#1




Possible bug in POIDEV
I noticed this whilst using POIDEV function, of the F90 NR version.
POIDEV relies on a subroutine called assert 1, given in the module nrutil. As part of this, assert 1 tests whether the value input into POIDEV is greater than 0, a condition needed to fulfill the mathematical operation of POIDEV. However in it's current form, there is a single situation in which this can fail. this produces the error message: nrerror: an assertion failed with this tag:gammln_s arg The expression includes the calculation: y=tan(PI*harvest). In my simulation I found that if harvest, a random number, is equal to 0.5 then y takes a value of infinite (as tan(pi/2) = infinite). this will pass the test as to whether the value is larger than 0, but when it is converted to a integer later it can be assigned a negative value, as it cannot cope with infinite.In this situation the programme will stop, according to assert1. There is a simple solution to this, which avoids the issue, though I have not tested to see if there are any other implications. this is given below: Current code: call ran2(harvest) y=tan(PI*harvest) em=sq*y+xm if (em >= 0.0) exit end do em=int(em) Edit to: call ran2(harvest) y=tan(PI*harvest) em=sq*y+xm em=int(em) if ((em >= 0.0)) exit end do By adjusting em to an integer inside the loop the problem is solved. I suppose another simple solution would be to add a condition to draw another random number if 0.5 is drawn. Either way, I hope this stops some of you feeling the frustration I felt when I got this error! 
#2




This is a good fix for this problem.
By the way, the 3rd edition has much faster routines for Poisson and other deviates which don't have this problem. (C++ only.) Saul Teukolsky 
Tags 
assertion fail, gammln_s arg, nrerror, poidev 
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