octave: Basic Matrix Functions
18.2 Basic Matrix Functions
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-- : AA = balance (A)
-- : AA = balance (A, OPT)
-- : [DD, AA] = balance (A, OPT)
-- : [D, P, AA] = balance (A, OPT)
-- : [CC, DD, AA, BB] = balance (A, B, OPT)
Balance the matrix A to reduce numerical errors in future
calculations.
Compute ‘AA = DD \ A * DD’ in which AA is a matrix whose row and
column norms are roughly equal in magnitude, and ‘DD = P * D’, in
which P is a permutation matrix and D is a diagonal matrix of
powers of two. This allows the equilibration to be computed
without round-off. Results of eigenvalue calculation are typically
improved by balancing first.
If two output values are requested, ‘balance’ returns the diagonal
D and the permutation P separately as vectors. In this case, ‘DD =
eye(n)(:,P) * diag (D)’, where n is the matrix size.
If four output values are requested, compute ‘AA = CC*A*DD’ and ‘BB
= CC*B*DD’, in which AA and BB have nonzero elements of
approximately the same magnitude and CC and DD are permuted
diagonal matrices as in DD for the algebraic eigenvalue problem.
The eigenvalue balancing option OPT may be one of:
"noperm", "S"
Scale only; do not permute.
"noscal", "P"
Permute only; do not scale.
Algebraic eigenvalue balancing uses standard LAPACK routines.
Generalized eigenvalue problem balancing uses Ward’s algorithm
(SIAM Journal on Scientific and Statistical Computing, 1981).
-- : BW = bandwidth (A, TYPE)
-- : [LOWER, UPPER] = bandwidth (A)
Compute the bandwidth of A.
The TYPE argument is the string "lower" for the lower bandwidth and
"upper" for the upper bandwidth. If no TYPE is specified return
both the lower and upper bandwidth of A.
The lower/upper bandwidth of a matrix is the number of
subdiagonals/superdiagonals with nonzero entries.
See also: 
isbanded XREFisbanded, isdiag XREFisdiag,
istril XREFistril, istriu XREFistriu.
-- : cond (A)
-- : cond (A, P)
Compute the P-norm condition number of a matrix with respect to
inversion.
‘cond (A)’ is defined as ‘norm (A, P) * norm (inv (A), P)’.
By default, ‘P = 2’ is used which implies a (relatively slow)
singular value decomposition. Other possible selections are ‘P =
1, Inf, "fro"’ which are generally faster. See ‘norm’ for a full
discussion of possible P values.
The condition number of a matrix quantifies the sensitivity of the
matrix inversion operation when small changes are made to matrix
elements. Ideally the condition number will be close to 1. When
the number is large this indicates small changes (such as underflow
or round-off error) will produce large changes in the resulting
output. In such cases the solution results from numerical
computing are not likely to be accurate.
DONTPRINTYET See also: condest XREFcondest, rcond XREFrcond, *noteDONTPRINTYET See also: condest XREFcondest, rcond XREFrcond,
condeig XREFcondeig, norm XREFnorm, svd XREFsvd.
-- : C = condeig (A)
-- : [V, LAMBDA, C] = condeig (A)
Compute condition numbers of a matrix with respect to eigenvalues.
The condition numbers are the reciprocals of the cosines of the
angles between the left and right eigenvectors; Large values
indicate that the matrix has multiple distinct eigenvalues.
The input A must be a square numeric matrix.
The outputs are:
• C is a vector of condition numbers for the eigenvalues of A.
• V is the matrix of right eigenvectors of A. The result is
equivalent to calling ‘[V, LAMBDA] = eig (A)’.
• LAMBDA is the diagonal matrix of eigenvalues of A. The result
is equivalent to calling ‘[V, LAMBDA] = eig (A)’.
Example
a = [1, 2; 3, 4];
c = condeig (a)
⇒ [1.0150; 1.0150]
DONTPRINTYET See also: eig XREFeig, cond XREFcond, *notebalance:
DONTPRINTYET See also: eig XREFeig, cond XREFcond, balance
XREFbalance.
-- : det (A)
-- : [D, RCOND] = det (A)
Compute the determinant of A.
