[R] classes and functions for qqnorm and stem

Peter Ehlers ehlers at ucalgary.ca
Thu Apr 15 13:06:37 CEST 2010

On 2010-04-14 21:20, Uwe Dippel wrote:
> Referring to "Using R for Data Analysis and Graphics" by J H Maindonald,
> and available from the R site, I found the example on p.30 non-working:
>  > stem(qqnorm(possum$hdlngth))
> Error in stem(qqnorm(possum$hdlngth)) : 'x' must be numeric
> Since qqnorm(possum$hdlngth) plots, and
>  > class(possum$hdlngth)
> [1] "numeric"
> , the problem must be here:
>  > class(qqnorm(possum$hdlngth))
> [1] "list"
> Does 'stem' require numerical input? The help says so.
> But how to render it numeric?:
>  > class(as.numeric(qqnorm(possum$hdlngth)))
> Error: (list) object cannot be coerced to type 'double'
> Can someone please enlighten me about what goes wrong/has changed here?

This is a perfect time to learn about str().
If you check


You will see that it is a list of two numeric vectors. But
stem() expects just one vector. The stem(....) call is an
error; it should be just stem(possum$hdlngth).

  -Peter Ehlers

> Uwe
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Peter Ehlers
University of Calgary

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