NAG Library Routine Document

F06ERF (DDOTI)

+− Contents

1 Purpose

9 Example

F06ERF (DDOTI) computes the scalar product of a sparse real vector, stored in compressed form, with a real vector.

FUNCTION F06ERF (

REAL (KIND=nag_wp) F06ERF

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

The routine may be called by its BLAS name ddoti.

F06ERF (DDOTI) returns, via the function name, the value of the scalar product

x^{T} y = x (1) \times y (indx (1)) + x (2) \times y (indy (2)) + \dots + x (n z) \times y (indx (n z))

where

x

is a sparse real vector, stored in compressed form and

y

is a real vector in full storage format.

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 sparse vector $x$ .
2: X( $*$ ) – REAL (KIND=nag_wp) arrayInput: Note: the dimension of the array X must be at least $\max (1, NZ)$ .
On entry: the nonzero elements of the sparse vector $x$ .
3: 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 of $X (i)$ in the sparse vector $x$ , for $i = 1, 2, \dots, NZ$ .
4: Y( $*$ ) – REAL (KIND=nag_wp) arrayInput: 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.