当r不按顺序时如何计算r中的行差 数据

问题描述

我有一个像这样的数据框：

name    count
 a        3  
 a        5   
 a        8   
 b        2   
 a        9  
 b        7  

所以我想按名称计算行差异组。所以我的代码是：

data%>%group_by(Name)%>%mutate(last_count = lag(count),diff = count - last_count)

但是，我得到如下表所示的结果

name    count    last_count   diff
 a        3          NA        NA
 a        5          3         2
 a        8          5         3
 b        2          NA        NA
 a        9          8         1
 b        7          2         5

但是我想要的应该是这样的：

name    count    last_count   diff
 a        3          NA        NA
 a        5          3         2
 a        8          5         3
 b        2          NA        NA
 a        9          NA        NA
 b        7          NA        NA

在此先感谢任何可以帮助我解决问题的人！

解决方法

这项工作：

> library(dplyr)
> df %>% mutate(last_count = case_when(name == lag(name) ~ lag(count),TRUE ~ NA_real_),diff = case_when(name == lag(name) ~ count - lag(count),TRUE ~ NA_real_))
# A tibble: 6 x 4
  name  count last_count  diff
  <chr> <dbl>      <dbl> <dbl>
1 a         3         NA    NA
2 a         5          3     2
3 a         8          5     3
4 b         2         NA    NA
5 a         9         NA    NA
6 b         7         NA    NA
> 

,

我们可以使用rleid根据“名称”列中相邻的匹配值创建一个分组列，然后应用diff

library(dplyr)
library(data.table)
data %>%
    group_by(grp = rleid(name)) %>% 
    mutate(last_count = lag(count),diff = count - last_count) %>% 
    ungroup %>% 
    select(-grp)

-输出

# A tibble: 6 x 4
#  name  count last_count  diff
#  <chr> <int>      <int> <int>
#1 a         3         NA    NA
#2 a         5          3     2
#3 a         8          5     3
#4 b         2         NA    NA
#5 a         9         NA    NA
#6 b         7         NA    NA

---

或者将base R与ave和rle一起使用

data$diff <-  with(data,ave(count,with(rle(name),rep(seq_along(values),lengths)),FUN = function(x) c(NA,diff(x)))

数据

data <- structure(list(name = c("a","a","b","b"),count = c(3L,5L,8L,2L,9L,7L)),class = "data.frame",row.names = c(NA,-6L))