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[Functions] source/IO/IO_getPrevScalar





  IO_getPrevScalar(char*(in) :: name,
                   real/int/str/log(out) :: value, 
                   integer(out),optional :: error)


  This function gets a 'previous' scalar from a 
  linked list implemented under the hood.  
  The scalar list can store 2 values, a 'current' value and
  a 'previous' value.  Previous values are used when restarting 
  a run and might also be used to compare
  a scalar value stored in a checkpoint to a new value set
  apon restart. (see example below)

  In FLASH3 what we mean by scalars are single value variables
  associated with an entire flash run.
  These scalars are in contrast to Grid scope variables which
  need to be stored at each zone of each block in the simulation.
  Density, pressure and temperature are examples of Grid scope 
  variables while simTime, dt, globalNumBlocks are single quantities
  associated with the entire run. 

  Scalars of this type can be integers, reals, strings or logical
  values.  An example of a string scalar might be the FLASH3 run
  comment, the name of the logfile, or the setup line.


 name:       name of scalar
 value:      scalar value
 error:      error code. 0 (NORMAL) for normal termination


  When restarting a run a user might want to change the timestep, dt
  or adjust the simulation time. The checkpoint read routine reads the
  scalar values from the checkpoint file and stores them in the 'previous'
  slot. The user can then compare the 'previous' value of the scalar to
  the new value.

     USE IO_interface : ONLY IO_getPrevScalar
     real :: dt

     call IO_getPrevScalar('dt', dt) 


  Because IO_getPrevScalar is an overloaded function a user calling
  the routine must use IO_interface.F90.
  (Under the hood, a call to the generic name IO_getPrevScalar is handled
  by IO_getPrevScalarReal, IO_getPrevScalarInt, IO_getPrevScalarStr, 
  or IO_getPrevScalarLog. A separate list is kept for each type.)

  An error value of NOTFOUND (-1) indicated that the given previous value
  was not found.  The calling subroutine should handle this error if the
  optional error dummy argument was provided.