isolating a special case

[Francesco Potorti`]

>What is the best way to isolate a special case?  I have a large collec-
>tion of surface patches contributing fields at an observation point.
>But that obs. point may be at one of the patches, in which case that
>patch's contribution has an analytic expression (otherwise it would be
>singular).  The results are put into a matrix whose indices represent
>the surface.  I know how to this using the approach
>
>    fieldmatrix = (range<0.1)*selffield + (0.1<=range)*otherfield
>
>where 'range' is the same size matrix as 'fieldmatrix', giving the
>distances between patch and obs. point.  But then I'd have to compute
>'selffield' in all the positions where it's not needed.  Is there a
>more efficient way of doing this?

Is suppose that you should just compute selffield where (range<0.1).

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