r - Rearrange words in strings depending on conditions

Question

I have this data:

df<- data.frame("position" = c("ante", "ex", "post", "post ante pre", "post pre", "ante post pre", "ex pre", "ante pre")) 

Now I want to move the word "pre" so that it's the first word in the string, but only for the strings containing two words and the word "pre", so row numbers 1, 2, 3, 4 and 6 should not be affected.

This should be the result:

df <- data.frame("position" = c("ante", "ex", "post", "post ante pre", "pre post", "ante post pre", "pre ex", "pre ante"))

I guess I can start by writing a grepl statement to only select the rows containing the word "pre" but after that I'm a bit lost.

question from:https://stackoverflow.com/questions/65830302/rearrange-words-in-strings-depending-on-conditions

Answer 1

You can use regex for this:

First I edited your example so that the starting and desired results are different (assuming this is your desired result here based on what you wrote)

library(dplyr)
library(stringr)


df <- data.frame("position" = c("ante", "ex", "post", "post pre ante", "post pre", "ante post pre", "ex pre", "pre ante")) 


df
#>        position
#> 1          ante
#> 2            ex
#> 3          post
#> 4 post pre ante
#> 5      post pre
#> 6 ante post pre
#> 7        ex pre
#> 8      pre ante
df2 <- data.frame("position" = c("ante", "ex", "post", "post pre ante", "pre post", "ante post pre", "pre ex", "pre ante"))
df2
#>        position
#> 1          ante
#> 2            ex
#> 3          post
#> 4 post pre ante
#> 5      pre post
#> 6 ante post pre
#> 7        pre ex
#> 8      pre ante

Then using regex:

df3 <- df %>%
  mutate(position = str_replace(position,'^([^\s]+) {1}(?=pre$)(pre)','\2 \1'))

df3
#>        position
#> 1          ante
#> 2            ex
#> 3          post
#> 4 post pre ante
#> 5      pre post
#> 6 ante post pre
#> 7        pre ex
#> 8      pre ante

identical(df2, df3)
#> [1] TRUE

Slight edit: I think the lookahead was unnecessary so we can reduce this to:

df3 <- df %>%
  mutate(position = str_replace(position,'^([^\s]+) {1}(pre)$','\2 \1'))