Hi,

With a data such as:
> z = data.frame(a = 1:5, b=10*a, c = c("a", "a", "b", "b", "b") )
* a  b  c*
 *1* 10 *a*
 *2* 20 *a*
 3 *30* *b*
 4 *40* *b*
 5 *50* *b*

Can anyone suggest a way to select [1, 2, 30, 40, 50],
ie. using column "c" to specify which column is selected for each row.

Many thanks

G

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