NAG FL Interface
f01vaf (dtrttp)

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1 Purpose

f01vaf copies a real triangular matrix, stored in a full format array, to a standard packed format array.

2 Specification

Fortran Interface
Subroutine f01vaf ( uplo, n, a, lda, ap, info)
Integer, Intent (In) :: n, lda
Integer, Intent (Out) :: info
Real (Kind=nag_wp), Intent (In) :: a(lda,*)
Real (Kind=nag_wp), Intent (Out) :: ap(n*(n+1)/2)
Character (1), Intent (In) :: uplo
C Header Interface
#include <nag.h>
void  f01vaf_ (const char *uplo, const Integer *n, const double a[], const Integer *lda, double ap[], Integer *info, const Charlen length_uplo)
The routine may be called by the names f01vaf, nagf_matop_dtrttp or its LAPACK name dtrttp.

3 Description

f01vaf packs a real n×n triangular matrix A, stored conventionally in a full format array, into an array of length n(n+1)/2. The matrix is packed by columns. This routine is intended for possible use in conjunction with routines from Chapters F06, F07, F08 and F16 where some routines use triangular matrices stored in the packed form. Packed storage format is described in Section 3.3.2 in the F07 Chapter Introduction.

4 References


5 Arguments

1: uplo Character(1) Input
On entry: specifies whether A is upper or lower triangular.
A is upper triangular.
A is lower triangular.
Constraint: uplo='U' or 'L'.
2: n Integer Input
On entry: n, the order of the matrix A.
Constraint: n0.
3: a(lda,*) Real (Kind=nag_wp) array Input
Note: the second dimension of the array a must be at least n.
On entry: the triangular matrix A.
  • If uplo='U', A is upper triangular and the elements of the array below the diagonal are not referenced.
  • If uplo='L', A is lower triangular and the elements of the array above the diagonal are not referenced.
4: lda Integer Input
On entry: the first dimension of the array a as declared in the (sub)program from which f01vaf is called.
Constraint: ldamax(1,n).
5: ap(n×(n+1)/2) Real (Kind=nag_wp) array Output
On exit: the n×n triangular matrix A, packed by columns.
More precisely,
  • if uplo='U', the upper triangle of A is stored with element Aij in ap(i+j(j-1)/2) for ij;
  • if uplo='L', the lower triangle of A is stored with element Aij in ap(i+(2n-j)(j-1)/2) for ij.
6: info Integer Output
On exit: info=0 unless the routine detects an error (see Section 6).

6 Error Indicators and Warnings

If info=-i, argument i had an illegal value. An explanatory message is output, and execution of the program is terminated.

7 Accuracy

Not applicable.

8 Parallelism and Performance

f01vaf is not threaded in any implementation.

9 Further Comments


10 Example

This example reads in a triangular matrix and copies it to packed format.

10.1 Program Text

Program Text (f01vafe.f90)

10.2 Program Data

Program Data (f01vafe.d)

10.3 Program Results

Program Results (f01vafe.r)