function pdf=dfexp(lamda, samples) 
% dfexp(lamda, samples)  Generates an exponential pdf with parameter lamda. 
%    samples sets the size of the numerical representation, and is optional. 
%    The default sample size is 1000. 
if nargin==1 
   samples=1000; 
end 
if lamda<=0 
   error('invalid value for lamda') 
end 
X=(-1/lamda)*log(1e-6/lamda)+0.5; 
x=[0:X/samples:X]; 
p=lamda*exp(-lamda*x); 
pdf=[0 1; x' p']; 
if ispf(pdf)==0 
   p=p./ttlprob(pdf); 
   pdf=[0 1; x' p']; 
end