迭代累计和

Question

以下是我的问题的指示，并允许一个易于重现的数据集：

如果我想确切知道在长途旅行中我需要在哪些英里停下来加油，我可以计算每英里使用的燃料加仑数。我假设我的车辆的燃油经济性是我的速度的函数，优化为 55 英里/小时，我给自己留了 1 加仑的油用于紧急搜索加油站。

例如：

require(data.table)
distance = c(1:3000)
speed = runif(length(distance),25,80)

avg_mpg = max(mtcars$mpg)
slow_mpg = ((55-speed)^(1/2))
fast_mpg = ((speed-55)^(1/2))
mpg = ifelse(speed<55, avg_mpg-slow_mpg, avg_mpg-fast_mpg)

gpm = 1 / mpg
tank_min = 1
full_tank = 13
fuel_use_function = function(x,y) x - cumsum(y)
fuel_use = fuel_use_function(full_tank, gpm)

trip_info = data.table(Mile_marker = distance,
                       Average_speed = speed,
                       Miles_per_gallon = mpg, 
                       Gallons_per_mile = gpm,
                       Fuel_use = fuel_use)
> trip_info
      Mile_marker Average_speed Miles_per_gallon Gallons_per_mile  Fuel_use
   1:           1      73.58330         29.58916       0.03379616  12.96620
   2:           2      55.09657         33.58925       0.02977143  12.93643
   3:           3      40.63387         30.10973       0.03321185  12.90322
   4:           4      69.86081         30.04503       0.03328338  12.86994
   5:           5      62.52757         31.15636       0.03209618  12.83784
  ---                                                                      
2996:        2996      62.83864         31.10024       0.03215409 -85.66442
2997:        2997      31.03928         29.00503       0.03447678 -85.69889
2998:        2998      30.81609         28.98229       0.03450383 -85.73340
2999:        2999      31.53397         29.05583       0.03441651 -85.76781
3000:        3000      34.27376         29.34739       0.03407458 -85.80189

我想在 trip_info 中添加另一列，记录我正在 "leg" 的旅程（即，我目前使用的是哪个油箱：第一、第二、第三...ETC。）。我可以像这样手动执行此操作：

first_tank_fuel_use = fuel_use_function(full_tank, trip_info$Gallons_per_mile)
first_tank_range = which(first_tank_fuel_use<tank_min)[1]
first_tank = rep(1,first_tank_range)
next_tanks = rep(2,nrow(trip_info) - length(first_tank))
tanks = c(first_tank, next_tanks)
trip_info$Tank_number = tanks

second_tank_fuel_use = 
  fuel_use_function(full_tank, trip_info$Gallons_per_mile[trip_info$Tank_number==2])
second_tank_range = which(second_tank_fuel_use<tank_min)[1]
second_tank = rep(2,second_tank_range)
tanks = c(first_tank, second_tank)
next_tanks = rep(3,nrow(trip_info) - length(tanks))
tanks = c(tanks, next_tanks)
trip_info$Tank_number = tanks

third_tank_fuel_use = ...

如何以矢量化格式自动执行此 'Tank number index' 生成器？

Answer 1

我认为这可能很简单，只需计算 tank=cumsum(Gallons_per_mile) %/% (full_tank - tank_min) 以指示您在哪个坦克上，然后过滤非零 diff(tank) 数字。

会引入一些舍入误差，使停止点的估计过于保守，但 "tank_min" 提供的摆动空间应该足以在实践中考虑到这一点。

迭代累计和

Iterative cumulative sum

r

data.table