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Re: simple array math question [message #33685 is a reply to message #33670]

Fri, 17 January 2003 20:21

Jeff Guerber
Messages: 41
Registered: July 2000

Member

On 16 Jan 2003, Craig Markwardt wrote:

> Heinz Stege <reply_to_posting@arcor.de> writes:
>> On Thu, 16 Jan 2003 14:05:27 -0600, "Sean Raffuse" <sean@me.wustl.edu>
>> wrote:
>>
>>>> > a=[[1,2,3],[4,5,6],[7,8,9]]
>>>
>>>> > b=[1,2,3]
>>>
>>> What is the best (read, fastest) way to multiply b by each individual row of
>>> a? I would like to return a result of:
>>>
>>> [[1,4,9],[4,10,18],[7,14,27]]
>>
>>
>> result=a*b(*,intarr(3))
>
> WOW! I've never seen that! It scares me how cool that is. :-)
>
> Craig

That IS way cool, but, uhhh, would someone mind explaining just what's
going on? I'm not getting it. It seems to only depend on the total
number of elements in the indexing array, not on its values (or even
dimensionality):

IDL> b=[4,5,6]
IDL> print,b[*,[10,10,10]]
4 5 6
4 5 6
4 5 6
IDL> print,b[*,[1000,1000]]
4 5 6
4 5 6
IDL> print,b[*,[[10,10,10],[10,10,10]]]
4 5 6
4 5 6
4 5 6
4 5 6
4 5 6
4 5 6
IDL> help,b[*,[[10,10,10],[10,10,10]]]
<Expression> INT = Array[3, 6]
IDL>

Oh, you can apply it multiple times, too:

IDL> print,b[*,[10,10,10,10],[10,10]]
4 5 6
4 5 6
4 5 6
4 5 6

4 5 6
4 5 6
4 5 6
4 5 6
IDL> help,b[*,[10,10,10,10],[10,10]]
<Expression> INT = Array[3, 4, 2]
IDL>

Thanks,

Jeff Guerber

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[Message index]

		Re: simple array math question By: marc schellens[1] on Sun, 19 January 2003 22:55
		Re: simple array math question By: Jeff Guerber on Fri, 17 January 2003 20:21
		Re: simple array math question By: Heinz Stege on Fri, 17 January 2003 16:51
		Re: simple array math question By: Sean Raffuse on Fri, 17 January 2003 14:10
		Re: simple array math question By: JD Smith on Fri, 17 January 2003 13:46
		Re: simple array math question By: Craig Markwardt on Thu, 16 January 2003 19:47
		Re: simple array math question By: Heinz Stege on Thu, 16 January 2003 16:26
		Re: simple array math question By: Craig Markwardt on Thu, 16 January 2003 13:30
		Re: simple array math question By: Pepijn Kenter on Fri, 24 January 2003 04:43
		Re: simple array math question By: JD Smith on Thu, 23 January 2003 08:53
		Re: simple array math question By: Heinz Stege on Sun, 26 January 2003 11:25
		Re: simple array math question By: Jeff Guerber on Sat, 25 January 2003 01:12

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