Initialize a nested data frame in R

Question

I have a nested data table called ldt. The highest dimension in each direction is

ldt[[44]][7200,4]

Meaning each nested data.table (DT) is 7200 rows by 4 columns, and there are 44 such nested DTs.

I want to create a second set of DTs based on the first.

Specifically, I want to execute the command

for (i in length(ldt)) {ldtSUM[[i]]<-ldt[[i]][, sum(V4), by="V1,V3"]}

If successful, this should generate a nested array of dimensionality

ldtSUM[[44]][120,4]

but instead doing so results in the error:

Error in ldtSUM[[i]] <- ldt[[i]][, sum(V4), by = "V1,V3"] : object 'ldtSUM' not found

presumably because I never initialized it.

So, to circumvent this, I attempted a variety of initialization techniques such as

ldtSUM=ldt[[1:44]][0,]

and many other statements, but they all failed for a variety of reasons, such as that I did not specify a number of empty rows, or "recursive indexing failed at level 3" etc.

So, in summary, what I would like to do is create a data.table wherein $V4 is summed by $V1,$V3 but I cannot seem to do this due to indexing and initialization issues.

Thank you very much!

Answer 1

根据agstudy的答案，正确的代码是：

ldtSUM<-lapply(ldt,function(x)x[, sum(V4), by = "V1,V3"]

Answer 2

You had two parts to your problem: The first part you have already solved by:

ldtSUM<-lapply(ldt,function(x)x[, sum(V4), by = "V1,V3"]

The second part is on initialization, and that can be solved by following:

ldtSUM <- list()

After this initialization if you run your original code:

for (i in length(ldt)) {ldtSUM[[i]]<-ldt[[i]][, sum(V4), by="V1,V3"]}

should also work.

Note: For your problem, the solution suggested by agstudy is best. But the initialization is a general trick that is useful in variety of situations.