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Re: Padding arrays - vector subscripts not working [message #71584 is a reply to message #71583]

Fri, 02 July 2010 12:29

penteado
Messages: 866
Registered: February 2018

Senior Member
Administrator

On Jul 2, 3:51 pm, James <donje...@gmail.com> wrote:
> Well, I see that this was already asked by hradliv in 2007 and
> answered by JD Smith, among others. Still, it seems strange that the
> natural method I tried to use doesn't work.

It is not strange.

X[1,1,1] = Y

and

X[diff] = Y with diff=[1,1,1]

are different things. X[diff] is the same as X[[1,1,1]].

To see the difference, consider this case (where I defined X so that
its elements are not all equal)

IDL> X = bindgen(5,5,5)

IDL> print,X[1,1,1]
31
IDL> print,X[31]
31

Both of these print lines are accessing the same (one) element of X,
the one at the (3D) indexes 1,1,1, which, as it happens, is the 32nd
element of the array:

IDL> print,array_indices(X,31)
1 1 1

Now,

IDL> diff=[1,1,1]
IDL> print,X[diff]
1 1 1

X[diff], which is the same as X[[1,1,1]], is a 3-element array, where
each element has the element of X with the (1D) index 1, that is, this
is 3 copies of the second element of X:

IDL> print,array_indices(X,1)
1 0 0
IDL> print,X[1,0,0]
1

X[diff] is the same as X[1,0,0], repeated 3 times.

Now, looking at your assignments:

X[1,1,1]=Y

is assigning 12 elements of X (Y has 12 elements), sequentially,
starting at X[1,1,1]. Which is working only because X has 125
elements, and this assignment is using 12 elements, starting at the
32nd, thus it fits inside X (it is assigning from to the 32nd to the
43rd element of X the 12 values in Y)

X[diff], since it is X[[1,1,1]] is attempting to assign the 12 values
of Y into 3 values (since X[[1,1,1]] is a 3-element array), thus it
fails. Only if you tried to assign to X[diff] a 3-element array (or a
scalar, which would be repeated), would this have worked.

If this is seeming too abstract, redo these examples with small 2D
arrays (which print nicely), and look at the whole X array, and how it
changes after assignments.

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[Message index]

		Re: Padding arrays - vector subscripts not working By: penteado on Sat, 03 July 2010 14:01
		Re: Padding arrays - vector subscripts not working By: James[2] on Sat, 03 July 2010 13:52
		Re: Padding arrays - vector subscripts not working By: penteado on Fri, 02 July 2010 13:01
		Re: Padding arrays - vector subscripts not working By: penteado on Fri, 02 July 2010 12:29
		Re: Padding arrays - vector subscripts not working By: James[2] on Fri, 02 July 2010 11:51
		Re: Padding arrays - vector subscripts not working By: James[2] on Tue, 06 July 2010 11:38

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