Subsetting in R using a list - r

Question

I have a large amount of data which I would like to subset based on the values in one of the columns (dive site in this case). The data looks like this:
site weather depth_ft depth_m vis_ft vis_m coral_safety coral_deep rate
alice rain 95 NA 50 NA 2 4 9
alice over NA 25 NA 25 2 4 9
steps clear NA 27 NA 25 2 4 9
steps NA 30 NA 20 1 4 9
andrea1 clear 60 NA 60 NA 2 4 5
I would like to create a subset of the data which contains only data for one dive site at a time (e.g. one subset for alice, one for steps, one for andrea1 etc...).
I understand that I could subset each individually using
alice <- subset(reefdata, site=="alice")
But as I have over 100 different sites to subset by would like to avoid having to individually specify each subset. I think that subset is probably not flexible enough for me to ask it to subset by a list of names (or at least not to my current knowledge of R, which is growing, but still in infancy), is there another command which I should be looking into?
Thank you

Answer 1

This will create a list that contains the subset data frames in separate list elements.
splitdat <- split(reefdata, reefdata$site)
Then if you want to access the "alice" data you can reference it like
splitdat[["alice"]]

Answer 2

I would use the plyr package.
library(plyr)
ll <- dlply(df,.variables = c("site"))
Result:
>ll
$alice
site weather depth_ft depth_m vis_ft vis_m coral_safety coral_deep rate
1 alice rain 95 NA 50 NA 2 4 9
2 alice over NA 25 NA 25 2 4 9
$andrea1
site weather depth_ft depth_m vis_ft vis_m coral_safety coral_deep rate
1 andrea1 clear 60 NA 60 NA 2 4 5
$steps
site weather depth_ft depth_m vis_ft vis_m coral_safety coral_deep rate
1 steps clear NA 27 NA 25 2 4 9
2 steps <NA> 30 NA 20 1 4 9 NA

Answer 3

split() and dlply() are perfect one shot solutions.
If you want a "step by step" procedure with a loop (which is frowned upon by many R users, but I find it helpful in order to understand what's going on), try this:
# create vector with site names, assuming reefdata$site is a factor
sites <- as.character( unique( reefdata$site ) )
# create empty list to take dive data per site
dives <- list( NULL )
# collect data per site into the list
for( i in 1:length( sites ) )
{
# subset
dive <- reefdata[ reefdata$site == sites[ i ] , ]
# add resulting data.frame to the list
dives[[ i ]] <- dive
# name the list element
names( dives )[ i ] <- sites[ i ]
}