2D array concatenation in fortran

Question

Fortran 2003 has square bracket syntax for array concatenation, Intel fortran compiler supports it too. I wrote a simple code here for matrix concatenation:

program matrix
implicit none
real,dimension (3,3) :: mat1,mat2
real,dimension(3,6):: mat3
integer i

mat1=reshape( (/1,2,3,4,5,6,7,8,9/),(/3,3/))
mat2=reshape( (/1,2,3,4,5,6,7,8,9/),(/3,3/))
mat3=[mat1,mat2]

!display
do i=1,3,1
write(*,10) mat3(i,:)
10 format(F10.4)
end do

end program

But I get error as

mat3=[mat1,mat2]
Error: Incompatible ranks 2 and 1 in assignment

I expect the output as

1 2 3 1 2 3
4 5 6 4 5 6
7 8 9 7 8 9

Can someone comment where am I going wrong? What is rank 2 and 1 here? I guess all arrays have rank 2.

Answer 1

The array concatenation in fortran 2003 doesn't work as you think. When you concatenate, it's not going to stack the two arrays side by side. It will pick elements from the first array one by one and put into a one-dimensional array. Then it will do the same thing with the second array but it will append this to the 1-D form of first array.

The following code works.

program matrix
implicit none
real,dimension (3,3) :: mat1,mat2
real,dimension(18) :: mat3
integer i

mat1=reshape( (/1,2,3,4,5,6,7,8,9/),(/3,3/))
mat2=reshape( (/1,2,3,4,5,6,7,8,9/),(/3,3/))
mat3=[mat1,mat2]

print*, shape([mat1,mat2])  !check shape of concatenated array
!display
do i=1,18,1
write(*,10) mat3(i)
10 format(F10.4)
end do

end program

However, the result you wanted can be achieved using following code

program matrix
implicit none
real,dimension (3,3) :: mat1,mat2
real,dimension(3,6) :: mat3
integer i

mat1=reshape( (/1,2,3,4,5,6,7,8,9/),(/3,3/))
mat2=reshape( (/1,2,3,4,5,6,7,8,9/),(/3,3/))

do i=1,3
mat3(i,:)=[mat1(:,i),mat2(:,i)]
enddo

!display
do i=1,3,1
write(*,*) mat3(i,:)
end do

end program

Answer 2

Fill it using 1-D arrays then reshape your mat3.

Answer 3

Another way could simply be to

mat3(:,1:3) = mat1
mat3(:,4:6) = mat2

I dont know which is faster, this or the do loop above...