Return an estimate of the reciprocal condition number if requested.
Programming Notes: Routines from LAPACK are used for full matrices
and code from UMFPACK is used for sparse matrices.
The determinant should not be used to check a matrix for
singularity. For that, use any of the condition number functions:
‘cond’, ‘condest’, ‘rcond’.
DONTPRINTYET See also: cond XREFcond, condest XREFcondest, *noteDONTPRINTYET See also: cond XREFcond, condest XREFcondest,
rcond XREFrcond.
-- : LAMBDA = eig (A)
-- : LAMBDA = eig (A, B)
-- : [V, LAMBDA] = eig (A)
-- : [V, LAMBDA] = eig (A, B)
-- : [V, LAMBDA, W] = eig (A)
-- : [V, LAMBDA, W] = eig (A, B)
-- : [...] = eig (A, BALANCEOPTION)
-- : [...] = eig (A, B, ALGORITHM)
-- : [...] = eig (..., EIGVALOPTION)
Compute the eigenvalues (LAMBDA) and optionally the right
eigenvectors (V) and the left eigenvectors (W) of a matrix or a
pair of matrices.
The flag BALANCEOPTION can be one of:
"balance"
Preliminary balancing is on. (default)
"nobalance"
Disables preliminary balancing.
The flag EIGVALOPTION can be one of:
"matrix"
Return the eigenvalues in a diagonal matrix. (default if 2 or
3 outputs are specified)
"vector"
Return the eigenvalues in a column vector. (default if 1
output is specified, e.g. LAMBDA = eig (A))
The flag ALGORITHM can be one of:
"chol"
Uses the Cholesky factorization of B. (default if A is
symmetric (Hermitian) and B is symmetric (Hermitian) positive
definite)
"qz"
Uses the QZ algorithm. (When A or B are not symmetric always
the QZ algorithm will be used)
no flag chol qz
-----------------------------------------------------------------------------
both are symmetric "chol" "chol" "qz"
at least one is not "qz" "qz" "qz"
symmetric
The eigenvalues returned by ‘eig’ are not ordered.
See also: eigs XREFeigs, svd XREFsvd.
-- : G = givens (X, Y)
-- : [C, S] = givens (X, Y)
Compute the Givens rotation matrix G.
The Givens matrix is a 2 by 2 orthogonal matrix
‘G = [C S; -S' C]’
such that
‘G [X; Y] = [*; 0]’
with X and Y scalars.
If two output arguments are requested, return the factors C and S
rather than the Givens rotation matrix.
For example:
givens (1, 1)
⇒ 0.70711 0.70711
-0.70711 0.70711
See also: planerot XREFplanerot.
-- : [G, Y] = planerot (X)
Given a two-element column vector, return the 2 by 2 orthogonal
matrix G such that ‘Y = G * X’ and ‘Y(2) = 0’.
See also: givens XREFgivens.
-- : X = inv (A)
-- : [X, RCOND] = inv (A)
Compute the inverse of the square matrix A.
Return an estimate of the reciprocal condition number if requested,
otherwise warn of an ill-conditioned matrix if the reciprocal
condition number is small.
In general it is best to avoid calculating the inverse of a matrix
directly. For example, it is both faster and more accurate to
solve systems of equations (A*x = b) with ‘Y = A \ b’, rather than
‘Y = inv (A) * b’.
If called with a sparse matrix, then in general X will be a full
matrix requiring significantly more storage. Avoid forming the
inverse of a sparse matrix if possible.
See also: ldivide XREFldivide, rdivide XREFrdivide,
pinv XREFpinv.
-- : X = linsolve (A, B)
-- : X = linsolve (A, B, OPTS)
-- : [X, R] = linsolve (...)
Solve the linear system ‘A*x = b’.
With no options, this function is equivalent to the left division
operator (‘x = A \ b’) or the matrix-left-divide function
(‘x = mldivide (A, b)’).
Octave ordinarily examines the properties of the matrix A and
chooses a solver that best matches the matrix. By passing a
structure OPTS to ‘linsolve’ you can inform Octave directly about
the matrix A. In this case Octave will skip the matrix examination
and proceed directly to solving the linear system.
