add a lammps_eval() function to the C library interface and all derived wrappers

python/lammps/core.py

@@ -339,6 +339,9 @@ class lammps(object):
     self.lib.lammps_extract_variable_datatype.argtypes = [c_void_p, c_char_p]
     self.lib.lammps_extract_variable_datatype.restype = c_int
 
+    self.lib.lammps_eval.argtypes = [c_void_p, c_char_p]
+    self.lib.lammps_eval.restype = c_double
+
     self.lib.lammps_fix_external_get_force.argtypes = [c_void_p, c_char_p]
     self.lib.lammps_fix_external_get_force.restype = POINTER(POINTER(c_double))
 
@@ -1533,6 +1536,30 @@ class lammps(object):
 
   # -------------------------------------------------------------------------
 
+  def eval(self, expr):
+    """ Evaluate a LAMMPS immediate variable expression
+
+    .. versionadded:: TBD
+
+    This function is a wrapper around the function :cpp:func:`lammps_eval`
+    of the C library interface.  It evaluates and expression like in
+    immediate variables and returns the value.
+
+    :param expr: immediate variable expression
+    :type name: string
+    :return: the result of the evaluation
+    :rtype: c_double
+    """
+
+    if expr: newexpr = expr.encode()
+    else: return None
+
+    with ExceptionCheck(self):
+      return self.lib.lammps_eval(self.lmp, newexpr)
+    return None
+
+  # -------------------------------------------------------------------------
+
   # return vector of atom properties gathered across procs
   # 3 variants to match src/library.cpp
   # name = atom property recognized by LAMMPS in atom->extract()