11 - Decompositions, factorisations, inverses and equation solvers (sparse matrices)

In armadillo4r: An 'Armadillo' Interface

Adapted from the official Armadillo documentation.

Eigen decomposition of a symmetric matrix {#eig_sym}

Obtain a limited number of eigenvalues and eigenvectors of sparse symmetric real matrix X.

Usage:

vec eigval = eigs_sym(X, k)
vec eigval = eigs_sym(X, k, form)
vec eigval = eigs_sym(X, k, form, opts)
vec eigval = eigs_sym(X, k, sigma)
vec eigval = eigs_sym(X, k, sigma, opts)

eigs_sym(eigval, X, k)
eigs_sym(eigval, X, k, form)
eigs_sym(eigval, X, k, form, opts)
eigs_sym(eigval, X, k, sigma)
eigs_sym(eigval, X, k, sigma, opts)

eigs_sym(eigval, eigvec, X, k)
eigs_sym(eigval, eigvec, X, k, form)
eigs_sym(eigval, eigvec, X, k, form, opts)
eigs_sym(eigval, eigvec, X, k, sigma)
eigs_sym(eigval, eigvec, X, k, sigma, opts)

k specifies the number of eigenvalues and eigenvectors.

The argument form is optional and is one of the following:

• "lm": obtain eigenvalues with largest magnitude (default operation).
• "sm": obtain eigenvalues with smallest magnitude (see the caveats below).
• "la": obtain eigenvalues with largest algebraic value.

The argument sigma is optional; if sigma is given, eigenvalues closest to sigma are found via shift-invert mode. Note that to use sigma, both ARMA_USE_ARPACK and ARMA_USE_SUPERLU must be enabled in armadillo/config.hpp.

The opts argument is optional; opts is an instance of the eigs_opts structure:

struct eigs_opts
{
  double       tol;     // default: 0
  unsigned int maxiter; // default: 1000
  unsigned int subdim;  // default: max(2*k+1, 20)
};

• tol specifies the tolerance for convergence.
• maxiter specifies the maximum number of Arnoldi iterations.
• subdim specifies the dimension of the Krylov subspace, with the constraint k < subdim <= X.n_rows; the recommended value is subdim >= 2*k.

The eigenvalues and corresponding eigenvectors are stored in eigval and eigvec, respectively.

If X is not square sized, an error is thrown.

If the decomposition fails:

• eigval = eigs_sym(X,k) resets eigval and throws an error.
• eigs_sym(eigval,X,k) resets eigval and returns a bool set to false (an error is not thrown).
• eigs_sym(eigval,eigvec,X,k) resets eigval and eigvec and returns a bool set to false (an error is not thrown).

Caveats:

• The number of obtained eigenvalues/eigenvectors may be lower than requested, depending on the given data.
• If the decomposition fails, try first increasing opts.subdim (Krylov subspace dimension), and, as secondary options, try increasing opts.maxiter (maximum number of iterations), and/or opts.tol (tolerance for convergence), and/or k (number of eigenvalues).
• For an alternative to the "sm" form, use the shift-invert mode with sigma set to 0.0.
• The implementation in Armadillo 12.6 is considerably faster than earlier versions; further speedups can be obtained by enabling OpenMP in your compiler (e.g., -fopenmp in GCC and clang).

Examples

[[cpp4r::register]] list eig_sym2_(const doubles_matrix<>& x,
                                   const char* method,
                                   const int& k) {
  sp_mat X = as_SpMat(x);

  sp_mat Y = X.t() * X;

  vec eigval;
  mat eigvec;

  eigs_opts opts;
  opts.maxiter = 10000;
  bool ok = eigs_sym(eigval, eigvec, Y, k, method, opts);

  writable::list out(3);
  out[0] = writable::logicals({ok});
  out[1] = as_doubles(eigval);
  out[2] = as_doubles_matrix(eigvec);

  return out;
}

Eigen decomposition of a general matrix {#eig_gen}

Obtain a limited number of eigenvalues and eigenvectors of sparse general (non-symmetric/non-hermitian) square matrix X.

Usage:

cx_vec eigval = eigs_gen(X, k)
cx_vec eigval = eigs_gen(X, k, form)
cx_vec eigval = eigs_gen(X, k, sigma)
cx_vec eigval = eigs_gen(X, k, form, opts)
cx_vec eigval = eigs_gen(X, k, sigma, opts)

eigs_gen(eigval, X, k)
eigs_gen(eigval, X, k, form)
eigs_gen(eigval, X, k, sigma)
eigs_gen(eigval, X, k, form, opts)
eigs_gen(eigval, X, k, sigma, opts)

eigs_gen(eigval, eigvec, X, k)
eigs_gen(eigval, eigvec, X, k, form)
eigs_gen(eigval, eigvec, X, k, sigma)
eigs_gen(eigval, eigvec, X, k, form, opts)
eigs_gen(eigval, eigvec, X, k, sigma, opts)

k specifies the number of eigenvalues and eigenvectors.

