[R] a function to compute the cumulative distribution function (cdf) of the gamma

Ben Bolker bolker at ufl.edu
Mon Oct 8 13:52:28 CEST 2007

elodie gillain wrote:
> Dear Forum,
> I would need to write a function to calculate the cumulative distribution
> function (cdf) of the gamma random variable. I am allowed to use the
> following precoded functions: rgamma, pgamma, and dgamma. I am a little
> overwhelmed by this project,  I am wondering how I should start.  Should I
> sum dgamma (or pgamma) from 0 to x, in order to obtain the cdf at x?

  This is a little bit confusing since pgamma() *is* the cumulative
distribution function of a gamma random variable.  (Also, "allowed
to use" makes this sound a little like a homework problem ...)
If you really wanted to do things the hard, inefficient way, you
could use integrate() to numerically integrate (not sum) dgamma()
from 0 to x -- but why not use pgamma() ??

  Ben Bolker

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