访问gregexpr的结果

Question

I would like to use the gregexpr function to find the start and finish positions of substrings within a string. The function works fine in the console, but I cannot access the results for either the start positions or string lengths:

g <- gregexpr("e", "cheese")

g

[[1]]
[1] 3 4 6
attr(,"match.length")
[1] 1 1 1
attr(,"index.type")
[1] "chars"
attr(,"useBytes")
[1] TRUE

g[[1]][1] merely brings up the first value (3), but I need to create a vector with ALL values for both start positions and length. Thanks.

Answer 1

You can use unlist and you will receive list of position. Once need to have only first and last, min and max can be used

unlist(g)

[1] 3 4 6

Answer 2

You can extract them in this way :

g <- gregexpr("e", "cheese")

# one liner for : starts <- g[[1]]
#                 attributes(starts) <- NULL
starts <- `attributes<-`(g[[1]],NULL) 

lens <- attr(g[[1]],'match.length')

> starts
[1] 3 4 6
> lens
[1] 1 1 1

Of course this works only if text is of length 1 (as in the example, since it contains only "cheese" ). Otherwise you'll need to iterate over the elements of g using g[[2]] , g[[3]] ... etc.

Answer 3

Another approach would be:

g <- gregexpr("e", "cheese")

g[[1]][1:length(g[[1]])]
#[1] 3 4 6

And the microbenchmarking with the unlist approach:

microbenchmark::microbenchmark(
   g[[1]][1:length(g[[1]])], 
   unlist(g)
)

#Unit: nanoseconds
#                     expr min  lq   mean median  uq   max neval
# g[[1]][1:length(g[[1]])] 378 378 653.80    379 756  8307   100
#                unlist(g)   0 378 544.32    378 378 15104   100