我有一个关于data.table的小问题。因为我不太擅长它，我不太确定如何在data.table中做到这一点。
基本上我有3列，并希望按前两列(键和日期)分组，然后对于每个键和每个日期，找到第三列(票价)中出现的最大值和最小值
我试过这样做，但它给了我一个错误
flights[, c("max_day", "min_day") := unlist(lapply(gross_fare, findr)), by = c("key", "created_date")]
Error in `[.data.table`(flights, , `:=`(c("max_day", "min_day"), unlist(lapply(gross_fare, :
Supplied 18 items to be assigned to group 1 of size 9 in column 'max_day'. The RHS length must either be 1 (single values are ok) or match the LHS length exactly. If you wish to 'recycle' the RHS please use rep() explicitly to make this intent clear to readers of your code.
findr是一个只查找最大值和最小值的函数，即
findr <- function(x) {list(max = max(x), min = min(x)}
我已经在dplyr中完成了我想要做的事情，我将为此附加代码，但由于我有数百万行，dplyr消耗了我的ram数据。
test <- flights %>%
select(key, created_date, gross_fare) %>%
group_by(key, created_date) %>%
summarise(
max_day = max(gross_fare),
min_day = min(gross_fare),
diff = max_day - min_day) %>%
arrange(created_date)
如果有人想要使用，我已经把输出放在了上面如果有人能帮忙，那就太好了，谢谢:)
data.table::setDT(structure(list(key = c("LHE_KHI_LHE+KHI_PA-405_15.0_1", "KHI_ISB_KHI+ISB_PK-370_20.0_0",
"LHE_KHI_LHE+KHI_PK-307_20.0_0", "ISB_KHI_ISB+KHI_PF-124_20.0_1",
"LHE_KHI_LHE+KHI_PK-307_20.0_0", "LHE_KHI_LHE+KHI_PA-405_15.0_1",
"KHI_LHE_KHI+LHE_PK-304_20.0_0", "KHI_ISB_KHI+ISB_PA-204_15.0_1",
"ISB_KHI_ISB+KHI_PA-207_15.0_1", "KHI_ISB_KHI+ISB_PA-200_20.0_1",
"KHI_LHE_KHI+LHE_PK-304_40.0_0", "ISB_KHI_ISB+KHI_PA-201_35.0_1",
"ISB_KHI_ISB+KHI_ER-501_20.0_1", "KHI_LHE_KHI+LHE_PF-145_20.0_2",
"KHI_ISB_KHI+ISB_PA-204_20.0_1", "LHE_KHI_LHE+KHI_PA-401_0.0_0",
"ISB_KHI_ISB+KHI_PK-309_40.0_0", "KHI_ISB_KHI+ISB_PF-123_20.0_2",
"ISB_KHI_ISB+KHI_PA-205_15.0_1", "LHE_KHI_LHE+KHI_PF-142_0.0_0",
"ISB_KHI_ISB+KHI_PA-223_15.0_1", "ISB_KHI_ISB+KHI_PF-126_20.0_2",
"ISB_KHI_ISB+KHI_PK-309_20.0_0", "KHI_ISB_KHI+ISB_PF-121_20.0_2",
"ISB_KHI_ISB+KHI_PK-373_20.0_0", "KHI_LHE_KHI+LHE_PF-145_20.0_2",
"KHI_LHE_KHI+LHE_PA-402_15.0_1", "LHE_KHI_LHE+KHI_PA-407_20.0_1",
"KHI_ISB_KHI+ISB_PK-308_40.0_0", "KHI_LHE_KHI+LHE_PF-145_20.0_2",
"LHE_KHI_LHE+KHI_PF-144_0.0_0", "ISB_KHI_ISB+KHI_PK-369_40.0_0",
"ISB_KHI_ISB+KHI_PF-124_20.0_2", "KHI_ISB_KHI+ISB_PA-204_15.0_1",
"KHI_ISB_KHI+ISB_PA-200_15.0_1", "ISB_KHI_ISB+KHI_PF-124_20.0_1",
"KHI_ISB_KHI+ISB_PK-300_20.0_0", "ISB_KHI_ISB+KHI_PF-122_20.0_2",
"KHI_ISB_KHI+ISB_PK-368_20.0_0", "KHI_ISB_KHI+ISB_PA-204_15.0_1",
