Example of using exceptions: correctly and incorrectly

The code, between the rows of # signs, executes a check after each call to track. If one skips this inside a loop (CORRECT=FALSE), the unstable particle will be dragged incorrectly to the end of the lattice.

N.B. If you use a NODE_LAYOUT you can look at lost_node as well as lost_fibre.

write(6,*) " Do you want a correct implementation of exceptions ->  yes=true "
read(5,*) CORRECT
WRITE(6,*) "   " 
WRITE(6,*) " GIVE A 'BIG' INITIAL VALUE (ONE NUMBER) FOR X(1:6)= R (example 0.1) " 
READ(5,*) R
X0(1:6)=R

x=x0
p=>als%start
do while(.not.associated(p%next,als%start)) ! this is one turn minus the last element

  call track(x,state,fibre1=p,fibre2=p%next)

############################################################
  if(.not.check_stable.and.CORRECT) then                                   
    write(6,*)  "In the loop"
    write(6,"(a255)")  messagelost
    write(6,*) lost_fibre%pos,lost_fibre%mag%name
    write(6,*)  "Unstable ray"
    WRITE(6,'(6(1x,E15.8))') Xlost
   exit
  endif
############################################################

p=>p%next
enddo

  if(.not.check_stable) then
   write(6,*)  "End of loop"
   write(6,"(a255)")  messagelost
   write(6,*) lost_fibre%pos,lost_fibre%mag%name
    write(6,*)  "Unstable ray"
    WRITE(6,'(6(1x,E15.8))') Xlost
   else
    write(6,*)  "stable ray"
    WRITE(6,'(6(1x,E15.8))') X
  endif

x=x0
  call track(x,state,fibre1=als%start,fibre2=als%end)  ! this is one turn minus the last element

  if(.not.check_stable) then
   write(6,*)  "Using one call to Track"
   write(6,"(a255)")  messagelost
   write(6,*) lost_fibre%pos,lost_fibre%mag%name
    write(6,*)  "Unstable ray"
    WRITE(6,'(6(1x,E15.8))') Xlost
   else
    write(6,*)  "stable ray"
    WRITE(6,'(6(1x,E15.8))') X
  endif