I'm trying to make a program that represents the birthday paradox. I understand the paradox and I'm pretty sure my code is wrong but I'm not sure where I went wrong. I've looked through related posts, but I haven't found anything that's helpful. I wrote the code when I was younger, so sorry if it's a bit messy. I know there are other ways to do it, and I understand why those work. I just want to know why my code doesn't work. Thanks!
EDIT: Sorry, it's late. Forgot to put what my actual problem was. I run it how it is and expect to get about 50.5%, which is the theoretical value. But, instead, I get about 21.1%.
public class Main {
static int trialsSucceeded = 0; // Stores number of successful trials
static final int TRIALS = 1000000; // 1,000,000 is a good number :) Biggest I've tried: 2,147,483,647, which is Integer.MAX_VALUE
static int numberOfPeople = 23; // The 'n' value for the birthday paradox
public static void main(String[] args) {
ArrayList<Integer> birthdays = new ArrayList<Integer>(); // Stores people's birthdays
// Runs until desired trials are completed
for (int trialNumber = 0; trialNumber < TRIALS; trialNumber++) {
// Provides progress updates to user
if (trialNumber % 1000 == 0)
System.out.printf("%.1f%% complete\n", (double) trialNumber * 100 / TRIALS);
// Populates the birthdays array
for (int personNumber = 0; personNumber < numberOfPeople; personNumber++) {
birthdays.add(getRandInt(1, 365));
}
// Used later to see if current trial should end
int prevTrialsSucceeded = trialsSucceeded;
// Checks each person's birthday against everyone else's
for (int i = 0; i < birthdays.size(); i++) {
for (int j = i + 1; j < birthdays.size(); j++) {
// If birthdays match, marks trial as a success jumps to next trail
if ((birthdays.get(i) == birthdays.get(j))) {
trialsSucceeded += 1;
break;
}
}
// Jumps to next trial if this one has already succeeded
if (prevTrialsSucceeded != trialsSucceeded) {
break;
}
}
// Clears list of birthdays to get ready for next trial
birthdays.clear();
}
// Tells user ratio of successful trials to total trials
System.out.println(((double) trialsSucceeded / TRIALS * 100) + "% of trials succeeded");
}
private static int getRandInt(int lowerBound, int upperBound) {
// Returns random integer between lowerBound and upperBound
Random random = new Random();
return random.nextInt(upperBound - lowerBound + 1) + lowerBound;
}
}
As others have figured correctly your problem is rooted in comparing references... But even when fixing that problem the solution here is neither efficient nor easy to grasp.
There is a simpler way to check this: just use a Set to store the random number birthdays instead of a list. But before adding the next number you use Set.contains() to check if that number is already in the set. If so you found a match... and you can stop right there!