Interrelated columns in R data.table - r

Question

To keep track of cash flow I have a number of interrelated columns in a data.table:
"Amount_spent" is always 5% of the "Balance".
"Revenue" is "Amount_spent" * "Price"
"Balance" is the cumulative sum of "Revenue" (starting at 100.00).
Transactions only happen on "Day" "a"
I am struggling to calculate these interrelated columns concurrently.
Example as I would like:
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
Here is my attempt so far:
# 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
As you will see it fails with this error message Error in eval(expr, envir, enclos) : object 'Revenue2' not found as the "Revenue2" column is yet to be created.
Thank you

Answer 1

You get mentioned error after the line my.try[, Balance2 := cumsum( Revenue2 )] which try to use the column Revenue2 which not exist in DT at that point in the code.
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
You did not produce expected results. I'm not sure what you mean by calculate the columns concurrently. If you want to assign/update multiple columns by reference in a single step you can use `:=()` function the same way as you would use .() or list() in data.table's j argument. For example: `:=`(col1=1+2, col2=2+3).
You can read more about update by reference in Reference semantics vignette.