[R] using subset() in data frame

[Chuck Cleland]

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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-- 
Chuck Cleland, Ph.D.
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