NAG Library Routine Document

F06EVF (DGTHRZ)

+− Contents

1 Purpose

9 Example

F06EVF (DGTHRZ) gathers specified (usually nonzero) elements of a real vector

y

in full storage form into a sparse real vector

x

in compressed form. The specified elements of

y

are set to zero.

SUBROUTINE F06EVF (

INTEGER	NZ, INDX(*)
REAL (KIND=nag_wp)	Y(), X()

The routine may be called by its BLAS name dgthrz.

F06EVF (DGTHRZ) gathers the specified elements of a vector,

y

, in full storage form, into the equivalent sparse vector compressed form. The gathered elements of

y

are set to zero.

Dodson D S, Grimes R G and Lewis J G (1991) Sparse extensions to the Fortran basic linear algebra subprograms ACM Trans. Math. Software 17 253–263

1: NZ – INTEGERInput: On entry: the number of nonzeros in the compressed sparse vector $x$ .
2: Y( $*$ ) – REAL (KIND=nag_wp) arrayInput/Output: Note: the dimension of the array Y must be at least $\max_{k} \{INDX (k)\}$ .
On entry: the vector $y$ . Only elements corresponding to indices in INDX are accessed.

On exit: the elements of $y$ corresponding to indices in INDX are set to zero.
3: X( $*$ ) – REAL (KIND=nag_wp) arrayOutput: Note: the dimension of the array X must be at least $\max (1, NZ)$ .
On exit: the compressed vector $x$ .
4: INDX( $*$ ) – INTEGER arrayInput: Note: the dimension of the array INDX must be at least $\max (1, NZ)$ .
On entry: $INDX (i)$ must contain the index $Y (i)$ , for $i = 1, 2, \dots, NZ$ , which is to be gathered into $x$ .
Constraint: the indices must be distinct.