[R] switching elements of a vector

[speretti]

Hi,

I would like to receive help for the following matter:

If I'm dealing with a numeric vectors containing increasing elements.
i.e.

a<-c(1,2,2,2,2,3,3,3,4,4,4,5,5,6,7,7,7) 

There exist an efficient way to obtain an vector that indicates the position
of the changing element of "a"?
In this case it would be something like:

index<-c(1,6,9,12,14,15)

usually I'm used cycles to obtain boolean vectors of the same length of "a"
indicating the changing elements ...later I've muliplied them for their
numeric sequence and after that I've selected elements different from zero
...it is quite long...
can you find an easier solution?

Thank you for you help
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