Developer Reference

ID 766877
Date 3/22/2024
Public

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

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/*
LAPACKE_ssyevd 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.
========================

LAPACKE_ssyevd (row-major, high-level) 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>
#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 LDA N

/* Main program */
int main() {
/* Locals */
MKL_INT n = N, lda = LDA, info;
/* Local arrays */
float w[N];
float a[LDA*N] = {
6.39f,  0.13f, -8.23f, 5.71f, -3.18f,
0.00f,  8.37f, -4.46f, -6.10f,  7.21f,
0.00f,  0.00f, -9.58f, -9.25f, -7.42f,
0.00f,  0.00f, 0.00f, 3.72f,  8.54f,
0.00f,  0.00f, 0.00f, 0.00f,  2.51f
};
/* Executable statements */
printf( "LAPACKE_ssyevd (row-major, high-level) Example Program Results\n" );
/* Solve eigenproblem */
info = LAPACKE_ssyevd( LAPACK_ROW_MAJOR, 'V', 'U', n, a, lda, w );
/* 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 );
exit( 0 );
} /* End of LAPACKE_ssyevd 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*lda+j] );
printf( "\n" );
}
}