How to store datasets in a list in a loop - R

Question

Suppose I have dataset A including information on the month ("Date"):

station ID	precipitation (mm)	LONG	LAT	Date
1	70	5	50	2010-01
1	60	5	50	2010-02
1	61	5	50	2010-03
2	75	10	47	2010-01
2	65	10	47	2010-02
2	70	10	47	2010-03

I have a while loop that creates separate datasets from dataset A based on the month. My aim is to create a list list_months that stores all information of each dataset (i.e. month).

#used as criterium for selecting data per loop
months = c("2010-01", "2010-02","2010-03")

#used in different variable names, based on month
abr = c("jan", "feb", "mar")

#set item from list to 1 (=January)
i = 1
j = 1
#create empty list to store individual datasets that are generated for each loop
list_months = list()

#while loop. Aim: create 3 separate datasets, based on month, and store each one per loop
while(i <= length(months) && j <= length(abr))
{
  #store each dataset to a different variable name, corresponding to the month
  assign(paste("dataset", abr[j], sep="_"),subset(A, Date == months[i])) 
  
  #assign variable name to variable that is appended to list
  ap <- paste("dataset", abr[j], sep="_")

  #append variable name to list for further data processing
  list_months <- append(list_months, ap)

  #next loop (i.e. following month)
  i = i+1
  j = j+1
}

However, when I try to view the first item via view(list_months[1]) the output is as follows:

	X.dataset_jan.
1	dataset_jan

How can I store the variable name in a list in such a way that it creates the output:

station ID	precipitation (mm)	LONG	LAT	Date
1	70	5	50	2010-01
2	75	10	47	2010-01

Creating a new list after the loop list_months = list(dataset_jan, dataset_feb, dataset_mar) does the trick. However, I would like to store the datasets during each loop.

Thanks

Answer 1

this is quite strfaightformward using data.table's split

library(data.table)
# make sure your data in in a data.table format
setDT(mydata)
# use split.data.table to split on a certain column
split(mydata, by = "Date")
# $`2010-01`
# station_ID precipitation LONG LAT    Date
# 1:          1            70    5  50 2010-01
# 2:          2            75   10  47 2010-01
# 
# $`2010-02`
# station_ID precipitation LONG LAT    Date
# 1:          1            60    5  50 2010-02
# 2:          2            65   10  47 2010-02
# 
# $`2010-03`
# station_ID precipitation LONG LAT    Date
# 1:          1            61    5  50 2010-03
# 2:          2            70   10  47 2010-03