获取每组的第一个和最后一个值

本文介绍了获取每组的第一个和最后一个值 - dplyr group_by with last() 和 first()的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 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) )