Date: 2018-11-30

# Before and After

#### 2018-11-30

Often, we want the part of a string that comes before or after a given pattern.

    library(strex)
    #> Loading required package: stringr

## Before

`str_before_nth()` gives you the part of a string before the `n`th appearance of a pattern. It has the friends `str_before_first()` and `str_before_last()`.

    string <- "ab..cd..de..fg..h"
    str_before_first(string, "e")
    #> [1] "ab..cd..d"
    str_before_nth(string, "\\.", 3)
    #> [1] "ab..cd"
    str_before_last(string, "\\.")
    #> [1] "ab..cd..de..fg."
    str_before_nth(string, ".", -3)
    #> [1] "ab..cd..de..fg"
    str_before_nth(rep(string, 2), fixed("."), -3)
    #> [1] "ab..cd..de." "ab..cd..de."

## After

`str_after_nth()` gives you the part of a string after the `n`th appearance of a pattern. It has the friends `str_after_first()` and `str_after_last()`.

    string <- "ab..cd..de..fg..h"
    str_after_first(string, "e")
    #> [1] "..fg..h"
    str_after_nth(string, "\\.", 3)
    #> [1] ".de..fg..h"
    str_after_last(string, "\\.")
    #> [1] "h"
    str_after_nth(string, ".", -3)
    #> [1] ".h"
    str_after_nth(rep(string, 2), fixed("."), -3)
    #> [1] "fg..h" "fg..h"

## A more concrete example

    string <- "James did the cooking, Harry did the cleaning."

Let’s write a function to figure out which task each of the lads did.

    library(magrittr)
    get_task <- function(string, name) {
      str_c(name, " did the ") %>%
        str_after_first(string, .) %>%
        str_before_first("[\\.,]")
    }
    get_task(string, "James")
    #> [1] "cooking"
    get_task(string, "Harry")
    #> [1] "cleaning"

`get_task()` would have been more difficult to write without `str_after_first()` and `str_before_first()`.