[r] Populating a data frame in R in a loop

I had a case in where I was needing to use a data frame within a for loop function. In this case, it was the "efficient", however, keep in mind that the database was small and the iterations in the loop were very simple. But maybe the code could be useful for some one with similar conditions.

The for loop purpose was to use the raster extract function along five locations (i.e. 5 Tokio, New York, Sau Paulo, Seul & Mexico city) and each location had their respective raster grids. I had a spatial point database with more than 1000 observations allocated within the 5 different locations and I was needing to extract information from 10 different raster grids (two grids per location). Also, for the subsequent analysis, I was not only needing the raster values but also the unique ID for each observations.

After preparing the spatial data, which included the following tasks:

1. Import points shapefile with the readOGR function (rgdap package)
2. Import raster files with the raster function (raster package)
3. Stack grids from the same location into one file, with the function stack (raster package)

Here the for loop code with the use of a data frame:

1. Add stacked rasters per location into a list

raslist <- list(LOC1,LOC2,LOC3,LOC4,LOC5)

2. Create an empty dataframe, this will be the output file

TB <- data.frame(VAR1=double(),VAR2=double(),ID=character())

3. Set up for loop function

L1 <- seq(1,5,1) # the location ID is a numeric variable with values from 1 to 5 

for (i in 1:length(L1)) {
  dat=subset(points,LOCATION==i) # select corresponding points for location [i] 
  t=data.frame(extract(raslist[[i]],dat),dat$ID) # run extract function with points & raster stack for location [i]
  names(t)=c("VAR1","VAR2","ID") 
  TB=rbind(TB,t)
}