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[QUOTE="Martin Ambuhl, post: 2449412"]
Your question has nothing to do with the C programming language.  Even 
so, I will provide you with some of the information that you should have 
already,  WARNING! Everything that follows is off-topic.

a) It's "Poisson", not "poison"
b) The Poisson distribution is not "a bell shaped curve".

A Poisson distribution is given by
    Pr(X = x) = exp(-lambda) * pow(lambda,x) / factorial(x)
There is *one* external parameter, lambda
The mean and variance of the distribution are each lambda,
the skewness is 1/sqrt(lambda), and the kurtosis is 3+1/lambda.

The distribution is most easily computed by an iterative procedure,
since
   Pr(X = 0) = exp(-lambda)
and
   Pr(X = x) = Pr(X = x-1) * lambda / x;

You want to compute this once, filling an array with the cumulative 
values, and using rand() to obtain a value to compare against this 
array.  So if
      x = rand()/(1. + RAND_MAX);
and if
      x > cum_freq_array[i-1]
and
      x <= cum_freq_array[i]
then there were i events in the time period.

Since this is not a C issue, I make no claim that any of the above is 
correct.  Check it yourself.[/i]
[/QUOTE]

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