How to Reverse a string in R

Question

I\'m trying to teach myself R and in doing some sample problems I came across the need to reverse a string.

Here\'s what I\'ve tried so far but the paste operation d

Answer 1

Here is one more base-R solution:

# Define function
strrev <- function(x) {
  nc <- nchar(x)
  paste(substring(x, nc:1, nc:1), collapse = "")
}

# Example
strrev("Sore was I ere I saw Eros")
[1] "sorE was I ere I saw eroS"

Solution was inspired by these U. Auckland slides.

Answer 2

So apparently front-end JS developers get asked to do this (for interviews) in JS without using built-in reverse functions. It took me a few minutes, but I came up with:

string <- 'hello'

foo <- vector()

for (i in nchar(string):1) foo <- append(foo,unlist(strsplit(string,''))[i])
paste0(foo,collapse='')

Which all could be wrapped in a function...

What about higher-order functionals? Reduce?

Answer 3

The following can be a useful way to reverse a vector of strings x, and is slightly faster (and more memory efficient) because it avoids generating a list (as in using strsplit):

x <- rep( paste( collapse="", LETTERS ), 100 )
str_rev <- function(x) {
  sapply( x, function(xx) { 
    intToUtf8( rev( utf8ToInt( xx ) ) )
  } )
}
str_rev(x)

If you know that you're going to be working with ASCII characters and speed matters, there is a fast C implementation for reversing a vector of strings built into Kmisc:

install.packages("Kmisc")
str_rev(x)

Answer 4

As @mplourde points out, you want the collapse argument:

paste(test_rev, collapse='')

Most commands in R are vectorized, but how exactly the command handles vectors depends on the command. paste will operate over multiple vectors, combining the ith element of each:

> paste(letters[1:5],letters[1:5])
[1] "a a" "b b" "c c" "d d" "e e"

collapse tells it to operate within a vector instead.

Answer 5

From ?strsplit, a function that'll reverse every string in a vector of strings:

## a useful function: rev() for strings
strReverse <- function(x)
        sapply(lapply(strsplit(x, NULL), rev), paste, collapse="")
strReverse(c("abc", "Statistics"))
# [1] "cba"        "scitsitatS"

Answer 6

Here is a function that returns the whole reversed string, or optionally the reverse string keeping only the elements specified by index, counting backward from the last character.

revString = function(string, index = 1:nchar(string)){
  paste(rev(unlist(strsplit(string, NULL)))[index], collapse = "")
}

First, define an easily recognizable string as an example:

(myString <- paste(letters, collapse = ""))

[1] "abcdefghijklmnopqrstuvwxyz"

Now try out the function revString with and without the index:

revString(myString)

[1] "zyxwvutsrqponmlkjihgfedcba"

revString(myString, 1:5)

[1] "zyxwv"