CODEWITHSUNDEEP
rdplyrgroup-by
Robin Kohrs
Robin Kohrs

Reputation: 697

Filter groups in grouped R dataframe by their group-index

Let's say i have the mtcars dataframe and I group the observations by the cyl. Now I want to select all the obsevations which fall in the first group. I'm not really sure how I can do that. I tried something like:

mtcars %>% 
  group_by(cyl) %>% 
  slice_head(n=1)

But I think this gives me the first observation of each group. I instead want all the observations from the first group.

Upvotes: 0

Views: 178

Answers (1)

Ronak Shah
Ronak Shah

Reputation: 389055

To get all the rows in the first group you can use cur_group_id() == 1.

library(dplyr)
mtcars %>%  group_by(cyl) %>% filter(cur_group_id() == 1)

#    mpg   cyl  disp    hp  drat    wt  qsec    vs    am  gear  carb
#   <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
# 1  22.8     4 108      93  3.85  2.32  18.6     1     1     4     1
# 2  24.4     4 147.     62  3.69  3.19  20       1     0     4     2
# 3  22.8     4 141.     95  3.92  3.15  22.9     1     0     4     2
# 4  32.4     4  78.7    66  4.08  2.2   19.5     1     1     4     1
# 5  30.4     4  75.7    52  4.93  1.62  18.5     1     1     4     2
# 6  33.9     4  71.1    65  4.22  1.84  19.9     1     1     4     1
# 7  21.5     4 120.     97  3.7   2.46  20.0     1     0     3     1
# 8  27.3     4  79      66  4.08  1.94  18.9     1     1     4     1
# 9  26       4 120.     91  4.43  2.14  16.7     0     1     5     2
#10  30.4     4  95.1   113  3.77  1.51  16.9     1     1     5     2
#11  21.4     4 121     109  4.11  2.78  18.6     1     1     4     2

Note that the output is different from the 1st value in cyl column.

mtcars %>%  filter(cyl == first(cyl))

#                mpg cyl  disp  hp drat    wt  qsec vs am gear carb
#Mazda RX4      21.0   6 160.0 110 3.90 2.620 16.46  0  1    4    4
#Mazda RX4 Wag  21.0   6 160.0 110 3.90 2.875 17.02  0  1    4    4
#Hornet 4 Drive 21.4   6 258.0 110 3.08 3.215 19.44  1  0    3    1
#Valiant        18.1   6 225.0 105 2.76 3.460 20.22  1  0    3    1
#Merc 280       19.2   6 167.6 123 3.92 3.440 18.30  1  0    4    4
#Merc 280C      17.8   6 167.6 123 3.92 3.440 18.90  1  0    4    4
#Ferrari Dino   19.7   6 145.0 175 3.62 2.770 15.50  0  1    5    6

Upvotes: 4

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