Apply function to a row in a data.frame using dplyr

Question

In base R I would do the following:

d <- data.frame(a = 1:4, b = 4:1, c = 2:5)
apply(d, 1, which.max)

With dplyr I could do the following:

library(dplyr)
d %>% mutate(u = purrr::pmap_int(list(a, b, c), function(...) which.max(c(...))))

If there’s another column in d I need to specify it, but I want this to work w/ an arbitrary amount if columns.

Conceptually, I’d like something like

pmap_int(list(everything()), ...)
pmap_int(list(.), ...)

But this does obviously not work. How would I solve that canonically with dplyr?

akrun · Accepted Answer · 2021-04-03T19:41:43.253

8

We just need the data to be specified as . as data.frame is a list with columns as list elements. If we wrap list(.), it becomes a nested list

library(dplyr)
d %>% 
  mutate(u = pmap_int(., ~ which.max(c(...))))
#  a b c u
#1 1 4 2 2
#2 2 3 3 2
#3 3 2 4 3
#4 4 1 5 3

Or can use cur_data()

d %>%
   mutate(u = pmap_int(cur_data(), ~ which.max(c(...))))

Or if we want to use everything(), place that inside select as list(everything()) doesn't address the data from which everything should be selected

d %>% 
   mutate(u = pmap_int(select(., everything()), ~ which.max(c(...))))

Or using rowwise

d %>%
    rowwise %>% 
    mutate(u = which.max(cur_data())) %>%
    ungroup
# A tibble: 4 x 4
#      a     b     c     u
#  <int> <int> <int> <int>
#1     1     4     2     2
#2     2     3     3     2
#3     3     2     4     3
#4     4     1     5     3

Or this is more efficient with max.col

max.col(d, 'first')
#[1] 2 2 3 3

Or with collapse

library(collapse)
dapply(d, which.max, MARGIN = 1)
#[1] 2 2 3 3

which can be included in dplyr as

d %>% 
    mutate(u = max.col(cur_data(), 'first'))

edited Apr 03 '21 at 19:41

answered Apr 03 '21 at 19:17

akrun

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1

I could have sworn that I tried `pmap_int(., ...)`, but overall `cur_data()` is what I was eventually looking for (also for some other use cases). Thx! – thothal Apr 03 '21 at 19:47
@thothal the `...` will be within a lambda call – akrun Apr 03 '21 at 19:48
1

Thank you dear @akrun for this thorough explanation. I could've solved this but only in a single `pmap` form. Your valuable explanations just gave me so much insight in this kinda solutions. Thank you very much. – Anoushiravan R Apr 03 '21 at 20:25
1

You almost covered everything and left no rooms to other answers :) Cool! +1 – ThomasIsCoding Apr 03 '21 at 20:52
2

Haha, I forgot `data.table` here. Thanks for reminder! I added that :) – ThomasIsCoding Apr 03 '21 at 21:05
1

@akrun, of course. I was just to lazy to type the full function call, hence I abbreviated it to `(., ...)` being well aware that this is not the proper code ;) – thothal Apr 06 '21 at 06:52
Can you explain the `c(...)` ? – Julien Jul 26 '22 at 15:31
Is this equivalent to `d%>% mutate(u = pmap_int(., ~ which.max(.x)))` ? – Julien Jul 26 '22 at 15:32
Apprently the code above is not equivalent to your solution – Julien Jul 26 '22 at 15:36

score 3 · Answer 2 · answered Apr 03 '21 at 21:04

3

Here are some data.table options

setDT(d)[, u := which.max(unlist(.SD)), 1:nrow(d)]

or

setDT(d)[, u := max.col(.SD, "first")]

answered Apr 03 '21 at 21:04

ThomasIsCoding

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Apply function to a row in a data.frame using dplyr

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