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Re: adding a 4th dimension to 3D array during concatenation [message #92872 is a reply to message #92869]

Wed, 16 March 2016 08:00

Paul Van Delst[1]
Messages: 1157
Registered: April 2002

Senior Member

Hello,

On 03/15/16 18:37, Wayana Dolan wrote:
> So outside a loop I start out with an empty array (ex. array=[ ] ).
> Then each time through the loop, I make an array with 3 dimensions
> (for example, array x has dimensions[91, 41, 33] ), and then
> concatinate it to the previous array. (ex. array=[array, x]).
>
> Lets say we run through the loop 16 times. What I'd like as a result
> is something that has dimensions like this [16, 91, 41, 33].
>
> I'm not sure how to do this... I've looked at IDL coyote's
> concatenation tutorial, and still am having trouble.
>
> I'm pretty new to coding period, so this is a challenge. Any ideas?
>

It depends on what you want to do with your monster array after
concatenation, and will each array be the same shape? (Even if the
answer right now is yes, will they always be?)

Concatenation is a slow operation in IDL, and I have always found
multi-dimensional concatenation similar to dealing with regular
expression - counting all the [[['s and ]]]'s to make sure they match
up, etc. This is not a fault with IDL, IMO it's just that arrays,
really, are not meant to have those sorts of things done to them.

So, why use an array?

Why not, say, a list?

IDL> array=list()
IDL> help, array
ARRAY LIST <ID=1 NELEMENTS=0>
IDL> x=findgen(91,43,33)
IDL> array.Add, x
IDL> x=findgen(14,17,36)
IDL> array.Add, x
IDL> help, array
ARRAY LIST <ID=1 NELEMENTS=2>
IDL> help, array[0]
<Expression> FLOAT = Array[91, 43, 33]
IDL> help, array[1]
<Expression> FLOAT = Array[14, 17, 36]

Or a hash? Works similarly.

Lists and hashes are data constructs that are designed to be added to
and extended. Arrays, not so much.

Anyhoo...

cheers,

paulv

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[Message index]

		adding a 4th dimension to 3D array during concatenation By: wdolan on Tue, 15 March 2016 15:37
		Re: adding a 4th dimension to 3D array during concatenation By: Paul Van Delst[1] on Wed, 16 March 2016 08:00
		Re: adding a 4th dimension to 3D array during concatenation By: Jim Pendleton on Wed, 16 March 2016 18:46
		Re: adding a 4th dimension to 3D array during concatenation By: wlandsman on Wed, 16 March 2016 20:46
		Re: adding a 4th dimension to 3D array during concatenation By: Guilherme Gualda on Wed, 16 March 2016 22:52

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