Of Eels, Feb 05 '17 at 15:26
@MikhailProkhorov okay i will change it , i will post the update later.. thankyou — rinaldy31, Feb 05 '17 at 15:46

score 1 · Accepted Answer · answered Feb 05 '17 at 15:54

Your problem is similar to Fibonnaci's one :

x0 = 0, x1 = 1, x(N) = x(N-2) + x(N-1)

If you need to do it with big numbers you should use a non-recursive method :

static long countBis(int n) {
        long fn, f1, f2, f3, f4, f5, f6;
        int i;
        f1=f2=f3=f4=f5=f6=fn = 1;
        for ( i = 2; i <= n; i++ ) {
            f6 = f5;      
            f5 = f4;      
            f4 = f3;
            f3 = f2;
            f2 = f1;
            f1 = fn;
            fn = f6 + f5 + f4 + f3 + f2 + f1;
        }
        return fn;
    }

At each iteration you just calculate the sum of the precedent ones

I've tested with n = 32 => with yours it took 8sec, with this one it took less than 1sec (I tried with n = 1000 => always 1sec, but i didn't try yours haha, after n = 35 it's a bit long ^^

WOW, it works ! thankyou very much sir. but can i ask u question? why i=2? — rinaldy31, Feb 05 '17 at 16:03
Thanks, just because i have `i<=n` then, it can be changed in : `i=1 ; i — azro, Feb 05 '17 at 16:21

I've got infinite looping

1 Answers1