This is the usual way of referencing row and column vectors. But it can also apply to a fully two-dimensional matrix, in which case the array is regarded as one long column vector formed from the columns of the original matrix. You can reduce the size of this expression using the colon operator.
Subscript expressions involving colons refer to portions of a matrix.
Using this notation, you can compute the sum of the fourth column of A more succinctly. The colon by itself refers to all the elements in a row or column of a matrix. The keyword end refers to the last row or column. Using the following syntax, you can compute this same column sum without having to specify row and column numbers. By adding an additional colon operator, you can refer to nonconsecutive elements in a matrix.