Why are leading singleton dimensions not dropped when displaying size of multidimensional arrays?

When displaying sizes of multidimensional arrays, the SIZE function retains the leading singleton dimensions, but drops the trailing singleton dimensions.

For example, consider the following multidimensional array:

for i=1:160

   myArr(:,:,i) = ones(128);

end

size(myArr(:,:,1))

For the array 'myArr', the SIZE function returns

 ans =

    128   128

However the commands:

size(myArr(:,1,:))

size(myArr(1,:,:))

return

 ans =

    128     1   160

 ans =

      1   128   160

respectively.

Answer 1

[MathWorks Support Team]

This is the expected behavior of multidimensional arrays.

Leading singleton dimensions are not dropped because they cannot be added on the fly (without a call to the RESHAPE function) to maintain memory integrity. However, trailing singleton dimensions are dropped because leaving them out will not affect memory integrity.

If the leading singleton dimensions are not required, the SQUEEZE function can be used. Hence for the example considered, the commands:

size(squeeze(myArr(:,1,:)))

size(squeeze(myArr(1,:,:)))

yield the results:

 ans =

    128   160

 ans =

    128   160

respectively, without displaying the singleton dimensions.

For more information regarding the SQUEEZE function, enter the following at the MATLAB Command Prompt:

doc squeeze