问R中的机车车辆分布

EN

library(tidyverse)

df <- data.frame(
  type = c("a","a","a","a","a"),
  date = as.Date(c("2020-01-01", "2020-02-01", "2020-03-01", "2020-04-01", "2020-05-01")),
  stock = c(3,7,4,4,4),
  arriving = c(4,1,1,3,2),
  consumption = c(0,7,5,5,3),
  safety_stock = c(4,4,4,4,4)
) %>% 
  mutate(
    to_purchase = safety_stock - (stock - consumption + arriving), # purchase equation
    to_purchase = ifelse(to_purchase < 0, 0, to_purchase) # if negative, change value to zero
  )

df <- data.frame(
  type = c("a","a","a","a","a"),
  date = as.Date(c("2020-01-01", "2020-02-01", "2020-03-01", "2020-04-01", "2020-05-01")),
  stock = c(3,NA,NA,NA,NA),
  arriving = c(4,1,1,3,2),
  consumption = c(0,7,5,5,3),
  safety_stock = c(4,4,4,4,4),
  to_purchase = NA
) 

for(i in 2:nrow(df)){
  # determine if you need to purchase anything from the previous month
  h = i-1
  df$to_purchase[h] = df$safety_stock[h] - (df$stock[h] - df$consumption[h] + df$arriving[h])
  df$to_purchase[h] = ifelse(df$to_purchase[h] < 0, 0, df$to_purchase[h])
  # determine initial stock of the current month
  df$stock[i] = df$stock[h] + df$arriving[h] - df$consumption[h] + df$to_purchase[h]
  # determine if you need to purchase anything for the current month
  df$to_purchase[i] = df$safety_stock[i] - (df$stock[i] - df$consumption[i] + df$arriving[i])
  df$to_purchase[i] = ifelse(df$to_purchase[i] < 0, 0, df$to_purchase[i])
}

df
#>   type       date stock arriving consumption safety_stock to_purchase
#> 1    a 2020-01-01     3        4           0            4           0
#> 2    a 2020-02-01     7        1           7            4           3
#> 3    a 2020-03-01     4        1           5            4           4
#> 4    a 2020-04-01     4        3           5            4           2
#> 5    a 2020-05-01     4        2           3            4           1