100 Days of Code Log R1D8

Today has been the beginning of Week 2 in Fortran for scientific Computing course. This had the introduction to arrays, mainly, along with elemental procedures. A word or two were mentioned about “pure functions” but those remain a mystery for now. But, here are some highlights.

Arrays

1. Arrays in fortran may have particular rank (upto 15 in case of Fortran 2008).

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real, dimension(5) :: x
real :: A(3,4)
real, dimension(-10:10) :: y

2. Arrays are arranged columnwise in Fortran.
3. Array can be indexed in slices in a similar manner to python
4. Array functions may be
   1. Elemental - operate on each value in array. Example sin(A) operates on each element.
   2. Operate on array as a whole - sum(A) adds all the elements in the A.
5. The 2nd category of array functions usually have the following optional parameters.
   1. dim - The dimension along which to perform the operations. For example, for A(m,n), the operation sum(A, dim=1) will return a vector of length n with each element being the sum of m rows in that column.
   2. mask - The condition to consider. Should be passed a logical array of same size of the array in consideration.

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real :: A(3,4), b(3)
A = reshape( [ (i, i=1,12) ], [3,4])
b = sum(A, dim=2, mask=mod(A,2)/=0)

There is count function which counts the number of instances which are true in logical array.

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count(mask=mod(A,2)/=0)
count(A<0)

Procedures

In a subroutine, if an array dimension n is specified, that dimension is the minimum requirement. But, if the passed array is larger, the first n values alone are considered.

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real :: a(5), b(8)
a = 5.
b = 0.
call print_arr(a) ! error
call print_arr(b) ! no error

subroutine print_arr(data)
    implicit none
    real,intent(in) :: data(6)
    print *, data
end subroutine print_arr

Also, the fortran allows one to index outside the array but it still doesn’t throw error. I still haven’t figured out why the (1,3) element comes out to be 8. May be it will be covered tomorrow!

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integer :: A(2,3)
call fill_array(A)

subroutine fill_array(A)
    implicit none
    integer,dimension(:,:) :: A
    integer :: i,j
    do j = 1, size(A, 1)
        do i = 1, size(A,2)
            A(i,j) = (i-1)*size(A,2) + j
            print *, A(i,j)
        end do
    end do
end subroutine fill_array

Also, elemental functions can have either have intent(in) or value not intent(out) nor intent(inout).