相关列data.table

Question

要跟踪现金流的我有许多相互关联的列在data.table：

• “Amount_spent”始终是“平衡”的5％。
• “收入”是“Amount_spent”*“价格”
• “余额”是“收入”（从100.00开始）的累计总和。
• 交易只发生在“天”“一”

我很努力的同时计算出这些相互关联的列。 例子，因为我想：

library(data.table) 
Day <- c("a", "c", "b", "a", "b", "a", "b", "c", "a", "a") 
Price <- c(0.6, 0.4, 0.9, -0.3, 0.8, 0.2, 0.3, 0.9, 0.9, -0.7) 
Balance <- c(100.00, 103.00, 103.00, 103.00, 101.46, 101.46, 102.47, 102.47, 102.47, 107.08) 
Amount_spent <- c(5.00, 0.00, 0.00, 5.15, 0.00, 5.07, 0.00, 0.00, 5.12, 5.35) 
Revenue <- c(3.00, 0.00, 0.00, -1.55, 0.00, 1.01, 0.00, 0.00, 4.61, -3.75) 

DT <- data.table(Day, Price, Balance, Amount_spent, Revenue) 
DT 

这是迄今为止我尝试：

# set initial balance 
Balance2 <- 100.00 
Day2 <- c("a", "c", "b", "a", "b", "a", "b", "c", "a", "a") 
Price2 <- c(0.6, 0.4, 0.9, -0.3, 0.8, 0.2, 0.3, 0.9, 0.9, -0.7) 
my.try <- data.table(Day2, Price2) 
my.try[, Balance2 := cumsum(Revenue2)] 
my.try[ Day2 == "a", Amount_spent2 := Balance2 * 0.05 ] 
my.try[is.na(Amount_spent2), Amount_spent2 := 0] 
my.try[, Revenue2 := Price2 * Amount_spent2 ] 
my.try 

正如你将看到它失败的“REVENUE2”列此错误消息Error in eval(expr, envir, enclos) : object 'Revenue2' not found是尚未建立。

谢谢

Answer 1

你得到它尝试使用其未在DT在代码中的那点存在列Revenue2行my.try[, Balance2 := cumsum(Revenue2)]后提到的错误。

library(data.table) 
Day <- c("a", "c", "b", "a", "b", "a", "b", "c", "a", "a") 
Price <- c(0.6, 0.4, 0.9, -0.3, 0.8, 0.2, 0.3, 0.9, 0.9, -0.7) 
Balance <- c(100.00, 103.00, 103.00, 103.00, 101.46, 101.46, 102.47, 102.47, 102.47, 107.08) 
Amount_spent <- c(5.00, 0.00, 0.00, 5.15, 0.00, 5.07, 0.00, 0.00, 5.12, 5.35) 
Revenue <- c(3.00, 0.00, 0.00, -1.55, 0.00, 1.01, 0.00, 0.00, 4.61, -3.75) 

DT <- data.table(Day, Price, Balance, Amount_spent, Revenue) 

Balance2 <- 100.00 
Day2 <- c("a", "c", "b", "a", "b", "a", "b", "c", "a", "a") 
Price2 <- c(0.6, 0.4, 0.9, -0.3, 0.8, 0.2, 0.3, 0.9, 0.9, -0.7) 
my.try <- data.table(Day2, Price2) 
my.try[, Balance2 := cumsum(Revenue2)] 
#Error in eval(expr, envir, enclos) : object 'Revenue2' not found 
"Revenue2" %in% names(DT) 
#[1] FALSE 

你并没有产生预期的效果。我不确定你的意思是同时计算列数。如果您想要通过一次引用分配/更新多列，则可以使用`:=()`函数，方法与在data.table的j参数中使用.()或list()的方法相同。例如：`:=`(col1=1+2, col2=2+3)。
您可以参阅更多更新于Reference semantics vignette。