program aug12
    implicit none
    interface
     subroutine display(n,f)
      integer,dimension(:)::n
      character(*)::f
     end subroutine
     logical function ok(n)
      integer,dimension(:)::n
     end function
     subroutine reverse(x,y)
      integer,dimension(:),intent(in)::x
      integer,dimension(:),intent(out)::y
     end subroutine
    end interface
    integer,dimension(4)::x,xr
    integer,dimension(6:8)::y,yr
    integer,dimension(2,3)::z
    integer::i,j
    character(23)::form = '(x,a2,"(",i2,") = ",i2)'
    print*,"Initialize x"
    do j = 1,4
     x(j) = j
    end do
    print*
    do j = 1,4
     print form,"x",j,x(j)
    end do
    print*
    print*,"Display x"
    print*
    call display(x,form)
    print*
    if (ok(x)) then
     print*,"x is ok"
    else
     print*,"x is not ok"
    end if
    print*
    print*,"xr = Reverse x"
    print*
    call reverse(x,xr);
    do j = 1,4
     print form,"xr",j,xr(j)
    end do
    print*
    print*,"Display xr"
    print*
    call display(xr,form)
    print*
    print*,"Initialize y"
    do j = 6,8
     y(j) = j+10
    end do
    y(7) = -1
    print*
    do j = 6,8
     print form,"y",j,y(j)
    end do
    print*
    print*,"Display y"
    print*
    call display(y,form)
    print*
    if (ok(y)) then
     print*,"y is ok"
    else
     print*,"y is not ok"
    end if
    print*
    print*,"yr = Reverse y"
    call reverse(y,yr);
    print*
    do j = 6,8
     print form,"yr",j,yr(j)
    end do
    print*
    print*,"Display yr"
    print*
    call display(yr,form)
    read*,((z(i,j),j = 1,3), i = 1,2)
    print*
    print*,"Read z and write z"
    print'(6i3)',((z(i,j),i = 1,2), j = 1,3)
   end program
  
   subroutine display(a,form)
    implicit none
    integer,dimension(:)::a
    character(*)::form
    integer::i
    do i = 1,size(a)
     print form,"a",i,a(i)
  !   print*,"a(",i,") =",a(i)
    end do
   end subroutine 
  
   logical function ok(a)
  ! output condition: ok = all entries of a are non-negative
    implicit none
    integer,dimension(:)::a
    logical::ok_so_far
    integer::i
    ok_so_far = .true.
    do i = 1,size(a)
     if (a(i) < 0) ok_so_far = .false.
    end do
    ok = ok_so_far
   end function
  
   subroutine reverse(x,y)
  !  input condition: size(x) = size(y)
    integer,dimension(:),intent(in)::x
    integer,dimension(:),intent(out)::y
    integer::i
    do i = 1,size(x)
     y(i) = x(size(x)+1-i)
    end do
   end subroutine