[Rd] Bug in 'pchisq' for x=0.0 (PR#9485)

[nothnagel at medinfo.uni-kiel.de]

The function 'pchisq' from the 'stats' library gives a wrong result if the
argument equals exactly zero:

# Upper tail of central 1-df chi^2 distribution
> pchisq(1         , 1, ncp=0, lower.tail = F, log.p = FALSE)
[1] 0.3173105
> pchisq(0.5       , 1, ncp=0, lower.tail = F, log.p = FALSE)
[1] 0.4795001
> pchisq(0.01      , 1, ncp=0, lower.tail = F, log.p = FALSE)
[1] 0.9203443
> pchisq(0.0001    , 1, ncp=0, lower.tail = F, log.p = FALSE)
[1] 0.9920213
> pchisq(0.00000001, 1, ncp=0, lower.tail = F, log.p = FALSE)
[1] 0.9999202
> pchisq(0.00      , 1, ncp=0, lower.tail = F, log.p = FALSE)
[1] 0   ## Should be 1.0

The error occurred with R version 2.4.0 (windows) and version 2.3.1 (linux).


Best regards,

Michael Nothnagel

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Dr. Michael Nothnagel
Institute of Medical Informatics and Statistics, 
University of Kiel
http://capella.uni-kiel.de/
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