相關列data.table

Question

要跟蹤現金流的我有許多相互關聯的列在data.table：

• 「Amount_spent」始終是「平衡」的5％。
• 「收入」是「Amount_spent」*「價格」
• 「餘額」是「收入」（從100.00開始）的累計總和。
• 交易只發生在「天」「一」

我很努力的同時計算出這些相互關聯的列。 例子，因爲我想：

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 

這是迄今爲止我嘗試：

# 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 

正如你將看到它失敗的「REVENUE2」列此錯誤消息Error in eval(expr, envir, enclos) : object 'Revenue2' not found是尚未建立。

謝謝

Answer 1

你得到它嘗試使用其未在DT在代碼中的那點存在列Revenue2行my.try[, Balance2 := cumsum(Revenue2)]後提到的錯誤。

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 

你並沒有產生預期的效果。我不確定你的意思是同時計算列數。如果您想要通過一次引用分配/更新多列，則可以使用`:=()`函數，方法與在data.table的j參數中使用.()或list()的方法相同。例如：`:=`(col1=1+2, col2=2+3)。
您可以參閱更多更新於Reference semantics vignette。