7.5 Other useful tools

7.5.1 imap()

imap_xxx(x, ...) is short hand for map2(x, names(x), ...)if x has names, or map2(x, seq_along(x), ...) if it does not. This is useful if you need to compute on both the value and the position of an element.

Note that when using the formula shortcut, .x is the value, and the .y is the position:

x <- c("sheldon" = 150, "leonard" = 140, "raj" = 145, "howard" = 140)
imap(x, str_c)
#> $sheldon
#> [1] "150sheldon"
#> 
#> $leonard
#> [1] "140leonard"
#> 
#> $raj
#> [1] "145raj"
#> 
#> $howard
#> [1] "140howard"
imap(x, ~ str_c(.y, .x))
#> $sheldon
#> [1] "sheldon150"
#> 
#> $leonard
#> [1] "leonard140"
#> 
#> $raj
#> [1] "raj145"
#> 
#> $howard
#> [1] "howard140"

# on a tibble
imap_chr(mtcars, ~ str_c(.y, "median:", median(.x), sep = " "))
#>                  mpg                  cyl                 disp 
#>   "mpg median: 19.2"      "cyl median: 6" "disp median: 196.3" 
#>                   hp                 drat                   wt 
#>     "hp median: 123" "drat median: 3.695"   "wt median: 3.325" 
#>                 qsec                   vs                   am 
#> "qsec median: 17.71"       "vs median: 0"       "am median: 0" 
#>                 gear                 carb 
#>     "gear median: 4"     "carb median: 2"

7.5.2 adverbs

partial wraps a function:

mean(c(10, NA, 5, 7), na.rm = TRUE)
#> [1] 7.33

my_mean <- partial(mean, na.rm = TRUE)
my_mean(c(10, NA, 5, 7))
#> [1] 7.33

negate() negates a predicate function:

lst <- list("a", 3, 22, NULL, "q", NULL)
map_lgl(lst, ~ !is.null(.))
#> [1]  TRUE  TRUE  TRUE FALSE  TRUE FALSE

is_not_null <- negate(is.null)
map_lgl(lst, is_not_null)
#> [1]  TRUE  TRUE  TRUE FALSE  TRUE FALSE

safely() and possibly():

add_ten <- function(x) {
  x + 10
}

add_ten_safely <- safely(add_ten)
map(lst, add_ten_safely)
#> [[1]]
#> [[1]]$result
#> NULL
#> 
#> [[1]]$error
#> <simpleError in x + 10: non-numeric argument to binary operator>
#> 
#> 
#> [[2]]
#> [[2]]$result
#> [1] 13
#> 
#> [[2]]$error
#> NULL
#> 
#> 
#> [[3]]
#> [[3]]$result
#> [1] 32
#> 
#> [[3]]$error
#> NULL
#> 
#> 
#> [[4]]
#> [[4]]$result
#> numeric(0)
#> 
#> [[4]]$error
#> NULL
#> 
#> 
#> [[5]]
#> [[5]]$result
#> NULL
#> 
#> [[5]]$error
#> <simpleError in x + 10: non-numeric argument to binary operator>
#> 
#> 
#> [[6]]
#> [[6]]$result
#> numeric(0)
#> 
#> [[6]]$error
#> NULL

# If you’re not interested in what the error is
add_ten_possibly <- possibly(add_ten, otherwise = "not numeric")
map(lst, add_ten_possibly)
#> [[1]]
#> [1] "not numeric"
#> 
#> [[2]]
#> [1] 13
#> 
#> [[3]]
#> [1] 32
#> 
#> [[4]]
#> numeric(0)
#> 
#> [[5]]
#> [1] "not numeric"
#> 
#> [[6]]
#> numeric(0)

compose() lets you string together multiple functions

add_ten_log_and_round <- compose(round, log, add_ten)
c(1, 5, 100) %>% add_ten_log_and_round()
#> [1] 2 3 5
# is equal to
c(1, 5, 10) %>% 
  add_ten() %>% 
  log() %>% 
  round()
#> [1] 2 3 3