```/* nag_dstevx (f08jbc) Example Program.
*
* Copyright 2017 Numerical Algorithms Group.
*
* Mark 26.1, 2017.
*/

#include <stdio.h>
#include <nag.h>
#include <nag_stdlib.h>
#include <nagf08.h>
#include <nagx02.h>
#include <nagx04.h>

int main(void)
{
/* Scalars */
double abstol, vl, vu;
Integer exit_status = 0, i, il = 0, iu = 0, j, m, n, pdz;
/* Arrays */
double *d = 0, *e = 0, *w = 0, *z = 0;
Integer *index = 0;
/* Nag Types */
Nag_OrderType order;
NagError fail, fail_print;

#ifdef NAG_COLUMN_MAJOR
order = Nag_ColMajor;
#else
order = Nag_RowMajor;
#endif

INIT_FAIL(fail);

printf("nag_dstevx (f08jbc) Example Program Results\n\n");

/* Skip heading in data file */
scanf("%*[^\n]");
scanf("%" NAG_IFMT "%*[^\n]", &n);

pdz = n;
/* Allocate memory */
if (!(d = NAG_ALLOC(n, double)) ||
!(e = NAG_ALLOC(n, double)) ||
!(w = NAG_ALLOC(n, double)) ||
!(z = NAG_ALLOC(pdz * n, double)) || !(index = NAG_ALLOC(n, Integer)))
{
printf("Allocation failure\n");
exit_status = -1;
goto END;
}

/* Read the lower and upper bounds of the interval to be searched,
* and read the diagonal and off-diagonal elements of the matrix
* A from data file.
*/
scanf("%lf%lf%*[^\n]", &vl, &vu);
for (i = 0; i < n; ++i)
scanf("%lf", &d[i]);
scanf("%*[^\n]");

for (i = 0; i < n - 1; ++i)
scanf("%lf", &e[i]);
scanf("%*[^\n]");

/* nag_real_safe_small_number (X02AMC).
* Set the absolute error tolerance for eigenvalues. With abstol
* set to zero, the default value would be used instead.
*/
abstol = nag_real_safe_small_number * 2;

/* nag_dstevx (f08jbc).
* Solve the symmetric eigenvalue problem.
*/
nag_dstevx(order, Nag_DoBoth, Nag_Interval, n, d, e, vl, vu, il, iu,
abstol, &m, w, z, pdz, index, &fail);
if (fail.code != NE_NOERROR && fail.code != NE_CONVERGENCE) {
printf("Error from nag_dstevx (f08jbc).\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* Print solution */
printf("Number of eigenvalues found =%5" NAG_IFMT "\n", m);
printf("\nEigenvalues\n");
for (j = 0; j < m; ++j)
printf("%8.4f%s", w[j], (j + 1) % 8 == 0 ? "\n" : " ");
printf("\n\n");

/* nag_gen_real_mat_print (x04cac).
* Print selected eigenvectors.
*/
INIT_FAIL(fail_print);
fflush(stdout);
nag_gen_real_mat_print(order, Nag_GeneralMatrix, Nag_NonUnitDiag, n, m, z,
pdz, "Selected eigenvectors", 0, &fail_print);
if (fail_print.code != NE_NOERROR) {
printf("Error from nag_gen_real_mat_print (x04cac).\n%s\n",
fail_print.message);
exit_status = 1;
goto END;
}
if (fail.code == NE_CONVERGENCE) {
printf("eigenvectors failed to converge\n");
printf("Indices of eigenvectors that did not converge\n");
for (j = 0; j < m; ++j)
printf("%8" NAG_IFMT "%s", index[j], (j + 1) % 8 == 0 ? "\n" : " ");
printf("\n");
}

END:
NAG_FREE(d);
NAG_FREE(e);
NAG_FREE(w);
NAG_FREE(z);
NAG_FREE(index);
return exit_status;
}
```