LAPACK/BLAS matrix multiplication in fortran returns zeros

Question

I'm a fortran beginner and simply wanted to try if i get a matrix multiplication to work.

    program testlapack
    implicit none

    COMPLEX, DIMENSION(2, 2) :: A, B, Output

    A = reshape((/ 4, 0, 0, 2 /), shape(A))
    B = reshape((/ 6, 0, 0, 3 /), shape(B))
    Output = reshape((/ 1, 0, 0, 1 /), shape(Output))

    call DGEMM('n','n',2,2,2,1.0,A,2,B,2,0.0,Output,2)

    Write(*,*) Output

    end program testlapack

I was expecting a simple A * B multiplication written into the Output-array. Instead, lapack is returning a 2x2 matrix with only zeros. The compilation with ifort -llpack yields no errors.

Where's my problem? Thanks

High Performance Mark · Accepted Answer · 2013-05-17 15:22:23Z

2

I suspect that passing complex arguments to a subroutine expecting real arguments might be the cause of your problem. Perhaps you meant to use zgemm rather than dgemm ?

answered May 17, 2013 at 15:22

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1 Comment

user2394066 Over a year ago

This was certainly a good guess, and thanks. But it wasn't the only reason, the output has not changed :(

user2394066 · Accepted Answer · 2013-05-21 08:53:37Z

1

I changed the matrices to REAL and used DGEMM. It worked once i used double precision for every number entering DGEMM, including the 1.0 set as "ALPHA". I didn't expect this to be mandatory.

answered May 21, 2013 at 8:53

user2394066

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2 Comments

Kyle Kanos Over a year ago

That makes sense, given that the D in DGEMM means "double;" if you wanted single precision, you want the routine SGEMM.

swimfar Over a year ago

Note that there's also CGEMM for complex numbers. netlib documentation

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