Trep "Hello World"
==================
Let's begin by creating the trep equivalent of "Hello World".

Import the trep module into python
----------------------------------
Let us begin by importing the trep module into Python.

.. literalinclude:: ./code_snippets/trepHelloWorld.py
    :start-after: # Import the Trep module into python
    :end-before: # Create a new instance of a trep system

After importing trep if you type ``trep`` followed by pressing tab in IPython a
list of all of the methods that are available for the trep module are
displayed. As you can see that :class:`trep.System` is listed, which instantiates an
empty trep system. This is used in the next section.

    >>> trep.
    trep.CONST_SE3         trep.RZ                trep.constraint        trep.ryXRT
    trep.Config            trep.Spline            trep.constraints       trep.rz
    trep.Constraint        trep.System            trep.finput            trep.save_trajectory
    trep.ConvergenceError  trep.TX                trep.force             trep.spline
    trep.Force             trep.TY                trep.forces            trep.system
    trep.Frame             trep.TZ                trep.frame             trep.tapemeasure
    trep.Input             trep.TapeMeasure       trep.load_trajectory   trep.tx
    trep.MidpointVI        trep.WORLD             trep.midpointvi        trep.ty
    trep.Potential         trep.config            trep.potential         trep.tz
    trep.RX                trep.const_se3         trep.potentials        trep.util
    trep.RY                trep.const_txyz        trep.rx                trep.visual


Create a new instance of a trep system
--------------------------------------
Let us create a new instance of a trep system.

.. literalinclude:: ./code_snippets/trepHelloWorld.py
    :start-after: # Create a new instance of a trep system
    :end-before: # Visualize the system with trep's visualization tools

This creates a new instance of a trep system. Typing ``system.`` followed by
pressing tab will list all the methods and properties of the system object.

    >>> system.
    system.L                           system.get_frame
    system.L_ddq                       system.get_input
    system.L_ddqddq                    system.get_potential
    system.L_ddqddqdq                  system.hold_structure_changes
    system.L_ddqddqdqdq                system.import_frames
    system.L_ddqdq                     system.inputs
    system.L_ddqdqdq                   system.kin_configs
    system.L_ddqdqdqdq                 system.lambda_
    system.L_dq                        system.lambda_dddk
    system.L_dqdq                      system.lambda_dddkdq
    system.L_dqdqdq                    system.lambda_ddq
    system.add_structure_changed_func  system.lambda_ddqddq
    system.configs                     system.lambda_ddqdq
    system.constraints                 system.lambda_dq
    system.ddq                         system.lambda_dqdq
    system.ddqd                        system.lambda_du
    system.ddqk                        system.lambda_duddq
    system.dq                          system.lambda_dudq
    system.dqd                         system.lambda_dudu
    system.dqk                         system.masses
    system.dyn_configs                 system.nQ
    system.export_frames               system.nQd
    system.f                           system.nQk
    system.f_dddk                      system.nc
    system.f_dddkdq                    system.nu
    system.f_ddq                       system.potentials
    system.f_ddqddq                    system.q
    system.f_ddqdq                     system.qd
    system.f_dq                        system.qk
    system.f_dqdq                      system.resume_structure_changes
    system.f_du                        system.satisfy_constraints
    system.f_duddq                     system.set_state
    system.f_dudq                      system.t
    system.f_dudu                      system.test_derivative_ddq
    system.forces                      system.test_derivative_dq
    system.frames                      system.total_energy
    system.get_config                  system.u
    system.get_constraint              system.world_frame
    system.get_force                   

**Some examples:**

The current system time is
    >>> system.t
    0.0
The current inputs are (the system has no inputs)
    >>> system.u
    array([], dtype=float64)
The current generalized coordinates are (the system has no configuration)
    >>> system.q
    array([], dtype=float64) 

All of the properties and methods for the system object are documented in the
:ref:`System <trep_system>` documentation. We will explore more of these properties and
methods later in the tutorial.

Visualize the system with trep's visualization tools
----------------------------------------------------
Let us visualize the system.

.. literalinclude:: ./code_snippets/trepHelloWorld.py
    :start-after: Visualize the system with trep's visualization tools

Below is what you should see.

.. image:: trepHelloWorld.png

This brings up the trep visualization of the mechanical system. Since this
system has no components nothing is shown. The ``VisualItem3D`` method takes in
three arguments: system object, time vector, and configuration vector. In this
example the time vector and configuration vector are set to empty vectors. If
these vectors were not empty the visualization would play a movie of the system
going through the configurations at the given times.

trepHelloWorld.py code
----------------------
Below is the entire script used in this section of the tutorial.

.. literalinclude:: ./code_snippets/trepHelloWorld.py
    :linenos: