NAG Library Routine Document

f01vcf (dtpttr)

1
Purpose

f01vcf (dtpttr) unpacks a real triangular matrix, stored in a standard packed format array, to a full format array.

2
Specification

Fortran Interface
Subroutine f01vcf ( uplo, n, ap, a, lda, info)
Integer, Intent (In):: n, lda
Integer, Intent (Out):: info
Real (Kind=nag_wp), Intent (In):: ap(n*(n+1)/2)
Real (Kind=nag_wp), Intent (Inout):: a(lda,*)
Character (1), Intent (In):: uplo
C Header Interface
#include <nagmk26.h>
void  f01vcf_ (const char *uplo, const Integer *n, const double ap[], double a[], const Integer *lda, Integer *info, const Charlen length_uplo)
The routine may be called by its LAPACK name dtpttr.

3
Description

f01vcf (dtpttr) unpacks a real n by n triangular matrix A, stored in an array of length nn+1/2, to conventional storage in a full format array. 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

None.

5
Arguments

1:     uplo – Character(1)Input
On entry: specifies whether A is upper or lower triangular.
uplo='U'
A is upper triangular.
uplo='L'
A is lower triangular.
Constraint: uplo='U' or 'L'.
2:     n – IntegerInput
On entry: n, the order of the matrix A.
Constraint: n0.
3:     apn×n+1/2 – Real (Kind=nag_wp) arrayInput
On entry: the n by n triangular matrix A, packed by columns.
More precisely,
  • if uplo='U', the upper triangle of A must be stored with element Aij in api+jj-1/2 for ij;
  • if uplo='L', the lower triangle of A must be stored with element Aij in api+2n-jj-1/2 for ij.
4:     alda* – Real (Kind=nag_wp) arrayOutput
Note: the second dimension of the array a must be at least n.
On exit: 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.
5:     lda – IntegerInput
On entry: the first dimension of the array a as declared in the (sub)program from which f01vcf (dtpttr) is called.
Constraint: ldamax1,n.
6:     info – IntegerOutput
On exit: info=0 unless the routine detects an error (see Section 6).

6
Error Indicators and Warnings

info<0
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

f01vcf (dtpttr) is not threaded in any implementation.

9
Further Comments

None.

10
Example

This example reads in a triangular matrix packed by columns and unpacks it to full format.

10.1
Program Text

Program Text (f01vcfe.f90)

10.2
Program Data

Program Data (f01vcfe.d)

10.3
Program Results

Program Results (f01vcfe.r)