The role of singleton elements in calls to the zeros function.

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Marshall 2014-10-2

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评论： Marshall 2014-10-2

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Hello MatLab'rs,

I'm confused by the return of zeros under certain circumstances.

For some vector V, calling zeros will create a length(V) dimensional matrix:

>> V = [ 1:10 ]
V =
       1     2     3     4     5     6     7     8     9    10
>> Z = zeros(V); % Z = <10-D double>

In my code I have a selectedDimLengths vector, each element of which can change from a singleton up to about 6 or 7, but often looks something like:

selectedDimLengths =
       1     1     1     5     5     1     1     1     3     1     1     1     1
>> Z = zeros(selectedDimLengths); % Z = <9-D double>

when I pass this to zeros I lose a couple of dimensions, down to 9 when I expect 13. What role do the singleton elements play here? Why?

Are the trailing singleton elements cropped somehow?

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Answer 1

Marshall 2014-10-2

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Um, so I'll answer my own question instead of deleting in case someone comes along looking.

I found the information required here in this article by the amazing Loren Shure:

http://blogs.mathworks.com/loren/2006/08/09/essence-of-indexing/

"Arrays in MATLAB are N-dimensional, with an infinite number of trailing singleton dimensions. Trailing singleton dimensions past the second are not displayed or reported on, e.g., with size. "

So I just now need to think of a different way to assess that vector... :)

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Marshall 2014-10-2

Oh, typed and went to lunch, submitted and then found Iain's answer too.

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Answer 2

Iain 2014-10-2

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Matlab only really needs to know which dimensions are "nonsingular". - A scalar has infinite dimensions, all with a size of 1. - A vector has infinite dimensions, only one of which has a size other than 1.