Extracting nth element from a nested list following strsplit - R

Question

I've been trying to understand how to deal with the output of strsplit a bit better. I often have data such as this that I wish to split:

mydata <- c("144/4/5", "154/2", "146/3/5", "142", "143/4", "DNB", "90")

#[1] "144/4/5" "154/2"   "146/3/5" "142"     "143/4"   "DNB"     "90"     

After splitting that the results are as follows:

strsplit(mydata, "/")

#[[1]]
#[1] "144" "4"   "5"  

#[[2]]
#[1] "154" "2"  

#[[3]]
#[1] "146" "3"   "5"  

#[[4]]
#[1] "142"

#[[5]]
#[1] "143" "4"  

#[[6]]
#[1] "DNB"

#[[7]]
#[1] "90"

I know from the strsplit help guide that final empty strings are not produced. Therefore, there will be 1, 2 or 3 elements in each of my results based on the number of "/" to split by

Getting the first element is very trivial:

sapply(strsplit(mydata, "/"), "[[", 1)

#[1] "144" "154" "146" "142" "143" "DNB" "90" 

But I am not sure how to get the 2nd, 3rd... when there are these unequal number of elements in each result.

sapply(strsplit(mydata, "/"), "[[", 2)

# Error in FUN(X[[4L]], ...) : subscript out of bounds

I would hope to return from a working solution, the following:

#[1] "4" "2" "3" "NA" "4" "NA" "NA" 

This is a relatively small example. I could do some for loop very easily on these data, but for real data with 1000s of observations to run the strsplit on and dozens of elements produced from that, I was hoping to find a more generalizable solution.

Answer 1

Try this:

> read.table(text = mydata, sep = "/", as.is = TRUE, fill = TRUE)
   V1 V2 V3
1 144  4  5
2 154  2 NA
3 146  3  5
4 142 NA NA
5 143  4 NA
6 DNB NA NA
7  90 NA NA

If you want to treat DNB as an NA then add the argument na.strings="DNB" .

If you really want to use strsplit then try this:

> do.call(rbind, lapply(strsplit(mydata, "/"), function(x) head(c(x,NA,NA), 3)))
     [,1]  [,2] [,3]
[1,] "144" "4"  "5" 
[2,] "154" "2"  NA  
[3,] "146" "3"  "5" 
[4,] "142" NA   NA  
[5,] "143" "4"  NA  
[6,] "DNB" NA   NA  
[7,] "90"  NA   NA  

Note: Using alexis_laz's observation that x[i] returns NA if i is not in 1:length(x) the last line of code above could be simplified to:

t(sapply(strsplit(mydata, "/"), "[", 1:3))

Answer 2

(at least regarding 1D vectors) [ seems to return NA when "i > length(x)" whereas [[ returns an error.

x = runif(5)
x[6]
#[1] NA
x[[6]]
#Error in x[[6]] : subscript out of bounds

Digging a bit, do_subset_dflt (i.e. [) calls ExtractSubset where we notice that when a wanted index ("ii") is "> length(x)" NA is returned (a bit modified to be clean):

if(0 <= ii && ii < nx && ii != NA_INTEGER)
    result[i] = x[ii];
else
    result[i] = NA_INTEGER;

On the other hand do_subset2_dflt (i.e. [[) returns an error if the wanted index ("offset") is "> length(x)" (modified a bit to be clean):

if(offset < 0 || offset >= xlength(x)) {
    if(offset < 0 && (isNewList(x)) ...
    else errorcall(call, R_MSG_subs_o_b);
}

where #define R_MSG_subs_o_b _("subscript out of bounds")

(I'm not sure about the above code snippets but they do seem relevant based on their returns)

Answer 3

You could use regex (if it is allowed)

 library(stringr)
 str_extract(mydata , perl("(?<=\\d/)\\d+"))
 #[1] "4" "2" "3" NA  "4" NA  NA 
 str_extract(mydata , perl("(?<=/\\d/)\\d+"))
#[1] "5" NA  "5" NA  NA  NA  NA 

Answer 4

You can assign the length inside sapply, resulting in NA where the current length is shorter than the assigned length.

s <- strsplit(mydata, "/")
sapply(s, function(x) { length(x) <- 3; x[2] })
# [1] "4" "2" "3" NA  "4" NA  NA 

Then you can add a second indexing argument with mapply

m <- max(sapply(s, length))
mapply(function(x, y, z) { length(x) <- z; x[y] }, s, 2, m)
# [1] "4" "2" "3" NA  "4" NA  NA