On 2/22/2008 8:01 PM, Robert Walters wrote:> R folks,
> As an R novice, I struggle with the mystery of subsetting. Textbook and
> online examples of this seem quite straightforward yet I cannot get my
> mind around it. For practice, I'm using the code in MASS Ch. 6,
> "whiteside data" to analyze a different data set with similar
variables
> and structure.
> Here is my data frame:
>
> ###subset one of three cases for the variable 'position'
> >data.b<-data.a[data.a$position=="inrow",]
> > print(data.b)
> position porosity x y
> 1 inrow macro 1.40 16.5
> 2 inrow macro . .
> . . . .
> . . . .
> 7 inrow micro
> 8 inrow micro
>
> Now I want to do separate lm's for each case of porosity, macro and
> micro. The code as given in MASS, p.141, slightly modified would be:
>
> fit1 <- lm(y ~ x, data=data.b, subset = porosity == "macro")
> fit2 <- update(fit1, subset = porosity == "micro")
>
> ###simplest code with subscripting
> fit1 <- lm(y ~ x, data.b[porosity=="macro"])
Assuming data.b has two dimensions, you need a comma after
porosity=="macro" to indicate that you are selecting a subset of rows
of
the data frame:
fit1 <- lm(y ~ x, data.b[porosity=="macro",])
> ###following example in ?subset
> fit1 <- lm(y ~ x, data.b, subset(data.b, porosity, select=macro))
The select argument to subset is meant to select variables (i.e., it
indicates "columns to select from a data frame") and you are misusing
it
by specifying the level of a factor. If you make your call to subset by
itself (a good idea when you are learning how a function works), you
should get an error like this:
> subset(whiteside, Insul, select=Before)
Error in subset.data.frame(whiteside, Insul, select = Before) :
'subset' must evaluate to logical
What I think you intended was this:
subset(data.b, porosity == "macro")
Even with the correct call to subset, you also don't want both data.b
and the subset piece, because subset returns a data frame. In other
words, you would be passing lm() two different data frames. So try this
instead:
fit1 <- lm(y ~ x, subset(data.b, porosity == "macro"))
> None of th above, plus many permutations thereof, works.
> Can anyone educate me?
>
> Thanks,
>
> Robert Walters
>
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