Soliciting analogies for arguments to "["

I have come around to thinking of arguments to "[" as being similar to machine gun belts:

set.seed(10)
datfrm <- data.frame(a= rev(letters[1:10]), b = runif(10), cc = letters[1:10] )
datfrm[datfrm$b < 0.5, "b"] <- 0

so the machine gun belt is only running through the first argument to "[":

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datfrm[datfrm$b < 0.5, "b"] <- 0

But if you want to only make assignments to a subset, say from another column:

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datfrm[ datfrm$b < .5, "cc"] <- 开发者_Python百科datfrm[ datfrm$b < .5, "a"]

The expression datfrm$b gets repeatedly evaluated and when they are paired on either side of an assignment... all proceeds as expected. So what's wrong or right with this and can it be improved?

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You can store the index first so it's only calculated once:

idx <- datfrm$b < .5
datfrm[idx, "cc"] <- datfrm[idx, "a"]

This is simpler to read and less prone to mistakes when making changes.

I think within probably provides the least typing, so using idx as above, and replacing the original with the output of within():

datfrm <- within(datfrm, cc[idx] <- a[idx])