program SWAPPA
       implicit real*8(a-h,r-z)
       implicit integer(i-n)
       implicit character*20(o-q)
       logical  logic
       parameter (MAXD=1000)
       dimension a(MAXD),b(MAXD)
       write (*,*) ''
       write (*,*) ''
       write (*,950)
     . '**********************************************************',
     . '*                                                        *',
     . '*                       SWAPPA                           *',
     . '*                  by Lorenzo Fortunato                  *',
     . '*                     March `02                          *',
     . '*                                                        *',
     . '**********************************************************',
     . '*                                                        *',
     . '*  Utility to swap two columns of data in a given file.  *',
     . '*  Change the parameter MAXD to augment the maximum      *',
     . '*  number of entries (now is 1000).                      *',
     . '*                                                        *',
     . '**********************************************************'      

       write(*,*)'Which file to swap?'
       read(*,*) qinput
 10    write(*,*)'Which filename for the output (NONE will overwrite the 
     . old file)?'
       read(*,*) qout
       inquire(file=qout,exist=logic)
 20    if (logic.eqv..true.) then
              write(*,*)'ALREADY EXISTENT FILE'
              write(*,*)'Do you want to overwrite (yes/no) ?'
              read(*,*) ohohoh
              if (ohohoh.eq.'yes') then 
                                goto 30
              elseif (ohohoh.eq.'no') then 
                                goto 10
              else 
                   goto 20
              endif
       endif
 30    if (qout.eq.'NONE') then
                  n_ctrl=1
                  qout=qinput  
       endif
       open(unit=5,file=qinput,status='old')
       do kk=1,MAXD
          read(5,*,end=50) a(kk),b(kk)
       enddo
 50    nn=kk-1
       if (n_ctrl.eq.1) then
                    close(unit=5,status='delete')
       endif
       open(unit=6,file=qout)
    
       do kk=1,nn
          write(6,*)b(kk),a(kk)
       enddo
       close (unit=6)



 950   format (14(a58,/))  
 990   stop
       end