Unable to store the persistent variable

I am trying to store two persistent variables in order to use them in further iterations. The problem is with the second variable 'weight'. It has a default value 0.1. Once I give a value to my function's 2nd argument, it should take this value.

The code is

function out =exp_average(in,b)
persistent average weight
if isempty(weight) %It does not already exist
weight = 0.1;
end
if isempty(average) 
    average=in;
elseif nargin<2
    average=weight*in+(1-weight)*average;
elseif nargin==2
    clear exp_average
    average=in;
    average=b*in+(1-b)*average;
    weight=b;
else
    error('WTF')
end
out=average;
end

When I give two arguments to my function, it want my persistent variable 'weight' take the value of the input argument 'b'. With this code it does not keep its value. Any help will be appreciated.

Answer 1

[Azzi Abdelmalek]

function out =exp_average(in,b)
persistent average weight
if isempty(weight) %It does not already exist
 weight = 0.1;
 average=in;
end
if nargin==1
  average=weight*in+(1-weight)*average;
elseif nargin==2
  average=b*in+(1-b)*in;
  weight=b;
else
  error('WTF')
end
  out=average;