Java output issue? Program not getting desired result [format updated]

Question

This program is supposed to go through the number of 2 to 50 people and get the probability out of 100,000 trials whether 2 people will have the same birthday or not.

import java.util.Random;

public class birthday {
    public static void main(String[] args) {
        int N = 2;
        while (N != 51) {
            double probability = one_probability(N); 
            System.out.println(N + "  " + probability);
            N++;
        }
    }

    public static boolean one_group(int N) {
        int A[] = new int[N]; 
        Random random = new Random();
        boolean have_match = false; 
        for (int i = 0; i < N; i++) {  //assigns each person a birthday
            int k = random.nextInt(365);
            A[i] = k;
        }
        for (int i = 0; i < N; i++) {  //testing to see if birthdays match up
            for (int j = i+1; j < N; j++) {
                if (A[i] == A[j]) { have_match = true; break; }
            }
        }
        return have_match;
    }

    public static double one_probability(int N) { 
        int x = 0;
        for (int i = 0; i < 100000; i++) {  //repeating 100,000 times to get average probability
            boolean have_match = one_group(N);
            if (have_match == true) { x++; }
        }
        double probability = x / 100000;  //getting the average probability
        return probability;
    }
}

Here's the result (it goes from 2-50), it keeps giving me zeros so I know something is wrong. Please help :) Output

Have you debug your code ? Having always zero let me think you are doing some division on an integer. — AxelH, Nov 28 '17 at 13:22
`x` is lesser than 100000 and you calculate `probability` as a integer => should be a float. — Stéphane Ammar, Nov 28 '17 at 13:22
I tried switching the probability to a double and a float and both got me 0.0 as an answer. — Michael A., Nov 28 '17 at 13:32
This would be a duplicate of this post where I have done a complet answer : [Why is the result of 1/3 == 0?](https://stackoverflow.com/a/41822430/4391450) — AxelH, Nov 28 '17 at 14:04

AxelH · Accepted Answer · 2017-11-28T13:53:12.937

0

Try with

int probability = x / 1000; // 1/100 of the value to get an probability in percent (integer)

or

float probably =  x / 100000F; //F for a `float` 
double probability = x / 100000.0; //a decimal without a F is a `double`

Without that, this can't work :

float probably = x / 100000;

First, the division of two integer will be stored in memory like an integer then store in a float/double. That storage truncate the value. This is the operator logic to return the biggest type in an operation so :

int * int -> int
int * float -> float
int * double -> double
short * short -> int //that's a trick, int is the minimal value an operation can return

edited Nov 28 '17 at 13:53

answered Nov 28 '17 at 13:46

AxelH

13,322
2
21
50

omg ty tyty it worked – Michael A. Nov 28 '17 at 13:53
@MichaelA. I did a more complete answer [Why is the result of 1/3 == 0?](https://stackoverflow.com/a/41822430/4391450) if you are interested – AxelH Nov 28 '17 at 14:02

Shakhawat Hossain · Answer 2 · 2017-11-28T13:55:27.397

-1

int probability=x/100000; always return 0.

convert it to double probability=x/100000.0; the value of probability will always less than or equal one. because x will never be greater than 100000. And also change the return type of one_probability method to double.

edited Nov 28 '17 at 13:55

answered Nov 28 '17 at 13:35

Shakhawat Hossain

628
5
11

I tried switching the probability to a double and a float and both gave me 0.0 as an answer – Michael A. Nov 28 '17 at 13:45
then check the value of x before calculating the probability. it may be zero. – Shakhawat Hossain Nov 28 '17 at 13:48
ya but why would it be 0, what would be wrong with the for loop? – Michael A. Nov 28 '17 at 13:50
You don't know how the operator work in JAVA. `double = int / int` will return an `int` then be cast into a `double` (because it fits). But the value is already truncated... – AxelH Nov 28 '17 at 13:54
use x/100000.0 instead of x/100000. it is working. I checked it – Shakhawat Hossain Nov 28 '17 at 13:55

Java output issue? Program not getting desired result [format updated]

2 Answers2