r - 如何在 R 中的 data.table 中使用具有累积值的移位计算

Question

我有一个布局如下的 data.table

TestData <- data.table(Threshold = 20,
                       Upto = 100,
                       Demand = c(0,0,5,0,50,10,10,10,10,50,20,60),
                       Stock  = c(100,0,0,0,0,0,0,0,0,0,0,0))

股票值(value)应计算为累积值，公式为:

If Stock (previous row) minus Demand (current row) is less or equal than the threshold, than update the current cell in Stock with the value in "Upto". Else update the value of stock (current row) with stock (previous row) minus demand (current row).

然后程序应该重新开始。结果应如下所示:

TestData <- data.table(Threshold = 20,
                       Upto = 100,
                       Demand = c(0,0,5,0,50,10,10,10,10,50,20,60),
                       Stock = c(100,100,95,95,45,35,25,100,90,40,100,40))

    Threshold Upto Demand Stock
 1:        20  100      0   100
 2:        20  100      0   100
 3:        20  100      5    95
 4:        20  100      0    95
 5:        20  100     50    45
 6:        20  100     10    35
 7:        20  100     10    25
 8:        20  100     10   100
 9:        20  100     10    90
10:        20  100     50    40
11:        20  100     20   100
12:        20  100     60    40

我所做的如下:

TestData[, Stock:= ifelse(cumsum(shift(Stock, 0, type="lead") - Demand) <= Threshold,
                     Upto,
                     cumsum(shift(Stock, 0, type="lead") - Demand))]

但在第一次更新后，计算停止并显示 100 每次平铺结束。库存中的第一个值是我手动设置的初始值。谢谢!

Answer 1

这是一个 data.table 解决方案。创新在于 by 分组。请务必发布此解决方案失败的任何边界案例。

TestData <- data.table(Threshold = 20,
                       Upto = 100,
                       Demand = c(0,0,5,0,50,10,10,10,10,50,20,60),
                       Stock  = c(100,0,0,0,0,0,0,0,0,0,0,0))

#to see by grouping
#TestData[,trunc(cumsum(Demand)/(Threshold - Upto))]

TestData[, Stock2 := c(Upto[1], Upto[1] - cumsum(Demand[-1])),
    by=trunc(cumsum(Demand)/(Threshold - Upto))]
TestData