Question

This question is a follow up to this previous question.

I have a vector of id’s, sampleIDs.
I also have a data.table, rec_data_table, keyed by bid and containing a column,
A_IDs.list where each elements is a collection (a vector) of aIDs.

I would like to create a second data.table containing sampleIDs and where
For each aID, there is a corresponding vector of all the bIDs for which
that aID appears in the A_IDs.list column.

Example:

> rec_data_table
   bid counts names_list A_IDs.list
1: 301     21        C,E       3,NA
2: 302     21          E         NA
3: 303      5      H,E,G     8,NA,7
4: 304     10        H,D        8,4
5: 305      3          E         NA
6: 306      5          G          7
7: 307      6        B,C        2,3

> sampleIDs
[1] 3 4 8

AB.dt <- data.table(aID=sampleIDs, key="aID")

# unkown step
AB.dt[ , bIDs := ????  ]

# desired result:
> AB.dt
    aid     bIDs
1:    3  301,307
2:    4      304
3:    8  303,304

---

I tried several different lines inside the AB.dt[] call.
The closest I could get was

rec_data_table[sapply(A_IDs.list, function(lst) aID %in% lst), bID]

which will give me the desired result for a given aID, and I can lapply
over sampleIDs to create a list of vectors and build the desired result.

However, I suspect there must be a more “data.table appropriate” method to accomplish this. Any suggestions are appreciated.

---

#--------------------------------------------------#
#           SAMPLE DATA                            #

library(data.table)
set.seed(101)

  rows <- size <- 7
  varyingLengths <- c(sample(1:3, rows, TRUE))
  A <-  lapply(varyingLengths, function(n) sample(LETTERS[1:8], n))
  counts <- round(abs(rnorm(size)*12))   
rec_data_table <- data.table(bID=300+(1:size), counts=counts, names_list=A, key="bID")

A_ids.DT <- data.table(name=LETTERS[c(1:4,6:8,10:11)], id=c(1:4,6:8,10:11), key="name")
rec_data_table[, A_IDs.list := sapply(names_list, function(n) c(A_ids.DT[n, id]$id))]
sampleIDs <- c(3, 4, 8)

Answer 1

After the join of tmp to A_ids.DT in my answer to the previous question, you can get your desired output by looking up sampleIDs in tmp:

# ... from previous answer
# tmp <- A_ids.DT[tmp]

AB.dt <- setkey(tmp, id)[J(sampleIDs)][, list(bIDs = list(bID)),
                                       by = list(aid = id)]

# setkey(tmp, orig.order)
# previous answer continues ...

Note that the capitalization of your bID column is different in these two questions, however. This is assuming, of course, that you are not executing the second to last line in your sample data. This ought to be faster than %in%-based approaches when there are many records due to the wonders of data.table‘s binary search.