Split string based on alternating character in R

Question

I\'m trying to figure out an efficient way to go about splitting a string like

\"111110000011110000111000\"

into a vector

[1] \         


        

Answer 1

It's not really what the OP was looking for (concise R code), but thought I'd give it a try in Rcpp, and turned out relatively simple and about 5x faster than the fastest R-based answers.

library(Rcpp)

cppFunction(
  'std::vector split_str_cpp(std::string x) {

  std::vector parts;

  int start = 0;

  for(int i = 1; i <= x.length(); i++) {
      if(x[i] != x[i-1]) {
        parts.push_back(x.substr(start, i-start));
        start = i;
      } 
  }

  return parts;

  }')

And testing on these

str1 <- "111110000011110000111000"
x1 <- "1111100000222000333300011110000111000"
x2 <- "aaaaabbcccccccbbbad1111100000222aaabbccd11DaaBB"

Gives the following output

> split_str_cpp(str1)
[1] "11111" "00000" "1111"  "0000"  "111"   "000"  
> split_str_cpp(x1)
 [1] "11111" "00000" "222"   "000"   "3333"  "000"   "1111"  "0000"  "111"   "000"  
> split_str_cpp(x2)
 [1] "aaaaa"   "bb"      "ccccccc" "bbb"     "a"       "d"       "11111"   "00000"   "222"     "aaa"     "bb"      "cc"      "d"       "11"     
[15] "D"       "aa"      "BB"   

And a benchmark shows it's about 5-10x faster than R solutions.

akrun <- function(str1) strsplit(str1, '(?<=1)(?=0)|(?<=0)(?=1)', perl=TRUE)[[1]]

richard1 <- function(x3){
  cs <- cumsum(
    rle(stri_split_boundaries(x3, type = "character")[[1L]])$lengths
  )
  stri_sub(x3, c(1, head(cs + 1, -1)), cs)
}

richard2 <- function(x3) {
  cs <- cumsum(rle(strsplit(x3, NULL)[[1L]])[[1L]])
  stri_sub(x3, c(1, head(cs + 1, -1)), cs)
}

library(microbenchmark)
library(stringi)

set.seed(24)
x3 <- stri_rand_strings(1, 1e6)

microbenchmark(split_str_cpp(x3), akrun(x3), richard1(x3), richard2(x3), unit = 'relative', times=20L)

Comparison:

Unit: relative
              expr      min       lq     mean   median       uq      max neval
 split_str_cpp(x3) 1.000000 1.000000 1.000000 1.000000 1.000000 1.000000    20
         akrun(x3) 9.675613 8.952997 8.241750 8.689001 8.403634 4.423134    20
      richard1(x3) 5.355620 5.226103 5.483171 5.947053 5.982943 3.379446    20
      richard2(x3) 4.842398 4.756086 5.046077 5.389570 5.389193 3.669680    20