Developer Reference

ID 766877
Date 12/20/2021
Public

## DSYEVX Example Program in C

/*******************************************************************************
*  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
*  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" );
}
}