Question

This has got to be a simple answer. I want to subset my data for testing purposes. I have a data frame where I want to keep all columns of information, just simply reduce the number of observations PER individual. So, I have a unique Identifier and about 50 individuals. I want to select only 2 individuals AND and I want to select only 500 data points from those 2.

My data frame is called wloc08. There are 50 unique IDs. I am only taking 2 of those individuals but of those 2, I’d like only 500 data points from each.

subwloc08=subset(wloc08, subset = ID %in% c("F07001","F07005"))

somewhere in this statement can I use [?

 reduced= subwloc08$ID[1:500,]

Doesn’t work.

Answer 1

You could use lapply:

do.call("rbind",
        lapply(c("F07001", "F07005"),
               function(x) wloc08[which(wloc08$ID == x)[1:500], ]))

Your command reduced = subwloc08$ID[1:500,] didn’t work since subwloc08$ID is a vector. However, reduced = subwloc08$ID[1:500] would have worked but would have returned the first 500 values of subwloc08$ID (not the whole rows of subwloc08).

If you want to run this command for the first 30 subjects, you could use unique(wloc08$ID)[1:30] instead of c("F07001", "F07005"):

do.call("rbind",
        lapply(unique(wloc08$ID)[1:30],
               function(x) wloc08[which(wloc08$ID == x)[1:500], ]))