"ISB_KHI_ISB+KHI_ER-503_20.0_1", "ISB_KHI_ISB+KHI_PA-209_15.0_1",
"KHI_ISB_KHI+ISB_PK-308_40.0_0", "ISB_KHI_ISB+KHI_PF-124_20.0_1",
"ISB_KHI_ISB+KHI_PK-301_40.0_0", "KHI_LHE_KHI+LHE_PA-408_35.0_1",
"LHE_KHI_LHE+KHI_PF-144_20.0_2", "KHI_ISB_KHI+ISB_PF-121_20.0_2",
"KHI_ISB_KHI+ISB_PA-204_35.0_1", "ISB_KHI_ISB+KHI_PK-309_40.0_0",
"ISB_KHI_ISB+KHI_PA-223_20.0_1", "KHI_ISB_KHI+ISB_PA-206_35.0_1",
"LHE_KHI_LHE+KHI_PF-142_32.0_1", "LHE_KHI_LHE+KHI_PF-142_20.0_1",
"KHI_ISB_KHI+ISB_PF-123_20.0_2", "ISB_KHI_ISB+KHI_PA-209_15.0_1",
"KHI_ISB_KHI+ISB_PA-204_35.0_1", "ISB_KHI_ISB+KHI_PA-201_20.0_1",
"KHI_ISB_KHI+ISB_PK-368_20.0_0", "ISB_KHI_ISB+KHI_PA-205_20.0_1",
"KHI_ISB_KHI+ISB_PF-121_20.0_1", "ISB_KHI_ISB+KHI_PF-124_20.0_1",
"ISB_KHI_ISB+KHI_PA-205_15.0_1", "KHI_LHE_KHI+LHE_PF-145_20.0_2",
"KHI_LHE_KHI+LHE_PA-406_35.0_1", "KHI_ISB_KHI+ISB_PK-308_20.0_0",
"LHE_KHI_LHE+KHI_PA-401_20.0_1", "LHE_KHI_LHE+KHI_PA-401_15.0_1",
"KHI_ISB_KHI+ISB_PA-204_35.0_1", "KHI_LHE_KHI+LHE_PA-406_35.0_1",
"KHI_ISB_KHI+ISB_PA-206_35.0_1", "KHI_ISB_KHI+ISB_PF-121_20.0_1",
"ISB_KHI_ISB+KHI_PA-205_20.0_1", "LHE_KHI_LHE+KHI_PF-142_20.0_1",
"LHE_KHI_LHE+KHI_PF-146_20.0_2", "LHE_KHI_LHE+KHI_PA-401_35.0_1",
"ISB_KHI_ISB+KHI_PA-209_15.0_1", "ISB_KHI_ISB+KHI_PK-301_40.0_0",
"ISB_KHI_ISB+KHI_PA-205_35.0_1", "KHI_LHE_KHI+LHE_PA-406_15.0_1",
"KHI_ISB_KHI+ISB_PF-123_20.0_1", "ISB_KHI_ISB+KHI_PA-201_35.0_1",
"KHI_ISB_KHI+ISB_PK-300_40.0_0", "KHI_LHE_KHI+LHE_PA-402_35.0_1",
"ISB_KHI_ISB+KHI_ER-505_20.0_1", "ISB_KHI_ISB+KHI_PF-122_20.0_2",
"ISB_KHI_ISB+KHI_PA-207_15.0_1", "KHI_LHE_KHI+LHE_PA-404_35.0_1",
"KHI_ISB_KHI+ISB_PF-123_20.0_1", "ISB_KHI_ISB+KHI_ER-503_20.0_1",
"ISB_GIL_ISB+GIL_PK-605_20.0_0", "KHI_ISB_KHI+ISB_PF-123_20.0_1",
"KHI_ISB_KHI+ISB_PA-200_15.0_1", "ISB_KHI_ISB+KHI_PF-122_20.0_2",
"KHI_LHE_KHI+LHE_PA-404_35.0_1", "ISB_KHI_ISB+KHI_PF-122_20.0_2",
"PEW_KHI_PEW+KHI_PF-152_20.0_1", "LHE_KHI_LHE+KHI_PK-303_20.0_0",
"KHI_ISB_KHI+ISB_PA-222_35.0_1", "ISB_KHI_ISB+KHI_PF-124_20.0_1"
), created_date = c("2021-04-20", "2021-05-27", "2021-02-13",
"2021-08-14", "2021-08-11", "2021-08-21", "2021-01-26", "2021-08-21",
"2021-05-24", "2021-09-15", "2021-06-05", "2021-07-19", "2021-09-29",
"2021-07-02", "2021-08-10", "2021-01-04", "2021-07-15", "2021-07-14",
"2021-08-13", "2021-01-11", "2021-09-13", "2021-09-20", "2021-05-27",
