[R] IFELSE across large array?
sander at oomvanlieshout.net
Tue Nov 23 17:23:36 CET 2004
As our previous email did not get any response, we try again with a
We are trying to do something which needs an efficient loop over a huge
array, possibly functions such as apply and related (tapply,
lapply...?), but can't really understand syntax and examples in
practice...i.e. cant' make it work.
to be more specific:
we are trying to apply a mask to a 3D array.
By this I mean that when "overlaying" [i.e. comparing element by element]
the mask on to the array the mask should change array elements according to
the values of both array and mask elements
the mask has two values: 1 and 10.
the array elements have 3 values: 0, 1, or 10
sticking for the moment to the single 2d array case
[A= array] 10 0 10 1 10 0
1 10 1 0 0 10
[ M=mask] 1 10 10 1 1 1
10 1 1 1 10 10
I would like the array elements to:
a) IF A(ij) !=10 and Mij = 1
leave A(ij) unchanged
b) IF A(ij) != 10 and M(ij) =10
change A(ij) to M(ij) i.e mask value (10)
c)IF A(ij) = 10 and M(ij) = 10
leave (Aij) unchanged
d) IF A(ij) = 10 and M(ij) !=10
replace A(ij) with the majority value in the 8-neighborhood
(or whatever if it is an edge element) BUT ignoring 10s in this
neighborhood (i.e. with either 1 or 0, whichever is in majority)
because the array is 3d I would like to repeat the thing with all the k
elements (2d sub-arrays) of the array in question, using the same mask for
al k elements
Would you be able to suggest a strategy to do this?
thanks very much
Alessandro and Sander.
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