An index expression allows you to reference or extract selected elements of a matrix or vector.
Indices may be scalars, vectors, ranges, or the special operator `:', which may be used to select entire rows or columns.
Vectors are indexed using a single expression. Matrices may be
indexed using one or two indices (a warning is issued if a single
index is used unless the value of the built-in variable
warn_fortran_indexing is zero).
If the value of
warn_fortran_indexingis nonzero, a warning is printed for expressions which select elements of a two-dimensional matrix using a single index. The default value is 0.
Given the matrix
a = [1, 2; 3, 4]
all of the following expressions are equivalent
a (1, [1, 2]) a (1, 1:2) a (1, :)
and select the first row of the matrix.
Indexing a scalar with a vector of ones can be used to create a vector the same size as the index vector, with each element equal to the value of the original scalar. For example, the following statements
a = 13; a ([1, 1, 1, 1])
produce a vector whose four elements are all equal to 13.
Similarly, indexing a scalar with two vectors of ones can be used to create a matrix. For example the following statements
a = 13; a ([1, 1], [1, 1, 1])
create a 2 by 3 matrix with all elements equal to 13.
This is an obscure notation and should be avoided. It is better to
use the function
ones to generate a matrix of the appropriate
size whose elements are all one, and then to scale it to produce the
desired result. See Special Utility Matrices.
If the value of
warn_resize_on_range_erroris nonzero, print a warning when a matrix is resized by an indexed assignment with indices outside the current bounds. The default value is 0.
Note that it is quite inefficient to create a vector using a loop like the one shown in the example above. In this particular case, it would have been much more efficient to use the expression
a = sqrt (1:10);
thus avoiding the loop entirely. In cases where a loop is still
required, or a number of values must be combined to form a larger
matrix, it is generally much faster to set the size of the matrix first,
and then insert elements using indexing commands. For example, given a
[nr, nc] = size (a); x = zeros (nr, n * nc); for i = 1:n x(:,(i-1)*nc+1:i*nc) = a; endfor
is considerably faster than
x = a; for i = 1:n-1 x = [x, a]; endfor
particularly for large matrices because Octave does not have to repeatedly resize the result.