| Re: idl2matlab translate-o-matic [message #19070 is a reply to message #19024] |
Tue, 22 February 2000 00:00   |
Craig Markwardt
Messages: 1869
Registered: November 1996
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Senior Member |
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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.
Craig
--
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Craig B. Markwardt, Ph.D. EMAIL: craigmnet@cow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
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