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:

myd         


        

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

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 

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

(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)