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Re: Sky Falling, etc. : Array substitution + addition with plus-equal (+=) [message #53029 is a reply to message #52941]

Tue, 13 March 2007 05:26

Foldy Lajos
Messages: 268
Registered: October 2001

Senior Member

On Mon, 12 Mar 2007, Ed Hyer wrote:

> Tentatively filed under "The Sky Is Falling!," and I hope it will be
> resolved in the same fashion.
>
> Here is the test case, which gives the correct result:
>
> IDL> test=fltarr(2,2,4)
> IDL> testadd=fltarr(2,2,2) + 1
> IDL> print,total(test),total(testadd)
> 0.00000 8.00000
> IDL> test[0,0,1] += testadd
> IDL> print,total(test),total(testadd)
> 8.00000 8.00000
> IDL> test[0,0,1] += testadd
> IDL> print,total(test),total(testadd)
> 16.0000 8.00000
> IDL> print,test
> 0.00000 0.00000
> 0.00000 0.00000
>
> 2.00000 2.00000
> 2.00000 2.00000
>
> 2.00000 2.00000
> 2.00000 2.00000
>
> 0.00000 0.00000
> 0.00000 0.00000
>
> That is exactly what I expect from this operation.
>
> Here is another case, with results somewhat different
> IDL> test=fltarr(2,2,4)
> IDL> testadd=fltarr(2,2,2)
> IDL> testadd[0,0,*]=1
> IDL> print,total(test),total(testadd)
> 0.00000 2.00000
> IDL> test[0,0,1] += testadd
> IDL> print,total(test),total(testadd)
> 2.00000 2.00000
> IDL> print,test
> 0.00000 0.00000
> 0.00000 0.00000
>
> 1.00000 0.00000
> 0.00000 0.00000
>
> 1.00000 0.00000
> 0.00000 0.00000
>
> 0.00000 0.00000
> 0.00000 0.00000 ; So far, so good
> IDL> test[0,0,1] += testadd ; Now, let's add again
> IDL> print,total(test),total(testadd)
> 10.0000 2.00000 ; Pardon the
> expression, WTF?
> IDL> print,test
> 0.00000 0.00000
> 0.00000 0.00000
>
> 2.00000 1.00000
> 1.00000 1.00000
>
> 2.00000 1.00000
> 1.00000 1.00000
>
> 0.00000 0.00000
> 0.00000 0.00000 ; Not even JD Smith
> expected _that_.
>
> Any ideas?
>

FL gives the same result, so it is correct :-)))

test[0,0,1] += testadd is transformed to the following form:

tmp = test[0,0,1] + testadd
test[0,0,1] = tmp

The first line is scalar - array addition, so the scalar is added to every
element of the array (8 addition!), and tmp will be an array.

In the second line an array is assigned to a scalar subscripted array, so
tmp is inserted into test.

I hope this helps.

regards,
lajos

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[Message index]

		Sky Falling, etc. : Array substitution + addition with plus-equal (+=) By: MarioIncandenza on Mon, 12 March 2007 14:17
		Re: Sky Falling, etc. : Array substitution + addition with plus-equal (+=) By: JD Smith on Tue, 13 March 2007 13:37
		Re: Sky Falling, etc. : Array substitution + addition with plus-equal (+=) By: MarioIncandenza on Tue, 13 March 2007 11:35
		Re: Sky Falling, etc. : Array substitution + addition with plus-equal (+=) By: JD Smith on Tue, 13 March 2007 10:46
		Re: Sky Falling, etc. : Array substitution + addition with plus-equal (+=) By: Foldy Lajos on Tue, 13 March 2007 05:26
		Re: Sky Falling, etc. : Array substitution + addition with plus-equal (+=) By: Y.T. on Tue, 13 March 2007 00:19

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