[R] Subsetting problem

[Mark Difford]

Hi Massimo,

Professor Ripley has given you your answer.

It may help you further to know that factor levels aren't automatically
dropped when you subset a data set; you have to do it manually.  Some time
ago I scrounged the following command from Andy Liaw's randomForest package:
it removes all empty factor levels in a subsetted data set.

I subset a great deal, and find it extremely useful.

MyDat[] <- lapply(MyDat, function(x) if (is.factor(x)) x[, drop=T] else x)   
## Liaw's code for doing this

Regards,
Mark.

On Thu, 12 Jul 2007, Cressoni, Massimo (NIH/NHLBI) [F] wrote:

> I need to perform the Exact Wilcoxon Mann-Whitney on a subset of my
> database.
> Assuming that IPPO is my data frame and IPPOBIS is the subset my variable
> still
> have 3 different levels and the function wilcox_test (package "coin")
> does not accept it.
> I do not know how to overcome this problem.
>
> ippo <- c(rep("A",10),rep("B",10),rep("C",10))
> ippo2 <- c(rnorm(10,0,1),rnorm(10,10,10),rnorm(10,10,10))
> IPPO <- data.frame(ippo,ippo2)
>
> IPPOBIS <- IPPO[IPPO$ippo == "A" | IPPO$ippo == "B",]
>
> wilcox_test(ippo2 ~ ippo,data=IPPOBIS,distribution=exact())
> Error in check(itp) : 'object' does not represent a two sample problem
> levels(IPPOBIS$ippo)
> [1] "A" "B" "C"
>
> Massimo Cressoni
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
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> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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