[数值问题] 关于求平均值的问题

Program www_fcode_cn
  Implicit None
  integer :: ierr
  integer :: i , year , mon , day
  integer :: yearlast , monlast , daylast , c
  real :: d , a
  Open( 12 , File = '57517(4年).txt' )
  Open( 13 , File = 'out.txt' )
  read(12,*)
  yearlast = -1
  monlast = -1
  daylast = -1
  a = 0.0
  c = 0
  Do
    Read( 12 , * , ioStat=ierr ) i , year , mon , day , d
    if ( ( ierr== 0 ) .and. (year == yearlast) .and. (mon == monlast) .and. ( (day>15)==(daylast>15) ) ) then !// 同一个半月
      if ( ( d > 12.0 ) .and. ( d < 3000.0 ) ) then
        a = a + d 
        c = c + 1
      end if
    else
      if( c> 0 ) then
        a = a / c
        write(13,*) yearlast , monlast , a , c
      end if
      a = 0.0
      c = 0
      if ( ( d > 12.0 ) .and. ( d < 3000.0 ) ) then
        c = 1
        a = d
      end if
    end if
    yearlast = year
    monlast = mon
    daylast = day
    if ( ierr /= 0 ) Exit
  End Do
  Close( 12 )
  Close( 13 )
End Program www_fcode_cn  

  yearlast = -1
  monlast = -1
  daylast = -1

( (day>15)==(daylast>15) ) )