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Re: Assignment Time for a 3d Variable [message #46470 is a reply to message #46466]

Wed, 23 November 2005 09:51

Antonio Santiago
Messages: 201
Registered: February 2004

Senior Member

On Wed, 2005-11-23 at 08:52 -0800, David Streutker wrote:
> David Fanning wrote:
>> The speed differences have to do with how you access different
>> parts of the array in memory. If the parts you want are contiguous,
>> then you can get them faster than you can if they are far apart in
>> memory. (Think how much faster it is to pick up the poker
>> chips when they are stacked than when they are scattered all
>> around the table.)
>>
>> To make these kinds of assignments as fast as possible, use
>> the TRANSPOSE function to organize the data into the fastest
>> possible position:
>>
>> IDL> Help, data
>> DATA BYTE = Array[3, 227, 149]
>> IDL> data = Transpose( Temporary(data), [2,3,1] )
>> IDL> Help, data
>> DATA BYTE = Array[227, 149, 3]
>
> How does one know which is the fastest possible position? Should the
> largest dimension be first? Nuno's example seems to imply that the
> first dimension is not the fastest accessed.
>
> -Dave
>

Also I note that it depends on the language IDL is implemented. By
default all ANSI C compilers use row major mode.
See the other post for the link to an example :)

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[Message index]

		Re: Assignment Time for a 3d Variable By: Mark Hadfield on Wed, 23 November 2005 12:17
		Re: Assignment Time for a 3d Variable By: Foldy Lajos on Wed, 23 November 2005 11:13
		Re: Assignment Time for a 3d Variable By: David Fanning on Wed, 23 November 2005 11:09
		Re: Assignment Time for a 3d Variable By: James Kuyper on Wed, 23 November 2005 10:35
		Re: Assignment Time for a 3d Variable By: K. Bowman on Wed, 23 November 2005 10:16
		Re: Assignment Time for a 3d Variable By: Antonio Santiago on Wed, 23 November 2005 09:51
		Re: Assignment Time for a 3d Variable By: David Streutker on Wed, 23 November 2005 08:52
		Re: Assignment Time for a 3d Variable By: David Fanning on Wed, 23 November 2005 07:57
		Re: Assignment Time for a 3d Variable By: Antonio Santiago on Wed, 23 November 2005 04:46

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