Using NA as a break point for indicator variable?

Date: Wed, 23 May 2012 16:42:02 -0400
From: Max Brondfield<mbrondf at post.harvard.edu>
To:r-help at r-project.org
Subject: [R] Using NA as a break point for indicator variable?
Message-ID:
	<CADu+jDpcJUHZTXxrsxyQvjaEmw_N0iLbL6ZJjHZC-rSBCMneiw at mail.gmail.com>
Content-Type: text/plain

Hi all,
I am working with a spatial data set for which I am only interested in high
concentration values ("leaks"). The low values (<  90th percentile) have
already been turned into NA's, leaving me with a matrix like this:

<  CH4_leak

       lon            lat            CH4
1  -71.11954 42.35068 2.595834
2  -71.11954 42.35068 2.595688
3   NA           NA           NA
4   NA           NA           NA
5   NA           NA           NA
6  -71.11948 42.35068 2.435762
7  -71.11948 42.35068 2.491003
8  NA            NA           NA
9  -71.11930 42.35068 2.464475
10 -71.11932 42.35068 2.470865

Every time an NA comes up, it means the "leak" is gone, and the next valid
value would represent a different leak (at a different location). My goal
is to tag all of the remaining values with an indicator variable to
spatially distinguish the leaks. I am envisioning a simple numeric
indicator such as:

      lon            lat            CH4            leak_num
1  -71.11954 42.35068 2.595834   1
2  -71.11954 42.35068 2.595688   1
3   NA           NA           NA             NA
4   NA           NA           NA             NA
5   NA           NA           NA             NA
6  -71.11948 42.35068 2.435762   2
7  -71.11948 42.35068 2.491003   2
8  NA            NA           NA             NA
9  -71.11930 42.35068 2.064475   3
10 -71.11932 42.35068 2.070865  3

Does anyone have any thoughts on how to code this, perhaps using the NA
values as a "break point"? The data set is far too large to do this
manually, and I must admit I'm completely at a loss. Any help would be much
appreciated! Best,

Max

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