On 13/03/14 14:48, Manabu Sakamoto wrote:
Dear list
I'm trying to access values within a data.table column by matching to
elements in a vector and replacing with corresponding elements in a
second vector. But I want to loop through specific column names also
stored as a character vector.
So something like:
DT<- data.table(A=seq(1:10),B=seq(1:10),C=seq(1:10))
cnm <- c("A", "B", "C")
before <- c(4, 5, 6)
after <- c(3, 7, 8)
nm <- cnm[i]
bfr <- before[i]
afr <- after[i]
DT[nm==bfr, nm:=afr]
I'm sure this is completely wrong because it didn't work.
So does anyone know how to correctly do this data.table solution?
DT[get(nm)==bfr, (nm):=afr]
or
set(DT, i=DT[[nm]]==bfr, j=nm, value=afr)
I prefer the first way but if you're looping through many columns (say
1,000+) then using set() should be faster, see ?set.
HTH, Matt
Many thanks,
Manabu
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