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Re: Array multiplication: implicit loop query [message #26246 is a reply to message #26191]

Mon, 13 August 2001 06:58

Craig Markwardt
Messages: 1869
Registered: November 1996

Senior Member

george@apg.ph.ucl.ac.uk (george Millward) writes:
>
> I have inserted the "rebin" function and this works fine.
> I am still a little intrigued as to why IDL works this way - it still
> seems to me that my orginal combination of 3D and 1D arrays should
> yield a 3D array. Not a problem - we all live with "features" of
> programming languages - just wondering.

Heh, the reason is pretty simple, if not intuitive. When confronted
with operations between arrays of different sizes, IDL will *truncate*
the longest array to the shortest size. I think this rule is pretty
general but somebody will probably pipe in with an exception.

That "limitation" can sometimes be used to your advantage. For
example, when doing finite differencing, a[i+1] - a[i] for each
element, normally you would write that like:

diff = a(1:*) - a(0:n_elements(a)-2)

The expression A(0:N_ELEMENTS(A)-2) removes that last element of the
array A. But that is a little obscure. It's almost "better" to say,

diff = a(1:*) - a(0:*) ; or
diff = a(1:*) - a

It saves keystrokes, avoids subscript clutter, etc. But why does it
work? The answer is that A(1:*) has one less element than A, so when
A appears by itself in the above equation, it is automatically
truncated by one, achieve the desired result.

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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[Message index]

		Array multiplication: implicit loop query By: george on Fri, 10 August 2001 03:56
		Re: Array multiplication: implicit loop query By: bennetsc on Tue, 14 August 2001 23:57
		Re: Array multiplication: implicit loop query By: Richard Younger on Mon, 13 August 2001 13:59
		Re: Array multiplication: implicit loop query By: david[2] on Mon, 13 August 2001 11:52
		Re: Array multiplication: implicit loop query By: Richard Younger on Mon, 13 August 2001 11:37
		Re: Array multiplication: implicit loop query By: david[2] on Mon, 13 August 2001 07:55
		Re: Array multiplication: implicit loop query By: Craig Markwardt on Mon, 13 August 2001 06:58
		Re: Array multiplication: implicit loop query By: Paul van Delst on Mon, 13 August 2001 06:46
		Re: Array Multiplication By: Craig Markwardt on Sun, 25 March 2012 12:58
		Re: Array Multiplication By: lecacheux.alain on Sun, 25 March 2012 08:09

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