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DSYEV Example Program in C
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/*
DSYEV 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.
========================
DSYEV 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>
/* DSYEV prototype */
extern void dsyev( char* jobz, char* uplo, int* n, double* a, int* lda,
double* w, double* work, int* lwork, int* info );
/* Auxiliary routines prototypes */
extern void print_matrix( char* desc, int m, int n, double* a, int lda );
/* Parameters */
#define N 5
#define LDA N
/* Main program */
int main() {
/* Locals */
int n = N, lda = LDA, info, lwork;
double wkopt;
double* work;
/* Local arrays */
double w[N];
double a[LDA*N] = {
1.96, 0.00, 0.00, 0.00, 0.00,
-6.49, 3.80, 0.00, 0.00, 0.00,
-0.47, -6.39, 4.17, 0.00, 0.00,
-7.20, 1.50, -1.51, 5.70, 0.00,
-0.65, -6.34, 2.67, 1.80, -7.10
};
/* Executable statements */
printf( " DSYEV Example Program Results\n" );
/* Query and allocate the optimal workspace */
lwork = -1;
dsyev( "Vectors", "Upper", &n, a, &lda, w, &wkopt, &lwork, &info );
lwork = (int)wkopt;
work = (double*)malloc( lwork*sizeof(double) );
/* Solve eigenproblem */
dsyev( "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 DSYEV 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" );
}
}
Parent topic: DSYEV Example