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doc/examples/triangle.rst

author | Dmitriy Morozov <dmitriy@mrzv.org> |

Thu, 14 May 2009 17:43:19 -0700 | |

branch | dev |

changeset 140 | 9851fee5a33b |

parent 134 | c270826fd4a8 |

child 181 | 1ee6edc17cb6 |

permissions | -rw-r--r-- |

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.. _triangle-example: Triangle example ================ Simple example of a filtered triangle is given in :sfile:`examples/triangle/triangle.cpp`. Its equivalent in Python appears in :sfile:`examples/triangle/triangle.py`, and we describe it next. .. literalinclude:: ../../examples/triangle/triangle.py :language: python After the necessary imports, the ``complex`` is setup explicitly as a list of simplices. Each :class:`Simplex` constructor takes an iterable sequence of vertices, and optionally a data value. The complex must be sorted lexicographically to compute persistence using :class:`StaticPersistence`, and it is accomplished via a helper comparison function :func:`vertex_cmp`, which compares simplices with respect to the lexicographic ordering:: complex.sort(vertex_cmp) A filtration ``f`` is initialized using the :class:`Filtration` class, which takes a list of lexicographically sorted simplices and a comparison that defines in what order the simplices should come in the filtration. In this case we use :func:`data_cmp`, which simply compares simplices' :attr:`~Simplex.data` attributes. :class:`StaticPersistence` is initialized with the filtration, and its method :meth:`~StaticPersistence.pair_simplices` pairs the simplices of the filtration:: p = StaticPersistence(f) p.pair_simplices() Subsequently, we iterate over ``p`` to access a representation of each simplex in the filtration order. We output each simplex, its sign, and its pair. Note ``complex[f[p(i)]]``: ``p(i)`` gives the integer index of the iterator ``i`` in the filtration ``f``; in turn ``f[p(i)]`` gives the index of the simplex in the (lexicographically ordered) ``complex``. So the entire expression returns individual simplices. Naturally, one could use this to access the :attr:`~Simplex.data` attribute of the simplices to output the actual persistence diagram, as is done in the :ref:`alpha-shape-example` and the :ref:`rips-example`.