***********************
Modifying loaded graphs
***********************

This guide shows how to add and remove vertices and edges from already loaded graphs. This can be useful for "what-if" analyses.

Recap: Loading a graph
----------------------

At first we need to load a graph.

.. code-block:: python
    :linenos:

    session = pypgx.get_session()
    graph = session.read_graph_with_properties(self.graph_path)

This will load the following sample graph:

.. image:: /_static/images/pgx-sample-graph.jpg
	:alt: pgx sample graph

Adding, removing and modifying vertices
---------------------------------------

Now that we have a graph loaded we can start modifying it.
First we need a :class:`GraphChangeSet` - an object similar to the :class:`GraphBuilder`.
It provides methods for modifying all aspects of a graph.

We are going to add a new vertex ``42`` with the property value ``23`` for ``prop1``.
Additionally we are going to set the value of ``prop1`` for vertex ``128`` to ``5``.
Finally we will remove the vertex with ID ``1908``.

.. code-block:: python
    :linenos:

    change_set = graph.create_change_set()
    # Add, remove and modify vertices
    change_set.add_vertex(42).set_property("prop", 23)
    change_set.update_vertex(128).set_property("prop", 5)
    change_set.remove_vertex(1908)
    updated_graph = change_set.build()
    # Check if the new changes are applied to updated_graph
    self.assertTrue(updated_graph.has_vertex(42))
    self.assertFalse(updated_graph.has_vertex(1908))


Notice that vertex ``1908`` has one edge attached in the original graph. This edge will be deleted automatically with the deletion of the vertex.

The resulting graph will have the following form:

.. image:: /_static/images/pgx-sample-graph-updated.png
	:alt: pgx sample graph updated

Adding edges
------------

The same way edges can be added to the graph. For example we can add a new triangle to the graph:

.. code-block:: python
    :linenos:

    change_set_2 = graph.create_change_set()
    change_set_2.add_edge(128, 99).set_property("cost", 42.42)
    change_set_2.add_edge(333, 1908)
    updated_graph_2 = change_set_2.build()

Note that we do not provide a value for the property `cost` for the second edge we add. In this case the property will be assigned a default value of ``0.0``.

This is the resulting graph:

.. image:: /_static/images/pgx-sample-graph-updated-2.png
	:alt: pgx sample graph updated 2

Note that by calling :meth:`change_set_2.build()` we created a brand new graph with a unique name assigned by PGX;
if need be, we can specify a name argument to the :meth:`build()` method.