Dear Bryan,
You could try using spatial techniques to choose the contiguous areas of your
matrices.
> require(spatstat)
> set.seed(1520)
> x <- matrix(rnorm(25), nrow=5, ncol=5,
+ dimnames=list(c("A","B","C","D","E"), c("v","w","x","y","z")))
> x
v w x y z
A 0.3089046 -0.003350135 -0.4506777 0.7971787 -1.95078919
B -1.5895009 0.336233539 -0.3237293 0.7676754 -0.76756928
C -1.0324022 -1.119037223 -1.1525350 0.6057773 -0.28930702
D -0.8440912 -0.499994418 0.7664473 0.6367184 -0.09801227
E 0.6261038 0.391232210 0.4967601 1.0753439 -0.50998559
> z <- y <- x
>
> y[, ] <- x < 0.6
> y
v w x y z
A 1 1 1 0 1
B 1 1 1 0 1
C 1 1 1 0 1
D 1 1 0 0 1
E 0 1 1 0 1
> yi <- as.im(y)
> ycOut <- connected(yi, background = 0)
> yc <- ycOut$v
> yc
[,1] [,2] [,3] [,4] [,5]
[1,] 1 1 1 <NA> 2
[2,] 1 1 1 <NA> 2
[3,] 1 1 1 <NA> 2
[4,] 1 1 <NA> <NA> 2
[5,] <NA> 1 1 <NA> 2
Levels: 1 2
>
> z[yc != 1 | is.na(yc)] <- NA
> z
v w x y z
A 0.3089046 -0.003350135 -0.4506777 NA NA
B -1.5895009 0.336233539 -0.3237293 NA NA
C -1.0324022 -1.119037223 -1.1525350 NA NA
D -0.8440912 -0.499994418 NA NA NA
E NA 0.391232210 0.4967601 NA NA
Hope this helps.
Best wishes,
Chris Campbell
MANGO SOLUTIONS
Data Analysis that Delivers
+44 1249 767700
-----Original Message-----
From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On
Behalf Of Bryan McCloskey
Sent: 31 January 2012 19:58
To: r-help@r-project.org
Subject: [R] Selecting contiguous, irregularly-shaped sets of values from arrays
All,
I am attempting to select all of the contiguous elements of a matrix that meet
some criterion. I.e., values that would be contained within an irregular area
defined by a "contour" applied around point of interest. So, if I have a matrix
x as follows:
> x <- matrix(rnorm(25), nrow=5, ncol=5,
dimnames=list(c("A","B","C","D","E"), c("v","w","x","y","z"))
> x
v w x y z
A 0.5184795 1.9641285 0.8632044 1.5010397 0.8468490 B -1.2402866 0.5211307
-0.1474351 1.3264893 0.1087390 C 0.5910275 -1.1708906 0.9440755 1.0970971
-0.2784806 D 0.6377495 1.1594035 -0.4217621 1.4021680 -0.6487677 E -1.4590833
0.2065765 0.1623669 1.3598283 0.3742821
>
how can I select all values in the "pond" of contiguous matrix entries that
have values, say <0.6, if my entry of interest is x["A","v"]. In that case, I
would like to select the following starred entries:
v w x y z
A 0.5184795* 1.9641285 0.8632044 1.5010397 0.8468490 B -1.2402866*
0.5211307* -0.1474351* 1.3264893 0.1087390 C 0.5910275* -1.1708906*
0.9440755 1.0970971 -0.2784806 D 0.6377495 1.1594035 -0.4217621* 1.4021680
-0.6487677 E -1.4590833 0.2065765* 0.1623669* 1.3598283 0.3742821
But I would _not_ like to select any of the values in x[,"z"], because, even
though they may be <0.6, they are not contiguous with the pond that x["A","v"]
is in.
Is there an easy way to do this for many points of interest in a large matrix?
Thanks,
-bryan
------
Bryan McCloskey, Ph.D.
U.S. Geological Survey
St. Petersburg Coastal & Marine Science Center St. Petersburg
------
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