How to calculate the winning chance of the lottery in c#?

Question

using System;

namespace FirstApplication
{
    class Program
    {
        public static void Main()
        {
            int n = Convert.ToInt32(Console.ReadLine());
            int k = Convert.ToInt32(Console.ReadLine());
            string category = Console.ReadLine();
            double total = 0;

                switch (category)
                {
                    case "I":
                        total = bc(k, 6) * bc(n - k, k - 6) / bc(n, k);
                        Console.WriteLine("{0:F10}", total);
                        return;
                    case "II":
                        total = bc(k, 4) * bc(n - k, k - 4) / bc(n, k);
                        Console.WriteLine("{0:F10}", total);
                        return;
                    case "III":
                        total = bc(k, 2) * bc(n - k, k - 2) / bc(n, k);
                        Console.WriteLine("{0:F10}", total);
                        return;
                }
            Console.Read();

        }
        private static double bc(decimal n, decimal k)
        {
            if (k == 0 || k == n)
                return 1;
            return bc(n - 1, k - 1) + bc(n - 1, k);
        }
    }
}

I have a problem with my code.

The exercise is the following:

You participate at the lottery 6/49 with only one winning variant(simple) and you want to know what odds of winning you have:

-at category I (6 numbers)

-at category II (5 numbers)

-at category III (4 numbers)

Write a console app which gets from input the number of total balls, the number of extracted balls, and the category, then print the odds of winning with a precision of 10 decimals if you play with one simple variant.

For example if I input:

49

6

I

The result is ok, but when I input:

45

15

III

I don’t get any result.

Any suggestions what is wrong with my code?

Did you debug the issue step by step? I bet you´ll find the problem way faster than anyone here, as you know best what your app **should** do and what it does **instead**. — MakePeaceGreatAgain, Feb 02 '21 at 11:56
Sure, I debugged, but I still can't find out what is the problem. — Bodor Arianna, Feb 02 '21 at 11:58
So what **exactly** happens while debugging? What does `bc` return for case `"III"`? Put a breakpoint into `bc` and see if it gets hit. If it does step line by line through your code to see where it behaves different from what you expect. We can´t say you what you expect, only you can. — MakePeaceGreatAgain, Feb 02 '21 at 12:04
Think about: if your inputs for both `k` and `n` are odd, will `if (k == 0 || k == n)` ever become true? — 500 - Internal Server Error, Feb 02 '21 at 12:34

score 2 · Accepted Answer · answered Feb 02 '21 at 12:37

I don't think there's anything wrong with your code. It just runs for a very long time, because you call bc with the same values again and again. I added a dictionary to store and lookup the values already calculated:

private static Dictionary<(int N, int K),double> knownValues = new Dictionary<(int N, int K),double>();
private static double bc(int n, int k)
{
    var key = (n,k);
    if (!knownValues.ContainsKey(key))
    {
       if (k == 0 || k == n)
       {
           knownValues.Add(key, 1);
       }
       else 
       {
           knownvalues.Add(key, bc(n - 1, k - 1) + bc(n - 1, k));
       }
    }
    return knownValues[key];
}

And it returns in < 6 seconds with the value 0.0364626616 for your 45,15,III input. Whether that is right, I don't know, but at least it terminates. Maybe your teacher wanted you to try this and learn about recursion and calculation times.

The compiler knows and is trying to tell you. If your are using an older version of C# it might not know about named tupes `(int n, int k)`. — Palle Due, Feb 02 '21 at 12:53

How to calculate the winning chance of the lottery in c#?

1 Answers1