Developer Reference

ID 766877
Date 12/20/2021
Public

## SSYEV Example Program in C

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/*
SSYEV Example.
==============

Program computes all eigenvalues and eigenvectors of a real symmetric
matrix A:

1.96  -6.49  -0.47  -7.20  -0.65
-6.49   3.80  -6.39   1.50  -6.34
-0.47  -6.39   4.17  -1.51   2.67
-7.20   1.50  -1.51   5.70   1.80
-0.65  -6.34   2.67   1.80  -7.10

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.

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

SSYEV Example Program Results

Eigenvalues
-11.07  -6.23   0.86   8.87  16.09

Eigenvectors (stored columnwise)
-0.30  -0.61   0.40  -0.37   0.49
-0.51  -0.29  -0.41  -0.36  -0.61
-0.08  -0.38  -0.66   0.50   0.40
0.00  -0.45   0.46   0.62  -0.46
-0.80   0.45   0.17   0.31   0.16
*/
#include <stdlib.h>
#include <stdio.h>

/* SSYEV prototype */
extern void ssyev( char* jobz, char* uplo, int* n, float* a, int* lda,
float* w, float* work, int* lwork, int* info );
/* Auxiliary routines prototypes */
extern void print_matrix( char* desc, int m, int n, float* a, int lda );

/* Parameters */
#define N 5
#define LDA N

/* Main program */
int main() {
/* Locals */
int n = N, lda = LDA, info, lwork;
float wkopt;
float* work;
/* Local arrays */
float w[N];
float a[LDA*N] = {
1.96f,  0.00f,  0.00f,  0.00f,  0.00f,
-6.49f,  3.80f,  0.00f,  0.00f,  0.00f,
-0.47f, -6.39f,  4.17f,  0.00f,  0.00f,
-7.20f,  1.50f, -1.51f,  5.70f,  0.00f,
-0.65f, -6.34f,  2.67f,  1.80f, -7.10f
};
/* Executable statements */
printf( " SSYEV Example Program Results\n" );
/* Query and allocate the optimal workspace */
lwork = -1;
ssyev( "Vectors", "Upper", &n, a, &lda, w, &wkopt, &lwork, &info );
lwork = (int)wkopt;
work = (float*)malloc( lwork*sizeof(float) );
/* Solve eigenproblem */
ssyev( "Vectors", "Upper", &n, a, &lda, w, work, &lwork, &info );
/* 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 );
/* Free workspace */
free( (void*)work );
exit( 0 );
} /* End of SSYEV Example */

/* Auxiliary routine: printing a matrix */
void print_matrix( char* desc, int m, int n, float* 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" );
}
}