This is a painfully basic question. It's probably been answered before (or might be in one of the documentation packages), but I've been looking for some time and can't find it! (Though it could very easily be that I'm not exactly sure what to look for, so I might have missed it!)
My question is basically this: I'd like to get Mathematica to compute a simple probability given a distribution and return the result as a number.
Specifically, let's say I'm interested in getting the probability that $x=0$ when $x$ is distributed according to a Poisson Distribution with $\mu = 1$. So I enter (basically straight from the Wolfram website):
Probability[x == 0, x \[Distributed] PoissonDistribution[1.]]
But instead of getting something like $.368$, that returns:
Probability[True, 0 \[Distributed] PoissonDistribution[1.]]
Which is puzzling, since on the wolfram website, the input and output examples are:
Probability[x == 20, x \[Distributed] PoissonDistribution[20.]]
Can anyone explain why one returns a numerical result and the other doesn't? And how can I get my functions to actually return/store numerical results?