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DSYEVX Example Program in C
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/*
DSYEVX 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.
========================
DSYEVX 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>
/* DSYEVX prototype */
extern void dsyevx( char* jobz, char* range, char* uplo, int* n, double* a,
int* lda, double* vl, double* vu, int* il, int* iu, double* abstol,
int* m, double* w, double* z, int* ldz, double* work, int* lwork,
int* iwork, int* ifail, int* info );
/* Auxiliary routines prototypes */
extern void print_matrix( char* desc, int m, int n, double* a, int lda );
/* Parameters */
#define N 5
#define NSELECT 3
#define LDA N
#define LDZ N
/* Main program */
int main() {
/* Locals */
int n = N, il, iu, m, lda = LDA, ldz = LDZ, info, lwork;
double abstol, vl, vu;
double wkopt;
double* work;
/* Local arrays */
/* iwork dimension should be at least 5*n */
int iwork[5*N], ifail[N];
double w[N], z[LDZ*NSELECT];
double a[LDA*N] = {
6.29, 0.00, 0.00, 0.00, 0.00,
-0.39, 7.19, 0.00, 0.00, 0.00,
0.61, 0.81, 5.48, 0.00, 0.00,
1.18, 1.19, -3.13, 3.79, 0.00,
-0.08, -0.08, 0.22, -0.26, 0.83
};
/* Executable statements */
printf( " DSYEVX 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;
/* Query and allocate the optimal workspace */
lwork = -1;
dsyevx( "Vectors", "Indices", "Upper", &n, a, &lda, &vl, &vu, &il, &iu,
&abstol, &m, w, z, &ldz, &wkopt, &lwork, iwork, ifail, &info );
lwork = (int)wkopt;
work = (double*)malloc( lwork*sizeof(double) );
/* Solve eigenproblem */
dsyevx( "Vectors", "Indices", "Upper", &n, a, &lda, &vl, &vu, &il, &iu,
&abstol, &m, w, z, &ldz, work, &lwork, iwork, ifail, &info );
/* 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 );
/* Free workspace */
free( (void*)work );
exit( 0 );
} /* End of DSYEVX 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: DSYEVX Example