tidyverse:按组进行逐行计算

我们可以使用accumulate从purrr

library(tidyverse)
df %>% 
   group_by(Mat, Plant) %>% 
   mutate(EffectiveFcst =  accumulate(CumDailyFcst - lag(UU, default = 0),  ~ 
          .y , .init = first(CumDailyFcst))[-1] ) 
# A tibble: 8 x 6
# Groups:   Mat, Plant [3]
#  Mat   Plant   Day    UU CumDailyFcst EffectiveFcst
#                      
#1 A     P1        1     0           11            11
#2 A     P1        2    10           22            22
#3 A     P1        3     0           33            23
#4 A     P2        1     0            0             0
#5 A     P2        2     0            5             5
#6 A     P2        3   120           10            10
#7 B     P1        1    10           20            20
#8 B     P1        2     0           50            40