[R] extracting information from an object

[David Howell]

I was working on a project involving a linear model, and wanted to 
extract the standard error of a predictor. I am able to do so, but not 
in the way I would expect.

I would have expected that if a created a model such as Model1 <- 
lm(y~x,z,d), the object Model1 would contain that information even 
though it does not print it out when I simply type Model1. I would also 
have (wrongly) suspected that if I type summary(Model1) R would simply 
look at the object Model1 and find whatever it needs. But it doesn't 
work that way. If I want that standard error I have to first create a 
summary of Model1 and then extract the standard error from the summary 
with something like summary(Model1)$coefficients or, more specifically, 
summary(Model)$coefficients[2,2]. [I know that I can cram all of that 
into one line if I want to.] But doesn't that mean that when I ask for a 
summary R has to recreate the linear model all over again before pulling 
out the standard error. (Venables and Ripley, p. 77) suggest that this 
could happen if the method is not written correctly, but how is it not 
happening anyway?) And if so, if Model1 doesn't contain the raw data, 
how does summary produce an answer even if I delete one of the variables 
before calling it?

As you can see, I have figured out how to get what I want, but I don't 
understand the process of building objects, which is the important thing 
to understand. Perhaps I don't understand "methods" well enough.

Below is sample code:

#Sample for linear model

x <- c(3,7,9,15,18)
y <- c(5,4,8,6,9)
reg <- lm(y~x)
reg
#Produces only the regression coefficients and using str(reg) indicates 
that
# that is all that it has.
regsummary <- summary(reg)
#Produces what I need and str(regsummary) shows that st. errors are part 
of the object.
regsummary$coefficients[1:2, 1:4]

rm(y)
out <- summary(reg)
# works just fine although y is no longer available and reg doesn't look 
like it
# could supply it.

Thanks,
Dave Howell