"2021-02-20", "2021-08-15", "2021-07-27", "2021-08-26", "2021-09-15",
"2021-08-02", "2021-06-25", "2021-05-15", "2021-08-26", "2021-07-30",
"2021-06-27", "2021-08-07", "2021-03-19", "2021-03-02", "2021-06-06",
"2021-08-15", "2021-06-27", "2021-09-19", "2021-07-28", "2021-08-09",
"2021-08-16", "2021-09-09", "2021-06-04", "2021-08-12", "2021-05-15",
"2021-07-26", "2021-05-27", "2021-08-12", "2021-08-02", "2021-01-26",
"2021-04-20", "2021-08-26", "2021-08-26", "2021-03-21", "2021-01-09",
"2021-04-23", "2021-01-04", "2021-08-13", "2021-06-22", "2021-05-31",
"2021-08-18", "2021-06-16", "2021-08-14", "2021-08-10", "2021-06-16",
"2021-04-08", "2021-05-20", "2021-06-22", "2021-04-20", "2021-01-05",
"2021-02-27", "2021-07-07", "2021-03-26", "2021-08-16", "2021-05-01",
"2021-07-31", "2021-06-14", "2021-06-16", "2021-03-25", "2021-09-14",
"2021-06-06", "2021-09-02", "2021-08-06", "2021-07-18", "2021-02-28",
"2021-04-28", "2021-09-19", "2021-08-25", "2021-06-17", "2021-06-07",
"2021-06-17", "2021-07-07", "2021-08-23", "2021-07-09", "2021-07-19",
"2021-07-14", "2021-05-21"), gross_fare = c(7796, 7427, 11504,
6870, 6580, 14945, 8697, 7524, 7124, 6785, 11858, 7524, 11500,
9525, 6785, 8739, 8200, 13560, 9045, 7400, 7524, 12500, 7458,
14000, 6570, 9525, 6220, 10545, 8310, 7900, 7820, 8410, 11285,
19892, 6810, 9800, 11441, 11900, 6570, 13592, 11500, 8300, 20380,
8525, 7340, 9707, 7870, 10655, 10545, 11798, 14645, 10545, 8650,
8650, 7870, 12945, 10799, 10227, 6765, 10227, 20120, 11045, 9403,
7870, 7124, 6570, 6810, 6531, 8605, 7124, 11072, 7390, 10227,
13435, 10530, 12280, 18945, 11147, 10545, 6531, 6620, 10799,
18480, 32702, 5606, 13560, 23895, 8027, 9655, 11500, 11990, 6620,
9403, 7620, 14645, 19105, 9000, 6440, 12645, 8025)), row.names = c(NA,
-100L), class = c("data.table", "data.frame")))
我想这行代码应该可以完成工作:
library(data.table)
flights[, .(min_day = min(gross_fare), max_day = max(gross_fare), diff = max(gross_fare) - min(gross_fare)), by = .(key, created_date)][]
由于函数findr返回一个列表，因此没有必要使事情复杂化:
findr <- function(x) {list(max = max(x), min = min(x))}
flights[, c("max_day", "min_day") := findr(gross_fare), by = list(key, created_date)][]
也返回max和min之间的差值，使用
findr2 <- function(x) {
list(max = max(x), min = min(x), diff = diff(range(x)))
}
flights[, c("max_day", "min_day", "diff_day") := findr2(gross_fare), by = list(key, created_date)][]