# [PyAR2] Car Talk Puzzler (The Hall of Lights)

Greg Lindstrom gslindstrom at gmail.com
Wed Nov 2 21:14:59 CET 2011

```The Car Talk "Puzzler" this week is a classic (I've heard it
opening/closing lockers):

*RAY:* This puzzler is from my "ceiling light" series. Imagine, if you
will, that you have a long, long corridor that stretches out as far as the
eye can see. In that corridor, attached to the ceiling are lights that are
operated with a pull cord.

There are gazillions of them, as far as the eye can see. Let's say there
are 20,000 lights in a row.

They're all off. Somebody comes along and pulls on each of the chains,
turning on each one of the lights. Another person comes right behind, and
pulls the chain on every second light.

*TOM:* Thereby turning off lights 2, 4, 6, 8 and so on.

*RAY:* Right. Now, a third person comes along and pulls the cord on every
third light. That is, lights number 3, 6, 9, 12, 15, etcetera. Another
person comes along and pulls the cord on lights number 4, 8, 12, 16 and so
on. Of course, each person is turning on some lights and turning other
lights off.

If there are 20,000 lights, at some point someone is going to come skipping
along and pull every 20,000th chain.

When that happens, some lights will be on, and some will be off. Can you
predict which lights will be on?

I wrote a Python program to "solve" this is 8 (non-blank) lines long,
including one line, "MAX_LIGHTS = 20000" used to adjust the number of
lights for debugging/testing.  Let's see how YOU would do it (I'll post
mine in a couple days).

O'Reilly accidentally sent me an "extra" case of books to give away at
PyArkansas (the arrived after the conference).  If Marsee will let me give
them away, the entry judged "best" will win one of the titles.  "Best" is
totally subjective but will include (1) Correct Solution and (2) Elegance
(is it Pythonic/Understandable?).  Python 2 or 3.

--greg
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