HP MLIB User's Guide Vol. 1 7th Ed.

Chapter 2 Basic Vector Operations 125
Apply modified Givens rotation SROTM/DROTM
incy Increment for the array y, incy ≠ 0:
incy > 0 y is stored forward in array y; that is,
y
i
is stored in y((i−1)×incy+1).
incy < 0 y is stored backward in array y; that
is, y
i
is stored in y((i−n)×incy+1).
Use incy = 1 if the vector y is stored contiguously in
array y; that is, if y
i
is stored in y(i). Refer to “BLAS
Indexing Conventions” in the introduction to this
chapter.
param Array containing the matrix elements of the modified
Givens rotation matrix H and a flag indicating which
form the rotation matrix takes, and therefore which of
the elements of param are significant. param is usually
set by the companion subprogram SROTMG or
DROTMG; refer to the description of these companion
subprograms for the specific contents of param.
Output x and y If n ≤ 0 or if param(1) = −2, x and y are unchanged.
Otherwise, the result vectors overwrite the input.
Notes The result is unspecified if incx = 0 or incy = 0 or if x and y overlap such that
any element of x shares a memory location with any element of y.
VECLIB also contains subprograms that construct and apply regular Givens
rotations. They are documented elsewhere in this chapter. The modified Givens
subprograms are a little more difficult to use but are more efficient.
Example 1 Apply a modified Givens rotation to x and y, vectors 10 elements long stored in
one-dimensional arrays X and Y of dimension 20.
INTEGER*4 N,INCX,INCY
REAL*8 X(20),Y(20),PARAM(5)
N = 10
INCX = 1
INCY = 1
CALL DROTM (N,X,INCX,Y,INCY,PARAM)