获取每组的第一个和最后一个值 - dplyr group_by with last() 和 first() [英] Get first and last values per group – dplyr group_by with last() and first()

问题描述

下面的代码应该按年份对数据进行分组，然后使用每年的第一个和最后一个值创建两个新列.

The code below should group the data by year and then create two new columns with the first and last value of each year.

library(dplyr)

set.seed(123)

d <- data.frame(
    group = rep(1:3, each = 3),
    year = rep(seq(2000,2002,1),3),
    value = sample(1:9, r = T))

d %>% 
    group_by(group) %>%
    mutate(
        first = dplyr::first(value),
        last = dplyr::last(value)
    )

然而，它不能正常工作.预期的结果是

However, it does not work as it should. The expected result would be

  group  year value first  last
  <int> <dbl> <int> <int> <int>
1     1  2000     3     3     4
2     1  2001     8     3     4
3     1  2002     4     3     4
4     2  2000     8     8     1
5     2  2001     9     8     1
6     2  2002     1     8     1
7     3  2000     5     5     5
8     3  2001     9     5     5
9     3  2002     5     5     5

然而，我得到了这个(它在整个数据框中取第一个和最后一个值，而不仅仅是组):

Yet, I get this (it takes the first and the last value over the entire data frame, not just the groups):

  group  year value first  last
  <int> <dbl> <int> <int> <int>
1     1  2000     3     3     5
2     1  2001     8     3     5
3     1  2002     4     3     5
4     2  2000     8     3     5
5     2  2001     9     3     5
6     2  2002     1     3     5
7     3  2000     5     3     5
8     3  2001     9     3     5
9     3  2002     5     3     5

推荐答案

dplyr::mutate() 成功了

d %>% 
    group_by(group) %>%
    dplyr::mutate(
        first = dplyr::first(value),
        last = dplyr::last(value)
    )

这篇关于获取每组的第一个和最后一个值 - dplyr group_by with last() 和 first()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！