Lesson goal: The Birthday Surprise

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Whenever a small group of people get together, many are often surprised if two people have the same birthday. Karma? Fate? Some "energy field" of the universe? (No, no and no.)

Perhaps this shouldn't be so surprising since there are only 365 possible birthdays in a year. If person #1's birthday is on day X, then person #2 has a 1/365 chance of sharing #1's birthday. Person #3's chances rise to 2/365, and so on. Our point? These numbers, which are the chances of birthdays being the same, aren't that small!

All told, would you believe that if 23 people are together, the odds that two of them share a birthday are about 50%? Let's test this here.

Let's use random numbers to determine if a given day is someone's birthday. Our plan will be to loop through N people (you choose N), and use a random number to determine the day of the year (1..365) of their birthday. Using the hist[day]=hist[day]+1 line, we'll count the number of people whose birthday falls on a given day.

Run the program many times (like 20, 30, or 50). Think of each run as a "get together" of N people. After each run, count how many times two people share a birthday. If the screen ends up blank, it means a pair did not share a birthday. If your N=23, it should happen for around 50% of your runs.

You can review arrays here and histograms here

math.random(low,high)
Move the mouse over a dotted box for more information.

Now you try. Choose some number of people in the N= line and fix the day=math.random line to choose a day of the year between 1 and 365.

Type your code here:


See your results here:

Show a friend, family member, or teacher what you've done!

Here is a share link to your code:

Does your code work? Want to run it on your iPhone?

Here's your code:

  1. Use [Control]-[C] (Windows) or [⌘]-[C] (MacOS) to copy your code.

  2. Paste it using [Control]-[V] (Windows) or [⌘]-[V] (MacOS) into this page

  3. Then click the "Use on iPhone" button that you'll see.