[R] Retrieve indexes of the "first occurrence of numbers" in an effective manner

[Jeff Newmiller]

x <- c(1,1,2,2,3,3,4,4)
match(unique(x),x)
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Sent from my phone. Please excuse my brevity.

Emmanuel Levy <emmanuel.levy at gmail.com> wrote:

>Hi,
>
>That sounds simple but I cannot think of a really fast way of getting
>the following:
>
> c(1,1,2,2,3,3,4,4) would give c(1,3,5,7)
>
>i.e., a function that returns the indexes of the first occurrences of
>numbers.
>
>Note that numbers may have any order e.g., c(3,4,1,2,1,1,2,3,5), can
>be very large, and the vectors are also very large (which prohibits
>any loop).
>
>The best I could think of is:
>
>tmp = c(1,1,2,2,3,3,4,4)
>u = unique(tmp)
>sapply(u, function(x){ which(!is.na(match(tmp,x)))[1]} )
>
>But there might be a useful function I don't know ..
>
>Thanks for any hint.
>All the best,
>
>Emmanuel
>
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