[R] Selecting groups with R

[Glen Sargeant]

jlwoodard wrote:
> 
> 
> Each of the above lines successfully excludes the BLUE subjects, but the
> "BLUE" category is still present in my data set; that is, if I try
> table(Color)  I get 
> 
> RED  WHITE  BLUE
> 82     151      0
> 
> How can I eliminate the BLUE category completely so I can do a t-test
> using Color (with just the RED and WHITE subjects)?
> 
> 

A simpler example.  See "details" in the help file for factor() for an
explanation.

>#Factor with 3 levels
> x <- rep(c("blue","red","white"),c(1,1,2))
> 
> x <- factor(x)
> 
> table(x)
x
 blue   red white 
    1     1     2 
> 
>#Subset is still a factor with 3 levels
> y <- x[x!="blue"]
> 
> table(y)
y
 blue   red white 
    0     1     2 
> 
>#Drops unused levels; result a factor with 2 levels
> table(factor(y))

  red white 
    1     2 

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