Hi!  1st time I'm posting here.  I'm beginning to learn R and I've encountered a problem that I'm unable to solve so far.

I have  a 20 000 x 5 matrix.  In the 5th column, I have elevation.  Missing value are actually put to -99999.  I want to track down the index of those values and replace them with NA.  I've read that to replace, the command "replace" is enough.  I just don't know how to construct the index vector that contains the index of -99999 values.

Hope this makes sense,
Thanks!
Philippe

 		
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