MATLAB中sum函数

最近阅读代码发现一句： sum(3维数组, 常数)

查询matlab帮助文档后：

sum Sum of elements.
S = sum(X) is the sum of the elements of the vector X. If X is a matrix,
S is a row vector with the sum over each column. For N-D arrays,
sum(X) operates along the first non-singleton dimension.

S = sum(X,'all') sums all elements of X.

S = sum(X,DIM) sums along the dimension DIM.

S = sum(X,VECDIM) operates on the dimensions specified in the vector 
VECDIM. For example, sum(X,[1 2]) operates on the elements contained in
the first and second dimensions of X.

即：S = sum(X)是向量X的元素和。如果X是一个矩阵，S是一个行向量，每个列的和。对于N-D数组，sum(X)沿着第一个非单例维度运行。
S = sum(X,VECDIM)作用于向量VECDIM中指定的维数。例如，sum(X，[1 2])作用于X的第一维和第二维中包含的元素。

下面用一个实例来说明：

首先建立一个自己的三维数组：

>> a = zeros(2,3,2);
>> a(:,:,1)= [1 1 1 ;2 2 2];
>> a(