SPUG: Stick Riddle

Jonathan Gardner jgardn at alumni.washington.edu
Fri Jan 3 08:18:52 CST 2003

On Thursday 02 January 2003 10:41 pm, Creede Lambard wrote:
> On Thu, 2003-01-02 at 22:23, Jeremy Calvert wrote:
> > > > > ... if A + B = C ...
> > > >
> > > > ... the probability of this happening is 0 ...
> > >
> > > Not so. If the first break results in two sticks of
> > > equal length ...
> >
> > Is so so:).  The probability of your counter-example
> > happening is also 0.  Otherwise, what is the
> > probability that the first break results in two sticks
> > of equal length?
> I still respectfully state that it's not so. The probability of the
> first break being exactly in the middle of the stick is vanishingly
> small, but so is the probability that the break appears anywhere else in
> the stick. Does that mean the stick can never be broken, since the
> probability of it breaking at any particular point is the same as the
> probability of it breaking at the exact center? (This probably has
> something to do with dividing by zero.)
> Not to mention the many, many circumstances in which the first break
> appears at location $x along the stick, and the second break causes the
> longer of the two sticks to split such that the longest piece left over
> has length of .5 .
> This is obviously a problem for some sort of calculus, but I'm afraid my
> calculus years are, ahem, many years behind me.

The chances of the stick breaking exactly in some predefined way is 
infinitessimally small -- hence zero. My reasoning is as follows. A 
mathematical stick can be cut anywhere between the two ends. There is an 
infinite number of positions in which it can be cut, and so the chances for 
any one of those positions to be cut is infinitessimally small.

This is not so when using perl's scalars. There is a finite number of places 
you can make a cut along the stick, even if you use floating point numbers 
(which aren't mathematically real numbers but give us a pretty good 
approximation). If you were able to randomly generate *any* real number 
between 0 and 1 in perl (you can't, so don't try!), then the situation for 
the mathematical stick applies.

A stick that is real -- err, I mean a real stick, not a mathematical or perl 
one -- is made up of atomic matter that can only be subdivided in a finite 
number of ways. So, like the perl sticks, it can be broken in some predefined 
way. The chances are very, very small, but not zero.

Jonathan Gardner
jgardn at alumni.washington.edu

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