在数据框中按组选择第一行和最后一行 [英] Select the first and last row by group in a data frame

问题描述

如何在以下数据框中为每个唯一的id选择第一行和最后一行?

How can I select the first and last row for each unique id in the following dataframe?

tmp <- structure(list(id = c(15L, 15L, 15L, 15L, 21L, 21L, 22L, 22L, 
22L, 23L, 23L, 23L, 24L, 24L, 24L, 24L), d = c(1L, 1L, 1L, 1L, 
1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L), gr = c(2L, 1L, 
1L, 1L, 1L, 2L, 1L, 1L, 2L, 1L, 1L, 2L, 1L, 1L, 1L, 2L), mm = c(3.4, 
4.9, 4.4, 5.5, 4, 3.8, 4, 4.9, 4.6, 2.7, 4, 3, 3, 2, 4, 2), area = c(1L, 
2L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 3L, 2L, 3L)), .Names = c("id", 
"d", "gr", "mm", "area"), class = "data.frame", row.names = c(NA, 
-16L))
tmp
#>    id d gr  mm area
#> 1  15 1  2 3.4    1
#> 2  15 1  1 4.9    2
#> 3  15 1  1 4.4    1
#> 4  15 1  1 5.5    2
#> 5  21 1  1 4.0    2
#> 6  21 1  2 3.8    2
#> 7  22 1  1 4.0    2
#> 8  22 1  1 4.9    2
#> 9  22 1  2 4.6    2
#> 10 23 1  1 2.7    2
#> 11 23 1  1 4.0    2
#> 12 23 1  2 3.0    2
#> 13 24 1  1 3.0    2
#> 14 24 1  1 2.0    3
#> 15 24 1  1 4.0    2
#> 16 24 1  2 2.0    3

推荐答案

一个 plyr 解决方案(tmp是您的数据框):

A plyr solution (tmp is your data frame):

library("plyr")
ddply(tmp, .(id), function(x) x[c(1, nrow(x)), ])
#    id d gr  mm area
# 1  15 1  2 3.4    1
# 2  15 1  1 5.5    2
# 3  21 1  1 4.0    2
# 4  21 1  2 3.8    2
# 5  22 1  1 4.0    2
# 6  22 1  2 4.6    2
# 7  23 1  1 2.7    2
# 8  23 1  2 3.0    2
# 9  24 1  1 3.0    2
# 10 24 1  2 2.0    3

或与 dplyr 一起使用(另请参见

Or with dplyr (see also here):

library("dplyr")
tmp %>%
group_by(id) %>%
slice(c(1, n())) %>%
ungroup()
# # A tibble: 10 × 5
#       id     d    gr    mm  area
#    <int> <int> <int> <dbl> <int>
# 1     15     1     2   3.4     1
# 2     15     1     1   5.5     2
# 3     21     1     1   4.0     2
# 4     21     1     2   3.8     2
# 5     22     1     1   4.0     2
# 6     22     1     2   4.6     2
# 7     23     1     1   2.7     2
# 8     23     1     2   3.0     2
# 9     24     1     1   3.0     2
# 10    24     1     2   2.0     3

这篇关于在数据框中按组选择第一行和最后一行的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！