row-column confusion with squeeze

Question

Abdullah Fahim 2016-10-25

1
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评论： Abdullah Fahim 2016-10-26

Can anybody please explain why A has 3x1 dimension whereas B has 2x3 dimension?

I mean, in both cases, I put 3 as the last dimension in the 3D matrix. So, both A and B should have either 3 rows or 3 columns, right?

If the argument is, squeeze just get rid of all the single-tone dimensions without keeping the relative position, then A and C should be of same dimension? But that is not the case as well.

A=squeeze(zeros(1,1,3))
A =
       0
       0
       0
B=squeeze(zeros(1,2,3))
B =
       0     0     0
       0     0     0
C=squeeze(zeros(1,3,1))
C =
       0     0     0

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David Goodmanson 2016-10-25

Probably not real logical. It seems reasonable that (3,1,1,1) should become a column vector of size (3,1) (which happens) and (1,1,1,3) should become a row vector of size (1,3) (which doesn't happen). But then should ( [m ones] 3 [n ones] ) become a row vector or a column vector?

Abdullah Fahim 2016-10-26

Good question.. I have no idea :). I got your point, so dropping the subject.

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Answer 1

Thorsten 2016-10-25

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编辑：Thorsten 2016-10-25

在 MATLAB Online 中打开

Squeeze squeezes out all singleton dimensions of X, if X is not a matrix.

In case of

C = squeeze(zeros(1,3,1))
whos C
Name      Size            Bytes  Class     Attributes
C         1x3                24  double

you already have a matrix, namely a 1 x 3 matrix. The trailing singleton dimensions are ignored by zeros.

isequal(zeros(1,3,1), zeros(1,3))
ans =
     1

or

isequal(zeros(1,3,1,1,1,1,1), zeros(1,3))
ans =
     1

So in case of C, nothing is squeezed and C is left as is.

In case of A, you don't have a matrix, but a 1 x 1 x 3 array. The first two singleton dimensions are squeezed out, resulting in 3 as the first dimension. Because there is no vector in Matlab, a second dimension is needed, which is 1. So A 1 x 1 x 3 is squeezed to 3 x 1.

In case of B, you have a 1 x 2 x 3 array. The first dimension is a singleton dimension which is squeezed out, resulting in a 2 x 3 matrix.

One could see an inconsistency between the handling of A and C, because the squeezing algorithm is not applied to C, since C is a matrix. But the help of squeeze explicitly states

2-D arrays are unaffected by squeeze so that row vectors remain rows.

BTW, squeeze is not built-in, so you can view the code with

edit squeeze

or

type squeeze

and have a look at how it works.