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Re: The IDL way: Find last non-zero value [message #67746 is a reply to message #67745]

Sun, 23 August 2009 17:32

Chris[6]
Messages: 84
Registered: July 2008

Member

On Aug 24, 2:07 am, Eric Hudson <ehud...@mit.edu> wrote:
> Hi,
>
> I have a 2D array that looks something like:
> x x 0 x x 0 0 x 0 0 0 0 0
> x 0 x x x 0 x x x x 0 0 0
> x 0 0 0 0 0 0 0 0 0 0 0 0
> x x x x x 0 0 0 0 0 0 0 0
>
> where x is some non-zero (positive definite) value. You'll notice
> that each row ends with a string of zeros.
> What I'd like to know is the 'IDL way' of returning a vector of the
> location (column) of the last non-zero elements in each row. So in
> this case, [7,9,0,4]
>
> It's straight forward to program with loops, but I figure there must
> be a clever way. I thought that maybe reversing it and doing a
> cumulative total might be a start, but then I can't convince myself
> that that is really going to be faster than doing a loop.
>
> For a sense of scale, the real array is something like 200 x 160000
>
> Thanks,
> Eric

hmmmm.....

sz = size(array)
ncol = sz[1]
nrow = sz[2]
nonzero = where(array ne 0)
ind = array_indices(array, nonzero)
sorted = sort(ind[0, *])
result = fltarr(nrow) - 1
result[ind[1, sorted]] = ind[0, sorted]

kind of hacky, but heres the idea:

find all of the nonzero elements, and then use array_indices to give
their row/column numbers. Then, find sorting of the array_indices
array that puts the column indices in ascending order. Next, make a
result array with the correct size, and copy, _in this sorted order_,
the columns from the array_indices array into the representative rows
of the result vector. This way, low column indices will get
overwritten by higher column values during the copy.

My test on a small array:

IDL> print, array
1 2 0 0 0
0 0 0 0 1
1 1 3 1 0
0 0 0 0 0
IDL> print, result
1.00000 4.00000 3.00000 -1.00000

Of course, I'm not convinced that this is easier to read or faster
than using a loop....

Chris

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[Message index]

		Re: The IDL way: Find last non-zero value By: Jeremy Bailin on Mon, 24 August 2009 19:53
		Re: The IDL way: Find last non-zero value By: Jeremy Bailin on Mon, 24 August 2009 19:43
		Re: The IDL way: Find last non-zero value By: penteado on Mon, 24 August 2009 18:35
		Re: The IDL way: Find last non-zero value By: Jeremy Bailin on Mon, 24 August 2009 17:12
		Re: The IDL way: Find last non-zero value By: Eric Hudson on Mon, 24 August 2009 13:42
		Re: The IDL way: Find last non-zero value By: cgguido on Sun, 23 August 2009 20:39
		Re: The IDL way: Find last non-zero value By: penteado on Sun, 23 August 2009 19:10
		Re: The IDL way: Find last non-zero value By: Chris[6] on Sun, 23 August 2009 17:32

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