我的生日问题代码没有打印任何东西

Question

I am an absolute beginner to learning programming and I was given this assignment:

Birthday problem. Suppose that people enter a room one at a time. How people must enter until two share a birthday? Counterintuitively, after 23 people enter the room, there is approximately a 50–50 chance that two share a birthday. This phenomenon is known as the birthday problem or birthday paradox. Write a program Birthday.java that takes two integer command-line arguments n and trials and performs the following experiment, trials times:

• Choose a birthday for the next person, uniformly at random between 0 and n−1.
• Have that person enter the room.
• If that person shares a birthday with someone else in the room, stop; otherwise repeat.

In each experiment, count the number of people that enter the room. Print a table that summarizes the results (the count i, the number of times that exactly i people enter the room, and the fraction of times that i or fewer people enter the room) for each possible value of i from 1 until the fraction reaches (or exceeds) 50%.

For more information on the assignment

However, my code won't print. I would really appreciate if someone could help me find the problem to my assignment.

public class Birthday {
    public static void main(String[] args) {
        int n = Integer.parseInt(args[0]); //number of days
        int trials = Integer.parseInt(args[1]);
        boolean[] birthdays = new boolean[n];
        int[] times = new int[n + 2]; //number of times i people entered the room
        int r;

        for (int t = 1; t <= trials; t++) {

            for (int k = 0; k < n; k++) { //reset birthday
                birthdays[k] = false;
            }

            for (int i = 1; i <= n; i++) { //number of times

                r = (int) (Math.random() * (n - 1)); //random birthday

                if (birthdays[r] = false) {
                    birthdays[r] = true;
                    continue;
                }

                else if (birthdays[r] = true) {
                    times[i]++; //number of times i people entered the room + 1
                    break;
                }

            }

        }
        int j = 1;
        while ((double) times[j] / trials <= 0.5) {
            System.out.print(j + "\t" + times[j] + "\t" + ((double) times[j] / trials));
            j++;
            System.out.println("");

        }
    }
}

Answer 1

I can spot two errors from your code

1. As Scary Wombat pointed out, you are miss double equal sign inside of your if statement.

2. The assignment is asking you to calculate "fraction of times that i or fewer people enter the room", meaning you need to do a summation for the first i indices and divided by trials.

For example, among 1 million trials, the fraction in which first duplicate birthday happens when 4th person enters is

(times[0] + times[1] + times[2] + times[3])/ 1000000

Here is what I got:

1       0       0.0
2       2810    0.00281
3       5428    0.008238
4       8175    0.016413

As you can see the fraction is calculated by adding the first three elements together and then divided by 1000000 (2810 + 5428 + 8175 = 16413) / 1000000 = 0.016413

The way you are calculating the fraction ((double) times[j] / trials) is not correct.

Answer 2

You are not adding the previous counts as shown in the example. To do so, you can create a new variable to store the sums of previous counts. and use it as your while loop condition. For instance, see below..

csum += times[j]; // this adds the previous counts into a cumulative sum.

This cumulative sum is supposed to be the one u use to divide by trials to get your probability. Cheers!