# #984 – The Birthday Problem

To celebrate today’s Thanksgiving holiday in the United States, the post today is just a fun little code fragment for exploring the birthday problem.  The idea of the birthday problem is to look at the probability of having at least one shared birthday in a group of n people.

Instead of calculating the actual probability of a collision, the code below empirically creates a group of people, assigns them birthdays, and then looks for shared birthdays.  It does this 100,000 times and then reports what percentage of time there actually was a match.

```        static void Main(string[] args)
{
while (true)
{
Console.Write("Enter # people: ");
int numPeople = int.Parse(Console.ReadLine());

const double NumTrials = 100000.0;

int numBirthdayMatches = 0;
double numUniqueBirthdaySum = 0;

List<int> personBirthdays;
Random rnd = new Random();

for (int trialNum = 1; trialNum <= NumTrials; trialNum++)
{
// Generate birthdays
personBirthdays = new List<int>(numPeople);
for (int personNum = 1; personNum <= numPeople; personNum++)
personBirthdays.Add(rnd.Next(1, 366));

if (personBirthdays.Count != personBirthdays.Distinct().Count())
numBirthdayMatches++;

numUniqueBirthdaySum += personBirthdays.Distinct().Count();
}

double percentMatched =
numBirthdayMatches / NumTrials * 100.0;
double numUniquePerTrial = numUniqueBirthdaySum / NumTrials;
Console.WriteLine(string.Format("For {0} people, there was at least one matching birthday {1}% of the time.  Avg # unique birthdays={2}",
numPeople,
percentMatched,
numUniquePerTrial));
}

Console.ReadLine();
}
```

About Sean
Software developer in the Twin Cities area, passionate about software development and sailing.

### 2 Responses to #984 – The Birthday Problem

1. Brian says:

Very similar to the Monty Hall problem (http://en.wikipedia.org/wiki/Monty_Hall_problem). Another fun one that’s easy to implement and prove via code.