Often you may want to select the first row in each group using the dplyr package in R. You can use the following basic syntax to do so:

df %>%
  group_by(group_var) %>%
  arrange(values_var) %>%
  filter(row_number()==1)

The following example shows how to use this function in practice.

Example: Select the First Row by Group in R

Suppose we have the following dataset in R:

#create dataset
df <- data.frame(team=c('A', 'A', 'A', 'B', 'B', 'B', 'C', 'C', 'C', 'C'),
                 points=c(4, 9, 7, 7, 6, 13, 8, 8, 4, 17))

#view dataset
df

   team points
1     A      4
2     A      9
3     A      7
4     B      7
5     B      6
6     B     13
7     C      8
8     C      8
9     C      4
10    C     17

The following code shows how to use the dplyr package to select the first row by group in R:

library(dplyr)

df %>%
  group_by(team) %>%
  arrange(points) %>%
  filter(row_number()==1)

# A tibble: 3 x 2
# Groups:   team [3]
  team  points
    
1 A          4
2 C          4
3 B          6

By default, arrange() sorts the values in ascending order but we can easily sort the values in descending order instead:

df %>%
  group_by(team) %>%
  arrange(desc(points)) %>%
  filter(row_number()==1)

# A tibble: 3 x 2
# Groups:   team [3]
  team  points
    
1 C         17
2 B         13
3 A          9

Note that you can easily modify this code to select the n^th row by each group. Simply change row_number() == n.

For example, if you’d like to select the 2nd row by group, you can use the following syntax:

df %>%
  group_by(team) %>%
  arrange(desc(points)) %>%
  filter(row_number()==2)

Or you could use the following syntax to select the last row by group:

df %>%
  group_by(team) %>%
  arrange(desc(points)) %>%
  filter(row_number()==n())

How to Arrange Rows in R
How to Count Observations by Group in R
How to Find the Maximum Value by Group in R