Developer Reference

ID 766877
Date 3/22/2024
Public

## LAPACKE_ssyevx Example Program in C for Column Major Data Layout

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/*
LAPACKE_ssyevx 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.
========================

LAPACKE_ssyevx (column-major, high-level) 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>
#include "mkl_lapacke.h"

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

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

/* Main program */
int main() {
/* Locals */
MKL_INT n = N, il, iu, m, lda = LDA, ldz = LDZ, info;
float abstol, vl, vu;
/* Local arrays */
MKL_INT ifail[N];
float w[N], z[LDZ*NSELECT];
float a[LDA*N] = {
6.29f,  0.00f,  0.00f,  0.00f,  0.00f,
-0.39f,  7.19f,  0.00f,  0.00f,  0.00f,
0.61f,  0.81f,  5.48f,  0.00f,  0.00f,
1.18f,  1.19f, -3.13f,  3.79f,  0.00f,
-0.08f, -0.08f,  0.22f, -0.26f,  0.83f
};
/* Executable statements */
printf( "LAPACKE_ssyevx (column-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_ssyevx( LAPACK_COL_MAJOR, 'V', 'I', 'U', n, a, lda,
vl, vu, il, iu, abstol, &m, w, z, ldz, ifail );
/* 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_ssyevx Example */

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