function period = Zero_x(U,Time)

%  Zero_x -- calculate Zero Crossing Period
%
%      U is an oscillatory variable, a vector of values
%           spaced in time.
%      Time is the time interval spanned by the values
%           of U.
%      period is the zero crossing period, in the same
%           units as Time.
% 
%      Fran Hotchkiss, January 8, 1998.

whichzeroes = find(U==0.);
U(whichzeroes) = 1;
signofU = sign(U);
signchanges = diff(signofU);
changes = find(signchanges);
howmany = length(changes);
if howmany ~= 0
   period = 2*Time/howmany;
else
   period = 4*Time;
end