In R: how to sum a variable by group between two dates

Question

I have two data frames (DF1 and DF2):

(1) DF1 contains information on individual-level, i.e. on 10.000 individuals nested in 30 units across 11 years (2000

Answer 1

You were almost there, I just modified slightly your for loop, and also made sure that the date variables are considered as such:

DF1$date1 = as.Date(DF1$date1,"%Y-%m-%d")
DF1$date2 = as.Date(DF1$date2,"%Y-%m-%d")
DF2$date = as.Date(DF2$date,"%Y-%m-%d")

for(i in 1:nrow(DF1)){
  DF1$newvar[i] <-sum(DF2$x[which(DF2$unit == DF1$unit[i] & 
                                  DF2$date>= DF1$date1[i] &
                                  DF2$date<= DF1$date2[i])]) 
}

The problem was, that you were asking DF2$date to be simultaneously == DF1$date1 & DF1$date2. And also, length(DF1) gives you the number of columns. To have the number of rows you can either use nrow(DF1), or dim(DF1)[1].