*Warning:* If the matrix A does not have the properties listed in
the OPTS structure then the result will not be accurate AND no
warning will be given. When in doubt, let Octave examine the
matrix and choose the appropriate solver as this step takes little
time and the result is cached so that it is only done once per
linear system.
Possible OPTS fields (set value to true/false):
LT
A is lower triangular
UT
A is upper triangular
UHESS
A is upper Hessenberg (currently makes no difference)
SYM
A is symmetric or complex Hermitian (currently makes no
difference)
POSDEF
A is positive definite
RECT
A is general rectangular (currently makes no difference)
TRANSA
Solve ‘A'*x = b’ by ‘transpose (A) \ b’
The optional second output R is the inverse condition number of A
(zero if matrix is singular).
DONTPRINTYET See also: mldivide XREFmldivide, *notematrix_type:
DONTPRINTYET See also: mldivide XREFmldivide, matrix_type
XREFmatrix_type, rcond XREFrcond.
-- : TYPE = matrix_type (A)
-- : TYPE = matrix_type (A, "nocompute")
-- : A = matrix_type (A, TYPE)
-- : A = matrix_type (A, "upper", PERM)
-- : A = matrix_type (A, "lower", PERM)
-- : A = matrix_type (A, "banded", NL, NU)
Identify the matrix type or mark a matrix as a particular type.
This allows more rapid solutions of linear equations involving A to
be performed.
Called with a single argument, ‘matrix_type’ returns the type of
the matrix and caches it for future use.
Called with more than one argument, ‘matrix_type’ allows the type
of the matrix to be defined.
If the option "nocompute" is given, the function will not attempt
to guess the type if it is still unknown. This is useful for
debugging purposes.
The possible matrix types depend on whether the matrix is full or
sparse, and can be one of the following
"unknown"
Remove any previously cached matrix type, and mark type as
unknown.
"full"
Mark the matrix as full.
"positive definite"
Probable full positive definite matrix.
"diagonal"
Diagonal matrix. (Sparse matrices only)
"permuted diagonal"
Permuted Diagonal matrix. The permutation does not need to be
specifically indicated, as the structure of the matrix
explicitly gives this. (Sparse matrices only)
"upper"
Upper triangular. If the optional third argument PERM is
given, the matrix is assumed to be a permuted upper triangular
with the permutations defined by the vector PERM.
"lower"
Lower triangular. If the optional third argument PERM is
given, the matrix is assumed to be a permuted lower triangular
with the permutations defined by the vector PERM.
"banded"
"banded positive definite"
Banded matrix with the band size of NL below the diagonal and
NU above it. If NL and NU are 1, then the matrix is
tridiagonal and treated with specialized code. In addition
the matrix can be marked as probably a positive definite.
(Sparse matrices only)
"singular"
The matrix is assumed to be singular and will be treated with
a minimum norm solution.
Note that the matrix type will be discovered automatically on the
first attempt to solve a linear equation involving A. Therefore
‘matrix_type’ is only useful to give Octave hints of the matrix
type. Incorrectly defining the matrix type will result in
incorrect results from solutions of linear equations; it is
entirely *the responsibility of the user* to correctly identify the
matrix type.
Also, the test for positive definiteness is a low-cost test for a
Hermitian matrix with a real positive diagonal. This does not
guarantee that the matrix is positive definite, but only that it is
a probable candidate. When such a matrix is factorized, a
Cholesky factorization is first attempted, and if that fails the
matrix is then treated with an LU factorization. Once the matrix
has been factorized, ‘matrix_type’ will return the correct
classification of the matrix.
-- : norm (A)
-- : norm (A, P)
-- : norm (A, P, OPT)
Compute the p-norm of the matrix A.
If the second argument is not given, ‘p = 2’ is used.
If A is a matrix (or sparse matrix):
P = ‘1’
1-norm, the largest column sum of the absolute values of A.
P = ‘2’
Largest singular value of A.
P = ‘Inf’ or "inf"
Infinity norm, the largest row sum of the absolute values of
A.
