fortran 90 expected bounds specification during pointer assignment - pointers

Question

I am new to Fortran. I am writing a program in Fortran 90 to get non-zero elements of an array and put them into a new array using pointer function as following:
program prog
implicit none
integer, target :: a(5)
integer :: i
integer, pointer :: nz(:)
a(1) = 1
a(2) = 0
a(3) = 0
a(4) = 2
a(5) = 3
nz => non_zeros(a)
do i=1, size(nz)
write(*,*) nz(i)
end do
contains
function non_zeros(a)
integer, target :: a(:)
integer, pointer:: non_zeros(:)
integer :: n, i, j
n = count(a .ne. 0)
allocate(non_zeros(n))
j = 0
do i=1, m
if (a(i) .ne. 0) then
j = j + 1
non_zeros(j) => a(i)
end if
end do
end function non_zeros
end program prog
during compiling I got the error:
non_zeros(j) => a(i)
1
Error: Expected bounds specification for 'non_zeros' at (1)
Can you please tell me what did I do wrong? Thank you in advance!
Update of my question: According to the explanation of High Performance Mark, I defined a derived type:
program prog
implicit none
integer, target :: a(5)
type dt
integer, pointer :: x
end type
type(dt), allocatable :: nz(:)
a(1) = 1
a(2) = 0
a(3) = 0
a(4) = 2
a(5) = 3
nz = non_zeros(a)
contains
function non_zeros(a)
integer, target :: a(:)
type(dt), allocatable :: non_zeros(:)
integer :: n, i, j
n = count(a .ne. 0)
allocate(non_zeros(n))
j = 0
do i=1, m
if (a(i) .ne. 0) then
j = j + 1
non_zeros(j)%x => a(i)
end if
end do
end function non_zeros
end program prog
Now program works and gives the desired results. However, I did not use pointer function in this case, since my function returns an allocatable array of pointers, not pointer to an array. Is there any way to use pointer function here? Thank you

Answer 1

To get the non-zero elements of a into a new array you could simply declare
integer, dimension(:), allocatable :: non_zeros
and then populate that with the statement
non_zeros = pack(a,a/=0)
and avoid fiddling around with pointers entirely. This relies on a feature introduced in the 2003 standard, but it is implemented by all (I think) the current crop of Fortran compilers on the market.
The code that you have written looks to me as if you want nz to be an array of pointers, with each element in nz pointing to a non-zero element of a. If I'm right, you've misunderstood what a statement such as
integer, pointer :: nz(:)
declares. It does not declare an array of pointers to integers, it declares a pointer to an array of integers. When you write
non_zeros(j) => a(i)
you're making the mistake of trying to set an element of non_zeros to point to an element of a.
The error message is misleading here because the compiler interprets non_zeros(j) as a syntactically-incorrect bounds-spec or bounds-remapping, but the error is semantic, the compiler doesn't understand your misunderstanding of Fortran.