| Swap even/odd elements in array [message #44507] |
Wed, 22 June 2005 01:16  |
photosalex
Messages: 6
Registered: April 2005
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Junior Member |
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Hi All,
sorry if the question is trivial:
how could I swap even and odd elements of a 1-D 16-bit INT array
without using loops? That is, if the source array is
[a0,a1,a2,a3,..]
I want it to be
[a1,a0,a3,a2,..]
Thanks!
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| Re: Swap even/odd elements in array [message #44553 is a reply to message #44507] |
Tue, 28 June 2005 12:00  |
JD Smith
Messages: 850
Registered: December 1999
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Senior Member |
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On Wed, 22 Jun 2005 10:52:04 +0200, Xavier Llobet wrote:
> In article <1119428174.330105.170110@o13g2000cwo.googlegroups.com>,
> photosalex@freenetname.co.uk wrote:
>
>> Hi All,
>> sorry if the question is trivial:
>> how could I swap even and odd elements of a 1-D 16-bit INT array
>> without using loops? That is, if the source array is
>>
>> [a0,a1,a2,a3,..]
>>
>> I want it to be
>>
>> [a1,a0,a3,a2,..]
>>
>> Thanks!
>
> n = (size(a))(1)
> ind=2*indgen(n)
> b=reform(transpose(reform([a(ind+1),a(ind)],n/2,2)),n)
A similar approach using the new stride indexing operator would be:
b=reform(transpose([[a[1:*:2]],[a[0:*:2]]]),n_elements(a))
The problem with all of these dimensional-juggling approaches is that
they fail for vectors of odd length (or, more generally, require the
length be a strict multiple of p, where p is the transposition length).
Here's a method which doesn't have this requirement:
ind=lindgen(n_elements(a))/2*2 & ind[0:*:2]+=1
b=a[ind]
It can be generalized for any p like this:
ind=lindgen(n_elements(a))/p*p
for i=0,p-2 do ind[i:*:p]+=p-1-i
b=a[ind]
It does require the vector to have at least p-1 elements.
JD
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| Re: Swap even/odd elements in array [message #44576 is a reply to message #44507] |
Thu, 23 June 2005 00:52 |
Craig Markwardt
Messages: 1869
Registered: November 1996
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Senior Member |
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"Peter Mason" <peter.mason@deleteme.csiro.au> writes:
> photosalex@freenetname.co.uk wrote:
>> Hi All,
>> sorry if the question is trivial:
>> how could I swap even and odd elements of a 1-D 16-bit INT array
>> without using loops?
>
> One from the south-east:
> j=indgen(16)
> print,fix(swap_endian(long(swap_endian(j),0,8),0,16)
... and a sure route to insanity. :-)
Craig
--
------------------------------------------------------------ --------------
Craig B. Markwardt, Ph.D. EMAIL: craigmnet@REMOVEcow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
------------------------------------------------------------ --------------
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| Re: Swap even/odd elements in array [message #44589 is a reply to message #44507] |
Wed, 22 June 2005 06:22 |
K. Bowman
Messages: 330
Registered: May 2000
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Senior Member |
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In article <1119428174.330105.170110@o13g2000cwo.googlegroups.com>,
photosalex@freenetname.co.uk wrote:
> Hi All,
> sorry if the question is trivial:
> how could I swap even and odd elements of a 1-D 16-bit INT array
> without using loops? That is, if the source array is
>
> [a0,a1,a2,a3,..]
>
> I want it to be
>
> [a1,a0,a3,a2,..]
>
> Thanks!
IDL> x = lindgen(16)
IDL> print, x
0 1 2 3 4 5
6 7
8 9 10 11 12 13
14 15
IDL> x = reform(reverse(reform(x, 2, 8), 1), 16)
IDL> print, x
1 0 3 2 5 4
7 6
9 8 11 10 13 12
15 14
Cheers, Ken Bowman
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| Re: Swap even/odd elements in array [message #44594 is a reply to message #44507] |
Wed, 22 June 2005 01:52 |
Xavier Llobet
Messages: 7
Registered: April 2005
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Junior Member |
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In article <1119428174.330105.170110@o13g2000cwo.googlegroups.com>,
photosalex@freenetname.co.uk wrote:
> Hi All,
> sorry if the question is trivial:
> how could I swap even and odd elements of a 1-D 16-bit INT array
> without using loops? That is, if the source array is
>
> [a0,a1,a2,a3,..]
>
> I want it to be
>
> [a1,a0,a3,a2,..]
>
> Thanks!
n = (size(a))(1)
ind=2*indgen(n)
b=reform(transpose(reform([a(ind+1),a(ind)],n/2,2)),n)
Bear my IDL 3.6.1 notation...
--
_xavier
--
Only one "o" in my e-mail address
--
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?
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| Re: Swap even/odd elements in array [message #44595 is a reply to message #44589] |
Wed, 22 June 2005 07:06 |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Ken Bowman writes:
>> how could I swap even and odd elements of a 1-D 16-bit INT array
>> without using loops?
>
> IDL> x = lindgen(16)
> IDL> print, x
> 0 1 2 3 4 5
> 6 7
> 8 9 10 11 12 13
> 14 15
> IDL> x = reform(reverse(reform(x, 2, 8), 1), 16)
> IDL> print, x
> 1 0 3 2 5 4
> 7 6
> 9 8 11 10 13 12
> 15 14
Those of you who have been invited to answer the Test Question
(http://www.dfanning.com/misc_tips/iepa.html) at this year's
IEPA conference might want to take careful note of this answer.
I'd say the chances are pretty good you might see it again. ;-)
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/
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