Apply over rows of data.table: find rows where a subset of columns are all NA

Question

I am trying, in my quest to rewrite old (slow) code with the data.table package, to figure out the best way to use apply with a data.table.

I have a data.table with multiple id columns, then multiple columns that have dose-response data in a wide format. I need to generalize the answer because not all data.tables will have the same number of dose-response columns. For simplicity I think the following data.table addresses the issue:

library(data.table)
library(microbenchmark)
set.seed(1234)
DT1 =  data.table(unique_id = paste0('id',1:1e6),
                 dose1 = sample(c(1:9,NA),1e6,replace=TRUE),
                 dose2 = sample(c(1:9,NA),1e6,replace=TRUE)
                 )

> DT1
          unique_id dose1 dose2
       1:       id1     2     2
       2:       id2     7     4
       3:       id3     7     9
       4:       id4     7     4
       5:       id5     9     3
---                      
  999996:  id999996     4     3
  999997:  id999997    NA     3
  999998:  id999998     4     2
  999999:  id999999     8     5
 1000000: id1000000     6     7

So each row has a unique id, some other ids, and I have left out the response columns, because they will be NA where the dose columns are NA. What I need to do is remove rows where all of the dose columns are NA. I came up with the first option, then realized I could trim it down to the second option.

DT2 <- copy(DT1)
DT3 <- copy(DT1)

len.not.na <- function(x){length(which(!is.na(x)))}

option1 <- function(DT){
  DT[,flag := apply(.SD,1,len.not.na),.SDcols=grep("dose",colnames(DT))]
  DT <- DT[flag != 0]
  DT[ , flag := NULL ]
}

option2 <- function(DT){
  DT[ apply(DT[,grep("dose",colnames(DT)),with=FALSE],1,len.not.na) != 0 ]
}

> microbenchmark(op1 <- option1(DT2), op2 <- option2(DT3),times=25L)
Unit: seconds
                expr      min       lq   median       uq      max neval
 op1 <- option1(DT2) 8.364504 8.863436 9.145341 11.27827 11.50356    25
 op2 <- option2(DT3) 8.291549 8.774746 8.982536 11.15269 11.72199    25

Clearly they two options do about the same thing, with option 1 having a few more steps, but I wanted to test how calling .SD might slow things down as has been suggested by other posts (for example).

Either way both options are still on the slow side. Any suggestions to speeding things up?

EDIT with comment from @AnandaMahto

DT4 <- copy(DT1)
option3 <- function(DT){
  DT[rowSums(DT[,grep("dose",colnames(DT)),with=FALSE]) != 0]
}

> microbenchmark(op2 <- option2(DT3), op3 <- option3(DT4),times=5L)
Unit: milliseconds
               expr        min         lq    median        uq       max neval
op2 <- option2(DT3) 7738.21094 7810.87777 7838.6067 7969.5543 8407.4069     5
op3 <- option3(DT4)   83.78921   92.65472  320.6273  559.8153  783.0742     5

rowSums is definitely faster. I am happy with the solution unless anyone has something faster.

Answer 1

My approach would be as follows:

Use rowSums to find the rows you want to keep:

Dose <- grep("dose", colnames(DT1))
# .. menas "up one level
Flag <- rowSums(is.na(DT1[, ..Dose])) != length(Dose)
DT1[Flag]