How to initialize two-dimensional arrays in Fortran
For multidimensional (rank>1) arrays, the Fortran way for initialization differs from the C solution because in C multidimensional arrays are just arrays of arrays of etc.
In Fortran, each rank corresponds to a different attribute of the modified data type. But there is only one array constructor, for rank-1 arrays. From these two reasons, initialization through array constructor requires the RESHAPE
intrisic function.
In addition to what has already been answered, there is a more direct way of entering the value of a matrix by row instead as by column: reshape has an optional argument ORDER
which can be used to modify the order of filling the element of the multidimensional array with the entries of the array constructor.
For instance, in the case of the example in the first answer, one could write:
INTEGER, DIMENSION(3, 3) :: array=reshape( (/ 1, 2, 3, &
4, 5, 6, &
7, 8, 9 /), &
shape(array), order=(/2,1/) )
obtaining the filling of the matrix array exactly in the order shown by the lines of code.
The array (/2, 1/)
forces the column index (2)
to have precedence on the row index (1)
, giving the desired effect.
You can do that using reshape and shape intrinsics. Something like:
INTEGER, DIMENSION(3, 3) :: array
array = reshape((/ 1, 2, 3, 4, 5, 6, 7, 8, 9 /), shape(array))
But remember the column-major order. The array will be
1 4 7
2 5 8
3 6 9
after reshaping.
So to get:
1 2 3
4 5 6
7 8 9
you also need transpose intrinsic:
array = transpose(reshape((/ 1, 2, 3, 4, 5, 6, 7, 8, 9 /), shape(array)))
For more general example (allocatable 2D array with different dimensions), one needs size intrinsic:
PROGRAM main
IMPLICIT NONE
INTEGER, DIMENSION(:, :), ALLOCATABLE :: array
ALLOCATE (array(2, 3))
array = transpose(reshape((/ 1, 2, 3, 4, 5, 6 /), &
(/ size(array, 2), size(array, 1) /)))
DEALLOCATE (array)
END PROGRAM main
Array initialization can be done in the array declaration statement itself, as shown below:
program test
real:: x(3) = (/1,2,3/)
real:: y(3,3) = reshape((/1,2,3,4,5,6,7,8,9/), (/3,3/))
integer:: i(3,2,2) = reshape((/1,2,3,4,5,6,7,8,9,10,11,12/), (/3,2,2/))
end program test
It surprises me that
real:: y(3,3) = (/(/1,2,3/),(/4,5,6/),(/7,8,9/)/)
is not accepted by the compiler (tried g95, gfortran). It turns out that the shape
of
(/(/1,2,3/),(/4,5,6/),(/7,8,9/)/)
is 9
and not 3 3
!