title: Error: Unclassifiable statement in if conditional
body: <p>This is the program I wrote to calculate the distance between carbon and hydrogen atom coordinates as a practice. But some errors occurred and I don't know how to fix them.</p>
  <pre><code>    program C_H_bdlength
      implicit none
      integer                 :: ia            ! integer atoms number               
      integer                 :: na            ! number of atoms                    
      real*8                  :: x  ,  y , z   ! x y z coordinates of all atoms     
      real*8                  :: x1 , y1 , z1  ! x y z coordinates of C atoms       
      real*8                  :: x2 , y2 , z2  ! x y z coordinates of H atoms       
      character(len=2)        :: ele           ! element name                       
      real*8                  :: dis           ! distance between C and H           
  
      open(100,file='cnt.ini',action='read',status='old')
      na = 0
      do
       read(100,*,end=101)
       na = na + 1
      end do
      101 continue
  
      open(200, file='cnt.ini',action='wtire',status='replace')
      rewind(100)
  
      write(200,*)
  
      do ia = 1 , na
       read(100,*) ele, x, y, z
         if ele == 'C' then x1 = x, y1 = y, z1 = z
            do ia = ia , na
               read(100,*) ele, x, y, z
                 if ele == 'H' then x2 = x, y2 = y, z2 = z
                 dis = sqrt((x1 - x2)**2 + (y1 - y2)**2 + (z1 - z2)**2)
  
                 write(200,*) dis
                 end if
            end do
         end if
      end do
      
      end program C_H_bdlength
  </code></pre>
  <p>Errors:</p>
  <pre><code>  In file C_H_bdlength.f90:26
  
         if ele == 'C' then x1 = x, y1 = y, z1 = z
        1
      Error: Unclassifiable statement at (1)
   In file C_H_bdlength.f90:27
  
            do ia = ia , na
                          1
   In file C_H_bdlength.f90:24
  
      do ia = 1 , na
                 2
  Error: Variable 'ia' at (1) cannot be redefined inside loop beginning at (2)
   In file C_H_bdlength.f90:29
  
                 if ele == 'H' then x2 = x, y2 = y, z2 = z
                1
  Error: Unclassifiable statement at (1)
   In file C_H_bdlength.f90:33
  
                 end if
                   1
  Error: Expecting END DO statement at (1)
   In file C_H_bdlength.f90:35
  
         end if
           1
  Error: Expecting END DO statement at (1)
  </code></pre>