Any expression that evaluates to a matrix can have its elements
referenced. The simplest case occurs when a matrix has been created
and assigned to a variable. One can reference single elements, or
one can reference full or partial rows and/or columns of a matrix.
Element referencing is performed via the `[ ]
' operators,
using the `;
' to delimit row and column specifications, and
the `,
' to delimit individual row or column specifications.
To reference a single element:
> a = [1,2,3; 4,5,6; 7,8,9]; > a [ 2 ; 3 ] 6
To reference an entire row, or column:
> a [ 2 ; ] 4 5 6 > a [ ; 3 ] 3 6 9
To reference a sub-matrix:
> a [ 2,3 ; 1,2 ] 4 5 7 8
As stated previously, any expression that evaluates to a matrix can have its elements referenced. A very common example is getting the row or column dimension of a matrix:
> size (a)[1] 3
In the previous example the function size
returns a
two-element matrix, from which we extract the 1st element (the
value of the row dimension). Note that we referenced the return
value (a matrix) as if it were a vector. Referencing matrices in
``vector-fashion'' is allowed with all matrices. When
vector-indexing is used, the matrix elements are referenced in
column order. As with matrix indexing, a combination of vector
elements can be referenced:
> a[3] 7 > a[3,4,9] 7 2 9