在Matlab中对3D矩阵的特定维度求和

Question

I have a 3D matrix in Matlab of size NxMxD and i need to sum over a specific dimension:

x = rand(5,3,2);
sum1 = sum(x,1);
sum2 = sum(x,2);
sum3 = sum(x,3);

I would obviously expect in the 3 case that the result would be either a 2D matrix or a 3D matrix with 1 dimension of length 1. This is unfortunately not the case.

sum1 and sum2 are 3D matrix with the dimension over which the sum is done of length 1 but sum3 is a 2D matrix.

I would like to be able to get sum1 and sum2 as 2D matrix in the similar way sum3 is being computed. Is that possible to do using sum only or the only way is to further use for example the squeeze() function )

Answer 1

As @Tommaso says, the third output is 5x3x1, though when you ask for its size MATLAB cuts of trailing ones.

MATLAB, cannot automatically do this in other places, because this would change the matrix/tensor, remember that MATLAB is “primarily” for matrices.

A 1xn vector is not the same as an nx1 vector, when performing matrix operations.

Tl;dr, you will have to use squeeze or reshape