Digital

Name

sgthrs, dgthrs, cgthrs, zgthrs − Gathers and scales the specified elements of a vector

FORMAT

{S,D,C,Z}GTHRS ( nz, alpha, y, x, indx )

Arguments

nzinteger∗4
On entry, the number of elements to be gathered into the compressed form.
On exit, nz is unchanged. 

alphareal∗4 | real∗8 | complex∗8 | complex∗16
On entry, the scalar multiplier for the elements of vector y.
On exit, alpha is unchanged. 

yreal∗4 | real∗8 | complex∗8 | complex∗16
On entry, an array of the elements of vector y stored in full form.
On exit, y is unchanged.  Only the elements in the vector y corresponding to the indices in the INDX array are accessed. 

xreal∗4 | real∗8 | complex∗8 | complex∗16
On entry, an array that receives the specified elements of vector y after scaling.
On exit, if nz <= 0, x is unchanged. If nz > 0,  the array X contains the specified elements of vector y after scaling by the scalar, alpha. 

indxinteger∗4
On entry, an array containing the indices of the values to be gathered into compressed form.
On exit, indx is unchanged. 

Description

The _GTHRS subprograms gather specified elements of vector y in full form, multiply the elements by alpha, and store the result as elements of a sparse vector x in compressed form. 

For i=1,..., nz:

x(i)  =  alpha ∗ y(indx(i))

If nz <= 0, x is unchanged. 

SGTHRS and DGTHRS gather the elements from a real vector in full storage and scale them into a real vector in compressed storage.  CGTHRS and ZGTHRS gather the specified elements from a complex vector in full storage and scale them into a complex vector in compressed storage. 

The _GTHRS subprograms are not part of the original set of Sparse BLAS Level 1 subprograms. 

Example

INTEGER NZ, INDX(10)
REAL∗8 Y(40), X(10), ALPHA
NZ = 10
ALPHA = 1.5D0
CALL DGTHRS(NZ, ALPHA, Y, X, INDX)

This FORTRAN code shows how the nz  elements  of  the  vector y, corresponding to the indices in  the INDX array, are scaled by the scalar alpha and gathered in a compressed form into the vector x.