我有一个DT，并希望计算一些简单的股票余额，如图所示。这种计算在Excel中很容易和立即，允许一些递归计算。
library(data.table)
DT <- data.table(expand.grid(8:9, 0:59, 1:4))
names(DT) <- c("Hour", "Minute", "Shop")
set.seed(1234)
DT <- DT[, ':=' (Demand = rpois(1, lambda = 0.8),
Stock_in = rpois(1, lambda = 0.05)*250
), by=names(DT)]
DT$Stock_Before <- NA
DT$Stock_After <- NA
DT$Sells <- NA
DT$Lost_Sells <- NA
DT$Stock_Before[which(DT$Hour == 8 & DT$Minute == 0)] <- c(21, 45, 31, 50)
DT <- DT[order(DT$Hour, DT$Minute, DT$Shop),]
现在使用tapply完成了所需的格式，但速度很慢。所以我尝试使用数据表语法来做得更好。
由于Stock_Before of the minute是之前一分钟的Stock_After，我设计的行报告错误。我想使用的命令如下：
DT <- DT[, ':=' (
Stock_After = max(0, Stock_Before+Stock_in-Demand),
Stock_Before = shift(Stock_After, 1),
Sells = min(Stock_Before + Stock_in, Demand, na.rm=T),
Lost_Sells = max(Demand-Sells, 0)
), by=Shop]
错误如下：
Error in shift(Stock_After, 1) : object 'Stock_After' not found
In addition: Warning message:
In `[.data.table`(DT, , `:=`(Stock_After = max(0, Stock_Before + :
Invalid .internal.selfref detected and fixed by taking a (shallow) copy of
the data.table so that := can add this new column by reference. At an earlier
point, this data.table has been copied by R (or been created manually using structure() or similar). Avoid key<-,
names<- and attr<- which in R currently (and oddly)
may copy the whole data.table. Use set* syntax instead to avoid copying: ?set,
?setnames and ?setattr. Also, in R<=v3.0.2, list(DT1,DT2) copied the entire DT1
and DT2 (R's list() used to copy named objects); please upgrade to R>v3.0.2 if that
is biting. If this message doesn't help, please report to datatable-help so the root cause can be fixed.
结果如下所示：
head(DT, 12)
Hour Minute Shop Demand Stock_in Stock_Before Stock_After Sells Lost_Sells
1 8 0 1 0 0 21 21 0 0
2 8 0 2 0 0 45 45 0 0
3 8 0 3 1 0 31 30 1 0
4 8 0 4 2 0 50 48 2 0
5 8 1 1 2 0 21 19 2 0
6 8 1 2 3 0 45 42 3 0
7 8 1 3 0 0 30 30 0 0
8 8 1 4 1 250 48 297 1 0
9 8 2 1 1 0 19 18 1 0
10 8 2 2 3 0 42 39 3 0
11 8 2 3 1 0 30 29 1 0
12 8 2 4 0 250 297 547 0 0
任何人都可以帮助我吗？非常感谢！