[R] scaling-centering a vector using an index

[Thomas de Cornulier]

dear all,
I would like to center and scale some columns of a data frame (these
include NAs) according to the levels of an index (list of factors from
the same data frame).
I tried to code it using for loops but gave up. I guess there is a
straightforward way to achieve this: any hint welcome!
thanks a lot
thomas

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