# [R] IFELSE across large array?

Liaw, Andy andy_liaw at merck.com
Tue Nov 23 18:28:48 CET 2004

```I'll give it half a crack:

Steps a through c can be done via nested ifelse(), treating A and M as
vectors (as they really are).  Step d is the hard one.  I'd write a simple
Fortran code and use .Fortran() for that.

I don't see how any of the *apply() functions can help here, as your
operations are element-wise, not dimension-wise.

Andy

> From: Sander Oom
>
> Dear all,
>
> As our previous email did not get any response, we try again with a
> reformulated question!
>
> 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
>
> for example:
> [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
> al k elements
>
> Would you be able to suggest a strategy to do this?
>
> thanks very much
>
> Alessandro and Sander.
>
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