P = "fro"
Frobenius norm of A, ‘sqrt (sum (diag (A' * A)))’.
other P, ‘P > 1’
maximum ‘norm (A*x, p)’ such that ‘norm (x, p) == 1’
If A is a vector or a scalar:
P = ‘Inf’ or "inf"
‘max (abs (A))’.
P = ‘-Inf’
‘min (abs (A))’.
P = "fro"
Frobenius norm of A, ‘sqrt (sumsq (abs (A)))’.
P = 0
Hamming norm—the number of nonzero elements.
other P, ‘P > 1’
p-norm of A, ‘(sum (abs (A) .^ P)) ^ (1/P)’.
other P ‘P < 1’
the p-pseudonorm defined as above.
If OPT is the value "rows", treat each row as a vector and compute
its norm. The result is returned as a column vector. Similarly,
if OPT is "columns" or "cols" then compute the norms of each column
and return a row vector.
See also: normest XREFnormest, normest1 XREFnormest1,
cond XREFcond, svd XREFsvd.
-- : null (A)
-- : null (A, TOL)
Return an orthonormal basis of the null space of A.
The dimension of the null space is taken as the number of singular
values of A not greater than TOL. If the argument TOL is missing,
it is computed as
max (size (A)) * max (svd (A)) * eps
See also: orth XREForth.
-- : orth (A)
-- : orth (A, TOL)
Return an orthonormal basis of the range space of A.
The dimension of the range space is taken as the number of singular
values of A greater than TOL. If the argument TOL is missing, it
is computed as
max (size (A)) * max (svd (A)) * eps
See also: null XREFnull.
-- : [Y, H] = mgorth (X, V)
Orthogonalize a given column vector X with respect to a set of
orthonormal vectors comprising the columns of V using the modified
Gram-Schmidt method.
On exit, Y is a unit vector such that:
norm (Y) = 1
V' * Y = 0
X = [V, Y]*H'
-- : pinv (X)
-- : pinv (X, TOL)
Return the Moore-Penrose pseudoinverse of X.
Singular values less than TOL are ignored.
If the second argument is omitted, it is taken to be
tol = max ([rows(X), columns(X)]) * norm (X) * eps
See also: inv XREFinv, ldivide XREFldivide.
-- : rank (A)
-- : rank (A, TOL)
Compute the rank of matrix A, using the singular value
decomposition.
The rank is taken to be the number of singular values of A that are
greater than the specified tolerance TOL. If the second argument
is omitted, it is taken to be
tol = max (size (A)) * sigma(1) * eps;
where ‘eps’ is machine precision and ‘sigma(1)’ is the largest
singular value of A.
The rank of a matrix is the number of linearly independent rows or
columns and determines how many particular solutions exist to a
system of equations. Use ‘null’ for finding the remaining
homogenous solutions.
Example:
x = [1 2 3
4 5 6
7 8 9];
rank (x)
⇒ 2
The number of linearly independent rows is only 2 because the final
row is a linear combination of -1*row1 + 2*row2.
DONTPRINTYET See also: null XREFnull, sprank XREFsprank, *noteDONTPRINTYET See also: null XREFnull, sprank XREFsprank,
svd XREFsvd.
-- : C = rcond (A)
Compute the 1-norm estimate of the reciprocal condition number as
returned by LAPACK.
If the matrix is well-conditioned then C will be near 1 and if the
matrix is poorly conditioned it will be close to 0.
The matrix A must not be sparse. If the matrix is sparse then
‘condest (A)’ or ‘rcond (full (A))’ should be used instead.
See also: cond XREFcond, condest XREFcondest.
-- : trace (A)
Compute the trace of A, the sum of the elements along the main
diagonal.
The implementation is straightforward: ‘sum (diag (A))’.
See also: eig XREFeig.
-- : rref (A)
-- : rref (A, TOL)
-- : [R, K] = rref (...)
Return the reduced row echelon form of A.
TOL defaults to ‘eps * max (size (A)) * norm (A, inf)’.
The optional return argument K contains the vector of "bound
variables", which are those columns on which elimination has been
performed.
Info Catalog
octave: Techniques Used for Linear Algebra
octave: Linear Algebra
octave: Matrix Factorizations