[R] question about combining data in two frames

[Michael A. Miller]

Dear R Experts,

I am still learning the subtleties or R programming and have a
question about the best (==most elegant) way to combine data in
two frames.  One frame is a table of measurements of sizes of
objects in multiple image fields and the other is a table of
areas for each field.  I want to calculate densities by dividing
total number of objects in each field by the appropriate area.  I
can do this like this:

sizes <- read.table("marked/combined.meas", header=T)
areas <- read.table("areas", header=T)

# Number of objects in each field
counts <- tapply( sizes$Diameter, factor(sizes$Field), length)

# Density in each field
for ( field in names(counts) ) {
  print( counts[field]/areas$Area[areas$Field==field] )
}

I'd like to get my densities as a vector though.  It feels like
there might be some way for me to use tapply (or some other R
idiom) to do the same thing as this for-loop, but I haven't been
able to hit upon it.  Can anyone offer suggestions?

Also, are there other ways to get my sums than the way I'm using
tapply above?

Thanks, Mike

-- 
Michael A. Miller                      mmiller3 at iupui.edu
  Krannert Institute of Cardiology, IU School of Medicine
  Indiana Center for Vascular Biology and Medicine
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