R 用前一行值填充 NA 和条件

Question

我需要用前一行值填充 NA 行，但仅在条件未更改之前。
作为一周中的几天、膳食和价格的简单示例:

Day = c("Mon", "Tues", "Wed", "Thus", "Fri", "Sat","Sun","Mon", "Tues", 
       "Wed", "Thus", "Fri", "Sat","Sun") 
Meal = c("B","B","B","B","B","D","D","D","D","L","L", "L","L","L") 
Price = c(NA, 20, NA,NA,NA,NA,NA,15,NA,NA,10,10,NA,10) 
df = data.frame(Meal,Day ,Price )
df
   Meal  Day Price
1     B  Mon    NA
2     B Tues    20
3     B  Wed    NA
4     B Thus    NA
5     B  Fri    NA
6     D  Sat    NA
7     D  Sun    NA
8     D  Mon    15
9     D Tues    NA
10    L  Wed    NA
11    L Thus    10
12    L  Fri    10
13    L  Sat    NA
14    L  Sun    10

我需要在 NA 中填写上一个但仅适用于相同膳食类型的一周。

我试过了

     na.locf(df, fromLast = TRUE)
   Meal  Day Price
1     B  Mon    20
2     B Tues    20
3     B  Wed    15
4     B Thus    15
5     B  Fri    15
6     D  Sat    15
7     D  Sun    15
8     D  Mon    15
9     D Tues    10
10    L  Wed    10
11    L Thus    10
12    L  Fri    10
13    L  Sat    10
14    L  Sun    10

这是错误的，因为与膳食类型重叠。
数据应如下所示:

  Meal  Day Price
1     B  Mon    20
2     B Tues    20
3     B  Wed    20
4     B Thus    20
5     B  Fri    20
6     D  Sat    15
7     D  Sun    15
8     D  Mon    15
9     D Tues    15
10    L  Wed    10
11    L Thus    10
12    L  Fri    10
13    L  Sat    10
14    L  Sun    10

非常感谢

Answer 1

使用 data.table 的另一种选择

library(data.table)
library(xts)

dt <- data.table(df)

dt[, Price := na.locf(Price, fromLast = TRUE), by = Meal]