NAG Library Manual, Mark 27.3
```/* nag_matop_dtrttp (f01vac) Example Program.
*
* Copyright 2021 Numerical Algorithms Group.
*
* Mark 27.3, 2021.
*/

#include <nag.h>
#include <stdio.h>

int main(void) {
/* Scalars */
Integer exit_status = 0, indent = 0, ncols = 80, i, j, pda;
Integer lenap, n;
/* Arrays */
double *a = 0, *ap = 0;
char nag_enum_arg[40], form[] = "%5.2f";
/* Nag Types */
Nag_MatrixType matrix;
Nag_OrderType order;
Nag_UploType uplo;
NagError fail;

#ifdef NAG_COLUMN_MAJOR
#define A(I, J) a[J * pda + I]
order = Nag_ColMajor;
#define KU(I, J) (I + J * (J + 1) / 2)
#define KL(I, J) (J * (n - 1) - J * (J - 1) / 2 + I)
#else
#define A(I, J) a[I * pda + J]
order = Nag_RowMajor;
#define KL(I, J) (J + I * (I + 1) / 2)
#define KU(I, J) (I * (n - 1) - I * (I - 1) / 2 + J)
#endif

INIT_FAIL(fail);

printf("nag_matop_dtrttp (f01vac) Example Program Results\n\n");
/* Skip heading in data file */
scanf("%*[^\n] ");
scanf("%" NAG_IFMT "%*[^\n] ", &n);
scanf("%39s  %*[^\n] ", nag_enum_arg);
pda = n;
lenap = (n * (n + 1)) / 2;
if (!(a = NAG_ALLOC(pda * n, double)) || !(ap = NAG_ALLOC(lenap, double))) {
printf("Allocation failure\n");
exit_status = -1;
goto END;
}
uplo = (Nag_UploType)nag_enum_name_to_value(nag_enum_arg);
/* Read a triangular matrix of order n. */
for (i = 0; i < n; i++) {
if (uplo == Nag_Upper) {
for (j = i; j < n; j++)
scanf("%lf", &A(i, j));
scanf("%*[^\n] ");
} else {
for (j = 0; j <= i; j++)
scanf("%lf", &A(i, j));
scanf("%*[^\n] ");
}
}

/* Print the unpacked matrix. */
matrix = (uplo == Nag_Upper ? Nag_UpperMatrix : Nag_LowerMatrix);

/* nag_file_print_matrix_real_gen_comp (x04cbc).
* Print real general matrix (comprehensive).
*/
fflush(stdout);
nag_file_print_matrix_real_gen_comp(
order, matrix, Nag_NonUnitDiag, n, n, a, pda, form,
"Unpacked Matrix A:", Nag_IntegerLabels, NULL, Nag_IntegerLabels, NULL,
ncols, indent, NULL, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_file_print_matrix_real_gen_comp (x04cbc).\n%s\n",
fail.message);
exit_status = 1;
goto END;
}
printf("\n");

/* Convert to triangular matrix from full to packed vector form using
* nag_matop_dtrttp (f01vac).
*/
nag_matop_dtrttp(order, uplo, n, a, pda, ap, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_matop_dtrttp (f01vac).\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* Print the packed vector using macros KL or KU. */
printf(" Packed Array AP (printed using KL/KU macros):\n\n");
for (i = 0; i < n; i++) {
printf("  ");
if (uplo == Nag_Upper) {
for (j = 0; j < i; j++)
printf("%6s", " ");
for (j = i; j < n; j++)
printf("%6.2f", ap[KU(i, j)]);
} else {
for (j = 0; j <= i; j++)
printf("%6.2f", ap[KL(i, j)]);
}
printf("\n");
}

END:
NAG_FREE(a);
NAG_FREE(ap);
return exit_status;
}
```