Formula for Poisson distribution

Hi,

I've been trying to write a formula that will allow me to create a graph matching the Poisson Dist. for frequency as a function of the parameter (in this instance, K).

for some reason, I cant get Matlab to run it, and was wandering if any of you can see my problem:

function f = poisson (x,a) f = ((a(1)^x)/factorial(round(x)))*e^(-a(1))*a(2) + a(3);

thnaks

Answer 1

[Jos (10584)]

I am not sure why you have three parameters rather than 1. See https://en.wikipedia.org/wiki/Poisson_distribution

In your code, you might want to use element wise operators, and use exp

function f = poisson (k, lambda) 
f = (lambda.^k) .* exp(-lambda) ./ factorial(k)

Answer 2

[Christos Papadakis]

Can someone tell me what is the difference of the above mentioned formula with the matlag function poissrnd?

In the case of the formula above, as expected , for given k and lambda we get a number between 0 and 1. On the other hand, when I call poissrnd(lambda) I get positive integers lower than lambda.

I did a Google search on that but even Matlab says that :

"r = poissrnd(lambda) generates random numbers from the Poisson distribution specified by the rate parameter lambda. "

But I do not understand exactly why is there such a difference. Sorry if my question is stupid.

Thanks in advance!