Developer Reference

ID 766877
Date 12/20/2021
Public

## LAPACKE_dsyevr Example Program in C for Row Major Data Layout

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/*
LAPACKE_dsyevr Example.
=======================

Program computes the smallest eigenvalues and the corresponding
eigenvectors of a real symmetric matrix A using the Relatively Robust
Representations, where A is:

0.67  -0.20   0.19  -1.06   0.46
-0.20   3.82  -0.13   1.06  -0.48
0.19  -0.13   3.27   0.11   1.10
-1.06   1.06   0.11   5.86  -0.98
0.46  -0.48   1.10  -0.98   3.54

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.
========================

LAPACKE_dsyevr (row-major, high-level) Example Program Results

The total number of eigenvalues found: 3

Selected eigenvalues
0.43   2.14   3.37

Selected eigenvectors (stored columnwise)
-0.98  -0.01  -0.08
0.01   0.02  -0.93
0.04  -0.69  -0.07
-0.18   0.19   0.31
0.07   0.69  -0.13
*/
#include <stdlib.h>
#include <stdio.h>
#include "mkl_lapacke.h"

/* Auxiliary routines prototypes */
extern void print_matrix( char* desc, MKL_INT m, MKL_INT n, double* a, MKL_INT lda );

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

/* Main program */
int main() {
/* Locals */
MKL_INT n = N, il, iu, m, lda = LDA, ldz = LDZ, info;
double abstol, vl, vu;
/* Local arrays */
MKL_INT isuppz[N];
double w[N], z[LDZ*N];
double a[LDA*N] = {
0.67, -0.20, 0.19, -1.06, 0.46,
0.00,  3.82, -0.13,  1.06, -0.48,
0.00,  0.00, 3.27,  0.11, 1.10,
0.00,  0.00, 0.00,  5.86, -0.98,
0.00,  0.00, 0.00,  0.00, 3.54
};
/* Executable statements */
printf( "LAPACKE_dsyevr (row-major, high-level) 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;
/* Solve eigenproblem */
info = LAPACKE_dsyevr( LAPACK_ROW_MAJOR, 'V', 'I', 'U', n, a, lda,
vl, vu, il, iu, abstol, &m, w, z, ldz, isuppz );
/* 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 );
exit( 0 );
} /* End of LAPACKE_dsyevr Example */

/* Auxiliary routine: printing a matrix */
void print_matrix( char* desc, MKL_INT m, MKL_INT n, double* a, MKL_INT lda ) {
MKL_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*lda+j] );
printf( "\n" );
}
}