Developer Reference

Intel® oneAPI Math Kernel Library LAPACK Examples

ID 766877
Date 3/31/2023
Public

A newer version of this document is available. Customers should click here to go to the newest version.

Document Table of Contents

DSYEVX Example Program in C

/*******************************************************************************
*  Copyright (C) 2009-2015 Intel Corporation. All Rights Reserved.
*  The information and material ("Material") provided below is owned by Intel
*  Corporation or its suppliers or licensors, and title to such Material remains
*  with Intel Corporation or its suppliers or licensors. The Material contains
*  proprietary information of Intel or its suppliers and licensors. The Material
*  is protected by worldwide copyright laws and treaty provisions. No part of
*  the Material may be copied, reproduced, published, uploaded, posted,
*  transmitted, or distributed in any way without Intel's prior express written
*  permission. No license under any patent, copyright or other intellectual
*  property rights in the Material is granted to or conferred upon you, either
*  expressly, by implication, inducement, estoppel or otherwise. Any license
*  under such intellectual property rights must be express and approved by Intel
*  in writing.
*
********************************************************************************
*/
/*
   DSYEVX Example.
   ==============

   Program computes the smallest eigenvalues and the corresponding
   eigenvectors of a real symmetric matrix A:

     6.29  -0.39   0.61   1.18  -0.08
    -0.39   7.19   0.81   1.19  -0.08
     0.61   0.81   5.48  -3.13   0.22
     1.18   1.19  -3.13   3.79  -0.26
    -0.08  -0.08   0.22  -0.26   0.83

   Description.
   ============

   The routine computes selected 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.
   Eigenvalues and eigenvectors can be selected by specifying either a range
   of values or a range of indices for the desired eigenvalues.

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

 DSYEVX Example Program Results

 The total number of eigenvalues found: 3

 Selected eigenvalues
   0.71   0.82   6.58

 Selected eigenvectors (stored columnwise)
   0.22   0.09  -0.95
   0.21   0.08  -0.04
  -0.52  -0.22  -0.29
  -0.73  -0.21  -0.09
  -0.32   0.94   0.01
*/
#include <stdlib.h>
#include <stdio.h>

/* DSYEVX prototype */
extern void dsyevx( char* jobz, char* range, char* uplo, int* n, double* a,
                int* lda, double* vl, double* vu, int* il, int* iu, double* abstol,
                int* m, double* w, double* z, int* ldz, double* work, int* lwork,
      int* iwork, int* ifail, int* info );
/* Auxiliary routines prototypes */
extern void print_matrix( char* desc, int m, int n, double* a, int lda );

/* Parameters */
#define N 5
#define NSELECT 3
#define LDA N
#define LDZ N

/* Main program */
int main() {
        /* Locals */
        int n = N, il, iu, m, lda = LDA, ldz = LDZ, info, lwork;
        double abstol, vl, vu;
        double wkopt;
        double* work;
        /* Local arrays */
   /* iwork dimension should be at least 5*n */
        int iwork[5*N], ifail[N];
        double w[N], z[LDZ*NSELECT];
        double a[LDA*N] = {
            6.29,  0.00,  0.00,  0.00,  0.00,
           -0.39,  7.19,  0.00,  0.00,  0.00,
            0.61,  0.81,  5.48,  0.00,  0.00,
            1.18,  1.19, -3.13,  3.79,  0.00,
           -0.08, -0.08,  0.22, -0.26,  0.83
        };
        /* Executable statements */
        printf( " DSYEVX Example Program Results\n" );
        /* Negative abstol means using the default value */
        abstol = -1.0;
        /* Set il, iu to compute NSELECT smallest eigenvalues */
        il = 1;
        iu = NSELECT;
        /* Query and allocate the optimal workspace */
        lwork = -1;
        dsyevx( "Vectors", "Indices", "Upper", &n, a, &lda, &vl, &vu, &il, &iu,
                        &abstol, &m, w, z, &ldz, &wkopt, &lwork, iwork, ifail, &info );
        lwork = (int)wkopt;
        work = (double*)malloc( lwork*sizeof(double) );
        /* Solve eigenproblem */
        dsyevx( "Vectors", "Indices", "Upper", &n, a, &lda, &vl, &vu, &il, &iu,
                        &abstol, &m, w, z, &ldz, work, &lwork, iwork, ifail, &info );
        /* Check for convergence */
        if( info > 0 ) {
                printf( "The algorithm failed to compute eigenvalues.\n" );
                exit( 1 );
        }
        /* Print the number of eigenvalues found */
        printf( "\n The total number of eigenvalues found:%2i\n", m );
        /* Print eigenvalues */
        print_matrix( "Selected eigenvalues", 1, m, w, 1 );
        /* Print eigenvectors */
        print_matrix( "Selected eigenvectors (stored columnwise)", n, m, z, ldz );
        /* Free workspace */
        free( (void*)work );
        exit( 0 );
} /* End of DSYEVX 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" );
        }
}