[R] using subset() in data frame

[Robert Walters]

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"])

###following example in ?subset
fit1 <- lm(y ~ x, data.b, subset(data.b, porosity, select=macro))

None of th above, plus many permutations thereof, works.
Can anyone educate me?

Thanks,

Robert Walters