IntelĀ® oneAPI Math Kernel Library LAPACK Examples

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/*
   DSYEVD Example.
   ==============

   Program computes all eigenvalues and eigenvectors of a real symmetric
   matrix A using divide and conquer algorithm, where A is:

     6.39   0.13  -8.23   5.71  -3.18
     0.13   8.37  -4.46  -6.10   7.21
    -8.23  -4.46  -9.58  -9.25  -7.42
     5.71  -6.10  -9.25   3.72   8.54
    -3.18   7.21  -7.42   8.54   2.51

   Description.
   ============

   The routine computes all eigenvalues and, optionally, eigenvectors of an
   n-by-n real symmetric matrix A. The eigenvector v(j) of A satisfies

   A*v(j) = lambda(j)*v(j)

   where lambda(j) is its eigenvalue. The computed eigenvectors are
   orthonormal.
   If the eigenvectors are requested, then this routine uses a divide and
   conquer algorithm to compute eigenvalues and eigenvectors.

   Example Program Results.
   ========================

 DSYEVD Example Program Results

 Eigenvalues
 -17.44 -11.96   6.72  14.25  19.84

 Eigenvectors (stored columnwise)
  -0.26   0.31  -0.74   0.33   0.42
  -0.17  -0.39  -0.38  -0.80   0.16
  -0.89   0.04   0.09   0.03  -0.45
  -0.29  -0.59   0.34   0.31   0.60
  -0.19   0.63   0.44  -0.38   0.48
*/
#include <stdlib.h>
#include <stdio.h>

/* DSYEVD prototype */
extern void dsyevd( char* jobz, char* uplo, int* n, double* a, int* lda,
                double* w, double* work, int* lwork, int* iwork, int* liwork, int* info );
/* Auxiliary routines prototypes */
extern void print_matrix( char* desc, int m, int n, double* a, int lda );

/* Parameters */
#define N 5
#define LDA N

/* Main program */
int main() {
        /* Locals */
        int n = N, lda = LDA, info, lwork, liwork;
        int iwkopt;
        int* iwork;
        double wkopt;
        double* work;
        /* Local arrays */
        double w[N];
        double a[LDA*N] = {
            6.39,  0.00,  0.00,  0.00,  0.00,
            0.13,  8.37,  0.00,  0.00,  0.00,
           -8.23, -4.46, -9.58,  0.00,  0.00,
            5.71, -6.10, -9.25,  3.72,  0.00,
           -3.18,  7.21, -7.42,  8.54,  2.51
        };
        /* Executable statements */
        printf( " DSYEVD Example Program Results\n" );
        /* Query and allocate the optimal workspace */
        lwork = -1;
        liwork = -1;
        dsyevd( "Vectors", "Upper", &n, a, &lda, w, &wkopt, &lwork, &iwkopt,
                        &liwork, &info );
        lwork = (int)wkopt;
        work = (double*)malloc( lwork*sizeof(double) );
        liwork = iwkopt;
        iwork = (int*)malloc( liwork*sizeof(int) );
        /* Solve eigenproblem */
        dsyevd( "Vectors", "Upper", &n, a, &lda, w, work, &lwork, iwork,
                        &liwork, &info );
        /* Check for convergence */
        if( info > 0 ) {
                printf( "The algorithm failed to compute eigenvalues.\n" );
                exit( 1 );
        }
        /* Print eigenvalues */
        print_matrix( "Eigenvalues", 1, n, w, 1 );
        /* Print eigenvectors */
        print_matrix( "Eigenvectors (stored columnwise)", n, n, a, lda );
        /* Free workspace */
        free( (void*)iwork );
        free( (void*)work );
        exit( 0 );
} /* End of DSYEVD Example */

/* Auxiliary routine: printing a matrix */
void print_matrix( char* desc, int m, int n, double* a, int lda ) {
        int i, j;
        printf( "\n %s\n", desc );
        for( i = 0; i < m; i++ ) {
                for( j = 0; j < n; j++ ) printf( " %6.2f", a[i+j*lda] );
                printf( "\n" );
        }
}