Source code for qiskit_optimization.applications.vertex_cover

# This code is part of a Qiskit project.
#
# (C) Copyright IBM 2018, 2025.
#
# This code is licensed under the Apache License, Version 2.0. You may
# obtain a copy of this license in the LICENSE.txt file in the root directory
# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
#
# Any modifications or derivative works of this code must retain this
# copyright notice, and modified files need to carry a notice indicating
# that they have been altered from the originals.

"""An application class for the vertex cover."""
from __future__ import annotations


import networkx as nx
import numpy as np
from docplex.mp.model import Model

from qiskit_optimization.algorithms import OptimizationResult
from qiskit_optimization.problems.quadratic_program import QuadraticProgram
from qiskit_optimization.translators import from_docplex_mp
from .graph_optimization_application import GraphOptimizationApplication




class VertexCover(GraphOptimizationApplication):
    """Optimization application for the "vertex cover" [1] problem based on a NetworkX graph.

    References:
        [1]: "Vertex cover", https://en.wikipedia.org/wiki/Vertex_cover
    """



    def to_quadratic_program(self) -> QuadraticProgram:
        """Convert a vertex cover instance into a
        :class:`~qiskit_optimization.problems.QuadraticProgram`

        Returns:
            The :class:`~qiskit_optimization.problems.QuadraticProgram` created
            from the vertex cover instance.
        """
        mdl = Model(name="Vertex cover")
        n = self._graph.number_of_nodes()
        x = {i: mdl.binary_var(name=f"x_{i}") for i in range(n)}
        objective = mdl.sum(x[i] for i in x)
        for w, v in self._graph.edges:
            mdl.add_constraint(x[w] + x[v] >= 1)
        mdl.minimize(objective)
        op = from_docplex_mp(mdl)
        return op




    def interpret(self, result: OptimizationResult | np.ndarray) -> list[int]:
        """Interpret a result as a list of node indices

        Args:
            result : The calculated result of the problem

        Returns:
            A list of node indices whose corresponding variable is 1
        """
        x = self._result_to_x(result)
        vertex_cover = []
        for i, value in enumerate(x):
            if value:
                vertex_cover.append(i)
        return vertex_cover


    def _draw_result(
        self,
        result: OptimizationResult | np.ndarray,
        pos: dict[int, np.ndarray] | None = None,
    ) -> None:
        """Draw the result with colors

        Args:
            result : The calculated result for the problem
            pos: The positions of nodes
        """
        x = self._result_to_x(result)
        nx.draw(self._graph, node_color=self._node_colors(x), pos=pos, with_labels=True)

    def _node_colors(self, x: np.ndarray) -> list[str]:
        # Return a list of strings for draw.
        # Color a node with red when the corresponding variable is 1.
        # Otherwise color it with dark gray.
        return ["r" if x[node] else "darkgrey" for node in self._graph.nodes]