计算R数据表中前3行的总和（由网格平方） [英] calculating sum of previous 3 rows in R data.table (by grid-square)

问题描述

我想计算每个网格正方形的最后三天的降雨量，并将其添加为我的data.table中的一个新列。要清楚，我想总结当前和上一个两（2）天的降雨量，对于每个仪表网格广场

 库（动物园）
库（数据表）
 
 
＃制作数据表
 rain < -  c（NA，NA，NA， 0，5，1，0，3，10）＃降雨值使用
 square < -  c（1,1,1,1,1,1,1,1,1,2）＃用于降雨测量的地理网格正方形
 desired_result < -  c（NA，NA，NA，NA，NA，5,6,6,4，NA）＃这是我查找的结果NA，因为我们现在是第二个网格广场的第一天）
 weather<  -  data.table（rain，square，desired_result）＃making the data.table 
  
 
 
 
我尝试回答：此行以前工作，但不再
  weather [，rain_3：= filter（rain，rep（1,2），sides = 1），by = list（square）] 
  pre> 
 
 
这里我尝试另一种方法：
 ＃this下一行得到的数字正确，但总和以下值，而不是前面的值。 
 weather $ rain_3<  -  rollapply（zoo（weather $ rain），list（seq（-2,0）），sum）
 
＃这里我添加的天气$ square ，但仍然没有成功
 weather $ rain_3<  -  rollapply（zoo（weather $ rain），list（seq（-2,0）），sum，by = list（weather $ square））
  
我非常感谢您的任何见解或建议。 
 
 
 
非常感谢！
解决方案
 
  weather [，rain_3：= filter（rain，rep（1,3），sides = 1），by = list（square）] 
＃ = 1）：
＃'filter'长于时间序列
 weather [，rain_3：= if（.N> 2）filter（rain，rep（1,3），sides = 1） else NA_real_，
 by = square] 
＃rain square desired_result rain_3 
＃1：NA 1 NA NA 
＃2：NA 1 NA NA 
＃3：NA 1 NA NA 
＃4：0 1 NA NA 
＃5：0 1 NA NA 
＃6：5 1 5 5 
＃7：1 1 6 6 
＃8：0 1 6 6 
＃9：3 1 4 4 
＃10：10 2 NA NA 
  
小心dplyr没有加载，因为它掩盖了过滤器。如果你需要dplyr，你可以调用 stats :: filter 显式。
 
I would like to calculate the rainfall that has fallen over the last three days for each grid square, and add this as a new column in my data.table. To be clear, I want to sum up the current and PREVIOUS two (2) days of rainfall, for each meterological grid square
library ( zoo )
library (data.table)


# making the data.table
rain           <- c(NA, NA, NA, 0, 0, 5, 1, 0, 3, 10)  # rainfall values to work with
square         <- c(1,1,1,1,1,1,1,1,1,2)               # the geographic grid square for the rainfall measurement
desired_result <- c(NA, NA, NA, NA, NA, 5, 6, 6, 4, NA )  # this is the result I'm looking for (the last NA as we are now on to the first day of the second grid square)
weather <- data.table(rain, square, desired_result)  # making the data.table
My attempt to answer: this line used to work, but no longer does
weather[, rain_3 := filter(rain, rep(1, 2), sides = 1), by = list(square)]  
So here I am trying another method:
# this next line gets the numbers right, but sums the following values, not the preceeding ones. 
weather$rain_3 <- rollapply(zoo(weather$rain), list(seq(-2,0)), sum)

# here I add in the by weather$ square, but still no success
weather$rain_3 <- rollapply(zoo(weather$rain), list(seq(-2,0)), sum, by= list(weather$square))
I would greatly appreciate any insights or suggestions you may have. 

Many thanks!
解决方案
weather[, rain_3 := filter(rain, rep(1, 3), sides = 1), by = list(square)]  
#Error in filter(rain, rep(1, 3), sides = 1) : 
#  'filter' is longer than time series
weather[, rain_3 := if(.N > 2) filter(rain, rep(1, 3), sides = 1) else NA_real_, 
        by = square] 
#    rain square desired_result rain_3
# 1:   NA      1             NA     NA
# 2:   NA      1             NA     NA
# 3:   NA      1             NA     NA
# 4:    0      1             NA     NA
# 5:    0      1             NA     NA
# 6:    5      1              5      5
# 7:    1      1              6      6
# 8:    0      1              6      6
# 9:    3      1              4      4
#10:   10      2             NA     NA
Take care that dplyr is not loaded because it masks filter. If you need dplyr, you can call stats::filter explicitly.

                        
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