SAXPYI/DAXPYI/CAXPYI/ZAXPYI

 

Sparse elementary vector operation

 

indx

Array containing the indices {ki} of the interesting

 

 

elements of x. The indices must satisfy

 

 

1 ≤ indx(i) ≤ n

i = 1, 2, …, m

 

 

and

 

 

 

indx(i) ≠ indx( j)

1 ≤ i ≠ j ≤ m,

 

 

where n is the length of y.

 

y

Array containing the elements of y, y(i) = yi.

Output

y

If m ≤ 0 or a = 0, then y is unchanged. Otherwise, ax+y

 

 

overwrites the input. Only the elements of y whose

 

 

indices are included in indx are changed.

Notes The result is unspecified if any element of indx is out of range, if any two elements of indx have the same value, or if x, indx, and y overlap such that any element of x or any index shares a memory location with any element of y.

Fortran Equivalent

SUBROUTINE SAXPYI (M, A, X,INDX, Y) REAL*4 A,X(*),Y(*)

INTEGER*4 INDX(*)

IF ( M .LE. 0 ) RETURN

IF ( A .EQ. 0.0 ) RETURN DO 10 I = 1, M

Y(INDX(I)) = A * X(I) + Y(INDX(I))

10CONTINUE RETURN END

Example Compute the REAL*8 elementary vector operation

y← 2x + y,

where x is a sparse vector with interesting elements x1, x4, x5, and x9 stored in

one-dimensional array X, and y is stored in a one-dimensional array Y of dimension 20.

INTEGER*4

M,INDX(4)

REAL*8

A,X(4),Y(20)

DATA

INDX / 1, 4, 5, 9 /

M =

4

 

A =

2.0D0

 

CALL DAXPYI (M,A,X,INDX,Y)

70HP MLIB User’s Guide