filling in a data.table with information from multiple objects in R

Question

Within a large function, I would like to create a "summary table" of sorts. This summary table, summaries information from multiple R objects, that have been created within the function. The objects are:

Data table with the information on the limit

>     str(limit)
Classes ‘data.table’ and 'data.frame':  1 obs. of  3 variables:
 $ id   : num 6292
 $ type : chr "DAILY"
 $ value: chr "350"
 - attr(*, ".internal.selfref")=<externalptr>

vector with position of the element in mydata that is over the limit limits$value

n <- which(mydata$amount > as.double(limit$value))
str(n)
int [1:4960] 1 2 3 5 6 9 11 16 19 20 ...

I have now created an empty data.table problem with rows that I want to use to summarise the elements that are over the limit in mydata :

  problem <- data.table("LIMITid" = character(),
                        "LIMITtype" = character (), 
                        "LIMITvalue" = character (), 
                        "amount" = double(), 
                        "customerID" = character())

Finally, i want to populate my problem data.table with the corresponding information. I tried:

if(length(n) > 0){
 problem$LIMITid <- limit$id
 problem$LIMITtype <- limit$type
 problem$LIMITvalue <- limit$value
 problem$amount <- mydata$amount
 problem$customerID <- mydata$customerID
}

How can i populate the data.table ? I was thinking of using a loop, but i am unsure how to loop over positions in an element - n %in% nrownames(mydata) ?

Answer 1

We can specify the columns of interest in .SDcols and then subset the .SD (Subset of Data.table) with the index provided by 'n' and cbind with the 'limits' dataset

library(data.table)
cbind(limits, mydata[, .SD[n], .SDcols = c("amount", "customerID")])

Answer 2

Wouldn't this work?

library(data.table)
data.table(limits, amount = mydata$amount[n], customerID = mydata$customerID[n])