Re: idl2matlab translate-o-matic

["J.D. Smith" <jdsmith(at)astro.cornell.edu>]

Craig Markwardt wrote:
> 
> Pavel Romashkin <pavel@netsrv1.cmdl.noaa.gov> writes:
> 
> > > What can you say of a language that is purely array oriented, but
> > > cannot comprehend the existence of an empty array?
> >
> > Agreeing with D.F., I so far had no use for an empty array. I
> > understand it is not flexible, but I usually work on data other than
> > nothing.
> 
> Forgive him, he knows not what he says.
> 
> Empty arrays would be invaluable in both indexing (such as with WHERE)
> and array concatenation.  By invaluable, I mean that it would remove a
> lot of the special casing.  Consider these examples:
> 
> ARRAY INDEXING - indexing with where()
>  *With* an empty array:
>    wh = where(array GT thresh, /EMPTY)
>    array(wh) = 0   ;; indexing with empty array has no effect
>  *Without* an empty array
>    wh = where(array GT thresh, count)
>    if count GT 0 then array(wh) = 0
> 
> ARRAY CONCATENATION - growing an array
>  *With* an empty array:
>    l = empty_array()
>    for i = 0, 100 do if expression(values) then l = [l, values]
>  *Without* an empty array:
>    for i = 0, 100 do $
>      if expression then $
>         if n_elements(l) EQ 0 then l = [values] else l = [l, values]
> 
> As you can see, the "with" code is more simple and easy to read.  The
> "without" (which represents the status quo) has special cases which
> ruin the flow of thought.  For a vectorized language, this is a
> painful burden to bear sometimes.  If you don't believe me, try doing
> the following (apparently simple) problem:
> 
>  * given two arrays, A and B: concatenate all but the last two
>    elements of A, with B.  Don't try [A(0:n-3),B], or you will be in a
>    world of hurt.
> 

I wouldn't say a *world* or hurt.  Maybe a minor planetesimal of hurt:

C=n_elements(A)>2?[A[0:n_elements(A)-3],B]:B

JD


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