# How to calculate the dirichlet cdf

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xueqi on 14 Jun 2014
Commented: Star Strider on 14 Jun 2014
Dear Fellows,
Do you know how to write the cdf of dirichlet distribution? I could write the pdf but do not know how to formulate the cdf. I would like to calculate the area between x-0.5 to x+0.5, which is F(x+0.5)-F(x-0.5) where F refers to the cdf. Could you offer me some advice about this?
Thanks very much.
Xueqi

Star Strider on 14 Jun 2014
Edited: Star Strider on 14 Jun 2014
Wikipedia doesn’t give an analytic expression for the cdf. Since you already have a function written for the pdf, I suggest simply integrating it using the integral or integral2 function.
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Star Strider on 14 Jun 2014
It is difficult for me to follow your code. I have no experience with dirpdf.
If you are using vector-valued functions, or if your call to dirpdf returns a vector, it might be necessary for you to use two nested integral calls, one to evaluate the value for x1 and the other for x2. The integral function will take vectors, while integral2 will not.
If that is not the problem, you will likely need to go through your code and your calls to dirpdf with the arguments you present to it in your call to integral or integral2 to understand what it returns.
If all else fails, generate a vector of values for x, evaluate dirpdf in as many nested loops as necessary, and then use trapz to integrate it.