#======================================================================

#                    L I B R W A L K . P L 

#                    doc: Tue Oct  5 21:34:37 2010

#                    dlm: Tue Oct  5 21:47:28 2010

#                    (c) 2010 A.M. Thurnherr

#                    uE-Info: 26 26 NIL 0 0 72 2 2 4 NIL ofnI

#======================================================================

sub fac($)

{ return ($_[0] < 2) ? $_[0] : $_[0]*fac($_[0]-1); }

#----------------------------------------------------------------------

# From: http://mathworld.wolfram.com/RandomWalk1-Dimensional.html

# Let N steps of equal length be taken along a line. Let p be the

# probability of taking a step to the right, q the probability of taking a

# step to the left, n1 the number of steps taken to the right, and  n2 the

# number of steps taken to the left.

# The following calculates the probability of taking exactly n1 steps out

# of N (= n1+n2) to the right.

#----------------------------------------------------------------------

sub pNSteps(@)

{

	my($n1,$N,$p,$q) = @_;

	$p = $q = 0.5 unless defined($p);

	return fac($n1+($N-$n1)) / (fac($n1)*fac($N-$n1)) * $p**$n1 * $q**($N-$n1);

}

1;