[R] IFELSE across large array?

[Liaw, Andy]

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 
> 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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