The argument form is optional; form is one of the following:

• "lm": obtain eigenvalues with largest magnitude (default operation).
• "sm": obtain eigenvalues with smallest magnitude (see the caveats below).
• "lr": obtain eigenvalues with largest real part.
• "sr": obtain eigenvalues with smallest real part.
• "li": obtain eigenvalues with largest imaginary part.
• "si": obtain eigenvalues with smallest imaginary part.

The argument sigma is optional; if sigma is given, eigenvalues closest to sigma are found via shift-invert mode. Note that to use sigma, both ARMA_USE_ARPACK and ARMA_USE_SUPERLU must be enabled in armadillo/config.hpp.

The opts argument is optional; opts is an instance of the eigs_opts structure:

struct eigs_opts
{
  double       tol;     // default: 0
  unsigned int maxiter; // default: 1000
  unsigned int subdim;  // default: max(2*k+1, 20)
};

• tol specifies the tolerance for convergence.
• maxiter specifies the maximum number of Arnoldi iterations.
• subdim specifies the dimension of the Krylov subspace, with the constraint k < subdim <= X.n_rows; the recommended value is subdim >= 2*k.

The eigenvalues and corresponding eigenvectors are stored in eigval and eigvec, respectively.

If X is not square sized, an error is thrown.

If the decomposition fails:

• eigval = eigs_gen(X, k) resets eigval and throws an error.
• eigs_gen(eigval, X, k) resets eigval and returns a bool set to false (an error is not thrown).
• eigs_gen(eigval,eigvec, X, k) resets eigval and eigvec and returns a bool set to false (an error is not thrown).

Caveats:

• The number of obtained eigenvalues/eigenvectors may be lower than requested, depending on the given data.
• If the decomposition fails, try first increasing opts.subdim (Krylov subspace dimension), and, as secondary options, try increasing opts.maxiter (maximum number of iterations), and/or opts.tol (tolerance for convergence), and/or k (number of eigenvalues).
• For an alternative to the "sm" form, use the shift-invert mode with sigma set to 0.0.

Examples

[[cpp4r::register]] list eig_gen2_(const doubles_matrix<>& x,
                                   const char* method,
                                   const int& k) {
  sp_mat X = as_SpMat(x);

  cx_vec eigval;
  cx_mat eigvec;

  eigs_opts opts;
  opts.maxiter = 10000;

  bool ok = eigs_gen(eigval, eigvec, X, k, method, opts);

  writable::list out(3);
  out[0] = writable::logicals({ok});
  out[1] = as_complexes(eigval);
  out[2] = as_complexes_matrix(eigvec);

  return out;
}

Truncated singular value decomposition {#svds}

Obtain a limited number of singular values and singular vectors (truncated SVD) of a sparse matrix.

The singular values and vectors are calculated via sparse eigen decomposition of:

$$ \begin{bmatrix} 0_{n \times n} & X \ X^T & 0_{m \times m} \end{bmatrix} $$

where $n$ and $m$ are the number of rows and columns of X, respectively.

Usage:

vec s = svds(X, k)
vec s = svds(X, k, tol)

svds(vec s, X, k)
svds(vec s, X, k, tol)

svds(mat U, vec s, mat V, sp_mat X, k)
svds(mat U, vec s, mat V, sp_mat X, k, tol)

svds(cx_mat U, vec s, cx_mat V, sp_cx_mat X, k)
svds(cx_mat U, vec s, cx_mat V, sp_cx_mat X, k, tol)

k specifies the number of singular values and singular vectors.

The singular values are in descending order.

The argument tol is optional; it specifies the tolerance for convergence, and it is passed as tol / sqrt(2) to eigs_sym.

If the decomposition fails:

• s = svds(X, k) resets s and throws an error.
• svds(s, X, k) resets s and returns a bool set to false (an error is not thrown).
• svds(U, s, V, X, k) resets U, s, and V and returns a bool set to false (an error is not thrown).

Caveats:

• svds is intended only for finding a few singular values from a large sparse matrix; to find all singular values, use svd instead.
• Depending on the given matrix, svds may find fewer singular values than specified.
• The implementation in Armadillo 12.6 is considerably faster than earlier versions; further speedups can be obtained by enabling OpenMP in your compiler (e.g., -fopenmp in GCC and clang).

Examples

[[cpp4r::register]] list svds1_(const doubles_matrix<>& x, const int& k) {
  sp_mat X = as_SpMat(x);

  // convert all values below 0.1 to zero
  X.transform([](double val) { return (std::abs(val) < 0.1) ? 0 : val; });

  mat U;
  vec s;
  mat V;

  bool ok = svds(U, s, V, X, k);

  writable::list out(4);
  out[0] = writable::logicals({ok});
  out[1] = as_doubles(s);
  out[2] = as_doubles_matrix(U);
  out[3] = as_doubles_matrix(V);

  return out;
}

Solve a system of linear equations {#spsolve}

Solve a sparse system of linear equations, $A \cdot X = B$, where $A$ is a sparse matrix, $B$ is a dense matrix or vector, and $X$ is unknown.

The number of rows in $A$ and $B$ must be the same.

