How to sort letters in a string?

Question

Let's say I have a string s = "bcabca".

What is the simplest way to get "aabbcc" out of it, i.e., sort the letters in s?

Straightforward, natural, shortest :) For example, in Haskell you can say `sort "bcabca"`, and get "aabbcc". This is something I call simple :) — Leo, May 06 '11 at 03:01

score 33 · Accepted Answer · edited Feb 19 '15 at 21:05

33

Maybe not the most simple answer, but this will work:

paste(sort(unlist(strsplit(s, ""))), collapse = "")

Or modify the strReverse function that is defined in the help page for ?strsplit to suit our needs. We'll call it strSort:

strSort <- function(x)
        sapply(lapply(strsplit(x, NULL), sort), paste, collapse="")

edited Feb 19 '15 at 21:05

MichaelChirico

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answered May 05 '11 at 22:31

Chase

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1

A variation using `stri_flatten` instead of `paste`: `stri_flatten(sort(unlist(strsplit(s,""))))` – kdauria Feb 16 '16 at 00:54
1

of course the first answer fails on character _vectors_. and i suspect the second will be slower than `sapply(strsplit(x, NULL), function(x) paste(sort(x), collapse = '')` (which is already slow) – MichaelChirico May 05 '18 at 00:55

score 18 · Answer 2 · answered May 05 '11 at 23:45

18

Here's a variant of Chase's solution that handles a vector of strings and keeps the original strings as names. ...and I get a chance to promote the use of vapply over sapply :-)

> x=c('hello', 'world', NA, 'a whole sentence')
> vapply(x, function(xi) paste(sort(strsplit(xi, NULL)[[1]]), collapse=''), '')
             hello              world               <NA>   a whole sentence 
           "ehllo"            "dlorw"                 "" "  aceeeehlnnostw"

answered May 05 '11 at 23:45

Tommy

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Yes, never use sapply when you can use vapply! – hadley May 06 '11 at 02:33
Reading this again two years later, there's a very slight fix to this to make it work for vectors, see my edited answer (only after submitting the edit did I read your response again and see it's all but the same! convergent evolution...) – MichaelChirico May 12 '17 at 19:13

Rich Scriven · Answer 3 · 2015-02-19T21:39:07.160

8

It might be good to mention the stringi package for this problem. It's stri_order and stri_sort functions are very efficient, testing at half the time of the base R method mentioned above.

library(stringi)
## generate 10k random strings of 100 characters each
str <- stri_rand_strings(1e4, 100)
## helper function for vapply()
striHelper <- function(x) stri_c(x[stri_order(x)], collapse = "")
## timings
system.time({
  v1 <- vapply(stri_split_boundaries(str, type = "character"), striHelper, "")
})
#    user  system elapsed 
#   0.747   0.000   0.743 

system.time({
  v2 <- sapply(lapply(strsplit(str, NULL), sort), paste, collapse="")
})
#    user  system elapsed 
#   2.077   0.000   2.068 

identical(v1, v2)
# [1] TRUE

edited Feb 19 '15 at 21:39

answered Feb 19 '15 at 21:27

Rich Scriven

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is `stringi` passing things through C to speed up? – MichaelChirico Feb 19 '15 at 22:48
1

@MichaelChirico - Yes, the package is written almost entirely in C – Rich Scriven Feb 19 '15 at 23:28
1

I'll add that for `tidyverse` users `stringr::str_order` wraps `stri_order` so you don't need to explicitly load another package when you already have `stringr` loaded by `tidyverse`. – Calum You May 05 '18 at 01:00

MichaelChirico · Answer 4 · 2018-05-05T00:56:16.780

4

Revisiting this, my old answer wasn't so good. Here's a better version with base functions:

vapply(strsplit(x, NULL), function(x) paste(sort(x), collapse = ''), '')

Based off this test vector:

NN = 1000000L
starts = seq(1L, NN, by = 100L)
name = 
  substring(paste(sample(letters, size = NN, replace = TRUE), collapse = ""),
            starts, starts + 99L)

edited May 05 '18 at 00:56

answered Feb 19 '15 at 19:55

MichaelChirico

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How to sort letters in a string?

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