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# [Fortran]Multiplying Matrices Using dgemm

## [Fortran]Multiplying Matrices Using dgemm

(OP)
Hi!
I would like to multiply two arrays in Fortran using DGEMM (BLAS procedure).
I have written a simple program:

#### CODE

program matrix
implicit none
double precision mac(m,n),mac2(n,k),mac3(m,k)
integer R,L
integer m,n,k

m=4
n=2
k=3

call FILLMATRIX(m,n,mac)
call FILLMATRIX(n,k,mac2)
call DGEMM("N","N",m,k,n,1.d0,mac,m,mac2,n,0.d0,mac3,m)

WRITE(*,*)'Matrix C:'
DO R=1,m
write(*,*) (mac3(R,L),L=1,k)
end do

END

SUBROUTINE FILLMATRIX (M,N,MATRIX)
INTEGER M,N
DOUBLE PRECISION MATRIX(M,N)

do  I=1,M
do  J=1,N
MATRIX(I,J) = I+2*J
end do
end do

WRITE(*,*) 'Matrix:'
do I=1,M
write(*,*) (MATRIX(I,J),J=1,N)
end do

END 

And I get this result:

#### CODE

Matrix C:
29.00000000000000        0.0000000000000000        0.0000000000000000
36.00000000000000        0.0000000000000000        0.0000000000000000
27.00000000000000        7.2315834086755357E-308   0.0000000000000000
34.00000000000000        1.0037966311151816E-317   0.0000000000000000
Segmentation fault 
Could you point me where I have a mistake?
I will be very grateful for your help.

### RE: [Fortran]Multiplying Matrices Using dgemm

It's surprising that your code compiled ran at all.

You've declared three arrays, mac, mac2 and mac3 with unknown sizes. m, n & k must have actual values before you declare mac, mac2 & mac3.

Put this statement before your Double Precision line and you'll be fine:
(note: this assumes fortran90 or higher. if you're only using an old fortran77 compiler, you need the older syntax to declare you integer parameters)

integer, parameter :: m=4, n=2, k=3

Dan

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