LAPACK Archives

[Lapack] extra multiplications in daxpy


I have a problem with subroutine daxpy.
I would like to add simply two vectors x and y, without multiplying
x or y by a constant coefficient da.
If I use daxpy, I have to set da = 1.d0 The problem is that, even if da is 
equal to
one, da is used to multiply the coefficient of the x vector: 
 do 10 i = 1,n
        dy(iy) = dy(iy) + da*dx(ix)


So, instead of n additions, the subroutine does n additions AND n 
I would like to know the reason for these extra n multiplications.Why
not a test comparing alpha to 1.d0 ?
Is there another routine in BLAS to avoid these extra n multiplications ?

The same problems arises when I need to compute the product of a matrix and a 
vector (dgemv) and the product of two matrix. (dgemm).

Thank you very much for your answer to this question !

Yours faithfully,

Jerome Bureau

P.S. I apologize if this question has already been answered somewhere else, I 
wasn't able
to find the answer. 


D??couvrez une nouvelle fa??on d'obtenir des r??ponses ?? toutes vos questions 
Profitez des connaissances, des opinions et des exp??riences des internautes 
sur Yahoo! Questions/R??ponses

<Prev in Thread] Current Thread [Next in Thread>
  • [Lapack] extra multiplications in daxpy, Bureau Jerome <=

For additional information you may use the LAPACK/ScaLAPACK Forum.
Or one of the mailing lists, or