Usage:

// A = matrix, b = vector
vec x = spsolve(A, b)
vec x = spsolve(A, b, solver)
vec x = spsolve(A, b, solver, opts)

// A = matrix, B = matrix
mat X = spsolve(A, B)

spsolve(x, A, b)
spsolve(x, A, b, solver)
spsolve(x, A, b, solver, opts)

The solver argument is optional; solver is either "superlu" (default) or "lapack".

• For "superlu", ARMA_USE_SUPERLU must be enabled in armadillo/config.hpp.
• For "lapack", the sparse matrix $A$ is converted to a dense matrix before using the LAPACK solver. This considerably increases memory usage.

The opts argument is optional and applicable to the SuperLU solver, and is an instance of the superlu_opts structure:

struct superlu_opts
{
  bool             allow_ugly;   // default: false
  bool             equilibrate;  // default: false
  bool             symmetric;    // default: false
  double           pivot_thresh; // default: 1.0
  permutation_type permutation;  // default: superlu_opts::COLAMD
  refine_type      refine;       // default: superlu_opts::REF_NONE
};

• allow_ugly is either true or false; it indicates whether to keep solutions of systems singular to working precision.
• equilibrate is either true or false; it indicates whether to equilibrate the system (scale the rows and columns of $A$ to have unit norm).
• symmetric is either true or false; it indicates whether to use SuperLU symmetric mode, which gives preference to diagonal pivots.
• pivot_thresh is in the range [0.0, 1.0], used for determining whether a diagonal entry is an acceptable pivot (details in SuperLU documentation).
• permutation specifies the type of column permutation; it is one of:
• superlu_opts::NATURAL: natural ordering.
• superlu_opts::MMD_ATA: minimum degree ordering on structure of $A^T \cdot A$.
• superlu_opts::MMD_AT_PLUS_A: minimum degree ordering on structure of $A^T + A$.
• superlu_opts::COLAMD: approximate minimum degree column ordering.
• refine specifies the type of iterative refinement; it is one of:
• superlu_opts::REF_NONE: no refinement.
• superlu_opts::REF_SINGLE: iterative refinement in single precision.
• superlu_opts::REF_DOUBLE: iterative refinement in double precision.
• superlu_opts::REF_EXTRA: iterative refinement in extra precision.

If no solution is found:

• x = spsolve(A, b) resets x and throws an error.
• spsolve(x, A, b) resets x and returns a bool set to false (an error is not thrown).

The SuperLU solver is mainly useful for very large and/or highly sparse matrices.

To reuse the SuperLU factorisation of $A$ for finding solutions where $B$ is iteratively changed, see the spsolve_factoriser class.

If there is sufficient memory to store a dense version of matrix $A$, the LAPACK solver can be faster.

Examples

[[cpp4r::register]] doubles spsolve1_(const doubles_matrix<>& a,
                                      const doubles& b,
                                      const char* method) {
  sp_mat A = as_SpMat(a);
  vec B = as_Col(b);

  vec X = spsolve(A, B, method);

  return as_doubles(X);
}

Factorise a sparse matrix for solving linear systems {#spsolve_factoriser}

Class for factorisation of sparse matrix $A$ for solving systems of linear equations in the form $AX = B$.

Allows the SuperLU factorisation of $A$ to be reused for finding solutions in cases where $B$ is iteratively changed.

For an instance of spsolve_factoriser named as SF, the member functions are:

• SF.factorise(A. opts): factorise square-sized sparse matrix $A. Optional settings are given in theoptsargument as per thespsolve()` function. If the factorisation fails, a bool set to false is returned.
• SF.solve(X, B): using the given dense matrix $B$ and the computed factorisation, store in $X$ the solution to $AX = B`. If computing the solution fails, $X$ is reset and a bool set to false is returned.
• SF.rcond(): return the 1-norm estimate of the reciprocal condition number computed during the factorisation. Values close to 1 suggest that the factorised matrix is well-conditioned. Values close to 0 suggest that the factorised matrix is badly conditioned.
• SF.reset(): reset the instance and release all memory related to the stored factorisation; this is automatically done when the instance goes out of scope.

Caveats:

• If the factorisation of $A$ does not need to be reused, use spsolve() instead.
• This class internally uses the SuperLU solver; ARMA_USE_SUPERLU must be enabled in config.hpp.

Examples

[[cpp4r::register]] list spsolve_factoriser1_(const doubles_matrix<>& a,
                                              const list& b) {
  sp_mat A = as_SpMat(a);

  bool status = SF.factorise(A);

  if (status == false) {
    stop("factorisation failed");
  }

  double rcond_value = SF.rcond();

  vec B1 = as_Col(b[0]);
  vec B2 = as_Col(b[1]);

  vec X1, X2;

  bool ok1 = SF.solve(X1, B1);
  bool ok2 = SF.solve(X2, B2);

  if (ok1 == false) {
    stop("couldn't find X1");
  }

  if (ok2 == false) {
    stop("couldn't find X2");
  }

  writable::list out(3);
  out[0] = writable::logicals({status && ok1 && ok2});
  out[1] = as_doubles(X1);
  out[2] = as_doubles(X2);

  return out;
}

armadillo4r documentation built on Aug. 23, 2026, 5:07 p.m.