Here is a "brute strength and stupidity" approach that may help.
Note I renamed your data.table to DT for my own convenience.
##===========================================##
# Load packages ------------------------------------------------------------
suppressMessages(library(data.table))
# Load dada. --------------------------------------------------------------
DT <- fread("raw.data.csv"); DT
# Subset data and rbind into one new data.table. -------------------------
mynames <- c("date", "price1", "vol1", "price2","vol2", "id")
ABC <- DT[, c(1, 2:5)][, id := "ABC"] ; names(ABC) <- mynames
DEF <- DT[, c(1, 6:9)][, id := "DEF"] ; names(DEF) <- mynames
DT1 <- rbind(ABC, DEF)
# Create new variable "price". --------------------------------------------
DT1[, price := fcase(vol1 < vol2, price2, default = price1)][]
# Summary statistic using "by". -------------------------------------------
DT1[, .(mysum = sum(price)), by = id]
##===========================================##
On Wed, 18 Jun 2025 at 06:27, Naresh Gurbuxani <naresh_gurbux...@hotmail.com>
wrote:
> My data.table has many columns in groups of four. On each group, I want
> to perform the same set of calculations.
>
> mydt <- data.table(
> date = seq(as.IDate("2025-06-01"), by = 1, length.out = 10),
> ABC1_price = runif(10, 100, 120), ABC1_volume = runif(10, 200, 300),
> ABC2_price = runif(10, 100, 120), ABC2_volume = runif(10, 200, 300),
> DEF1_price = runif(10, 100, 120), DEF1_volume = runif(10, 200, 300),
> DEF2_price = runif(10, 100, 120), DEF2_volume = runif(10, 200, 300))
>
> mydt[
> , let(ABC_price = fifelse(ABC1_volume < ABC2_volume, ABC2_price,
> ABC1_price),
> DEF_price = fifelse(DEF1_volume < DEF2_volume, DEF2_price, DEF1_price))
> ]
>
> # Now use mydt[, list(ABC_price, DEF_price)] for subsequent calculations
>
> With many columns, above method is error-prone and tedious. Is it
> possible to do something like:
>
> myvars <- c("ABC", "DEF")
> for (avar in myvars) {
> # code for above calculations
> }
>
> Thanks,
> Naresh
>
> ______________________________________________
> R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see
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> PLEASE do read the posting guide
> https://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
--
John Kane
Kingston ON Canada
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