Preserve bounds in allocation in intrinsic assignment

Question

I am using automatic allocation on assignment to calculate the difference of two arrays, with bounds starting at 0:

program main
   implicit none
   integer, allocatable :: a(:), b(:), c(:)

   allocate(a(0:10))
   allocate(b(0:10))

   a = 1
   b = 2

   write (*,*) lbound(a)
   write (*,*) lbound(b)

   c = b - a

   write (*,*) lbound(c)
end program main

Both, gfortran and ifort give the output:

       0
       0
       1

Why doesn't c have the same bounds as a and b? Is there a short and concise (without an explicit allocate) way of making sure c has the same bounds?

Answer 1

Why doesn't c have the same bounds as a und b?

Because it shall be allocated to the same shape as a and b and starting at the lower bound of the expression on the right hand side. The b-a is an expression and it starts at 1 and any result f array arithmetic operations will start like that. What if b and a start at different indexes?)

Is there a short and concise (without an explicit allocate) way of making sure c has the same bounds?

No. But you can at least explicitly allocate with mold=a or source=a.