Editing Common Lisp (section)

===Birthday paradox===
The following program calculates the smallest number of people in a room for whom the probability of unique birthdays is less than 50% (the [[birthday paradox]], where for 1 person the probability is obviously 100%, for 2 it is 364/365, etc.). The answer is 23.

In Common Lisp, by convention, constants are enclosed with + characters.

<syntaxhighlight lang="lisp">
(defconstant +year-size+ 365)

(defun birthday-paradox (probability number-of-people)
  (let ((new-probability (* (/ (- +year-size+ number-of-people)
                               +year-size+)
                            probability)))
    (if (< new-probability 0.5)
        (1+ number-of-people)
        (birthday-paradox new-probability (1+ number-of-people)))))
</syntaxhighlight>

Calling the example function using the [[REPL]] (Read Eval Print Loop):
<syntaxhighlight lang="text">
CL-USER > (birthday-paradox 1.0 1)
23
</syntaxhighlight>