Change identifier by looping through folders -- R - r

Question

I have a loop related question. I have the following folder structure (excerpt):
├───Y2017
│ UDB_cSK17D.csv
│ UDB_cSK17H.csv
│ UDB_cSK17P.csv
│ UDB_cSK17R.csv
│ UDB_cUK17D.csv
│ UDB_cUK17H.csv
│ UDB_cUK17P.csv
│ UDB_cUK17R.csv
└───Y2018
│ UDB_cSK18D.csv
│ UDB_cSK18H.csv
│ UDB_cSK18P.csv
│ UDB_cSK18R.csv
│ UDB_cUK18D.csv
│ UDB_cUK18H.csv
│ UDB_cUK18P.csv
│ UDB_cUK18R.csv
All the files have the same structure. I would like to loop through them and extract data from a select number of columns. The file names also all have the same structure. All files have:
unique country identified (e.g. UK, SK in the examples above)
unique database type (D, H, P... - last character in file name)
I would like to construct a loop that iterates through the file names. For one country this would work like this:
library(data.table)
ldf<-list()
country_id<-"UK(.*)"
db_id<-"P.csv$"
listcsv<-dir(pattern = paste0(country_id,db_id), recursive = T, full.names = T)
for (k in 1:length(listcsv)){
ldf[[k]]<-fread(listcsv[k],select = c("PB010","PB020"))
}
uk_data<-bind_rows(as.data.frame(do.call(rbind,ldf[])))
This code extract all the columns I need based on the country identifier I give it (UK in this example). As I have numerous countries in my data set I would like to have a code that iterates through and updates the country identifier. I have tried the following:
ldf_new<-list()
countries <-c("SK", "UK")
for (i in 1:length(countries)) {
currcty1 <- countries[i]
listcsv<-dir(pattern = paste0(currcty1,"(.*)",db_id), recursive = T, full.names = T)
# print(listcsv)
ldf_new<-fread(listcsv[i],select = c("PB010","PB020"))
}
What happens here is that I get only the results of the last iteration in the variable ldf_new (i.e. UK in this case). Is there any way I could get the results for SK and UK.
Many thanks in advance!

Answer 1

Changing the last line of your loop so that a new element is added to the list should do the trick:
ldf_new<-list()
countries <-c("SK", "UK")
for (i in 1:length(countries)) {
currcty1 <- countries[i]
listcsv<-dir(pattern = paste0(currcty1,"(.*)",db_id), recursive = T, full.names = T)
# print(listcsv)
ldf_new<-c(ldf_new, fread(listcsv[i],select = c("PB010","PB020")))
}