CODEWITHSUNDEEP
rdplyr
W Barker
W Barker

Reputation: 312

Test the last value dyplr::mutated for calculating the current value

I am trying to mutate a new column in mtcars, but make the new entry (row) dependent in on the just-previous row in the same column (the example using mtcars is admittedly nonsense).

Simplify mtcars with...

m.subset <- mtcars[2:7, 10:11]  # top few rows and only the relevant columns

Similar questions have been asked...

Reuse value of previous row during dplyr::mutate

use the diff function with mutate at from dplyr

...and others, but all seem to refer to existing columns, not the subject one -- or they refer to multiple columns/rows as they currently exist.

For example, neither of the following work:

mtcars1 <- mtcars %>% # doesn't work
  mutate (
    newcol = gear + carb + 
      ifelse(shift(newcol, n=1, type = "lag") >4, 0, 1)
  )

mtcars1 <- mtcars %>% # doesn't work
  mutate (
    newcol = gear + carb + 
      ifelse(shift(., n=1, type = "lag") >4, 0, 1)
  )

...and while the following works, it's wrong:

mtcars1 <- mtcars %>% # works but wrong
  mutate (
    newcol = gear + carb + 
      ifelse(lag(.) >4, 0, 1)
  )

Result I am looking for is:

enter image description here

Upvotes: 1

Views: 75

Answers (1)

AndrewGB
AndrewGB

Reputation: 16856

One option doing it in two steps:

library(dplyr)

m.subset %>%
  mutate(newcol = ifelse(gear + carb <= 4, TRUE, FALSE),
         newcol = gear + carb + lag(newcol))

Output

                  gear carb temp
Mazda RX4 Wag        4    4   NA
Datsun 710           4    1    5
Hornet 4 Drive       3    1    4
Hornet Sportabout    3    2    6
Valiant              3    1    4
Duster 360           3    4    8

Upvotes: 2

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