[S] sampling from a vector subset

["Valerie Zartarian" <VZARTARIAN@usgs.gov>]

I have a vector of the following form:

temp_c("R","N","N","R","M","M","R","R","M","R","R","N","R","R","N")

I want to replace a random sample of 3 of the "R"s with a "Z".  I would greatly 
appreciate any insight into how to do this.  

Thank you very much.

Valerie Zartarian

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