[R] From strings to column names

[Bert Gunter]

If I understand correctly, see ?"[" : data tables inherit from data frames,
and data frame columns can be indexed by their names as character strings.

The following should give you the idea:

d <- data.frame(a1 = 1:3, a2 = 4:6)
x <- vector("numeric",2)
for(i in 1:2){
   nm <- paste0("a",i)
   x[i] <- sum(d[, nm])
}
x


Please note: This is **absoutely** awful R code just meant to illustrate
the sort of thing that you want to do, not as a strict model to be followed.

Also, as I don't use data tables, there may be better ways to do this using
data table's enhanced functionality directly.

Cheers,
Bert

On Wed, Jun 18, 2025 at 3:27 AM Naresh Gurbuxani <
naresh_gurbuxani using 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
>
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