Python/other/markov_chain.py
Christian Clauss 9200a2e543
from __future__ import annotations (#2464)
* from __future__ import annotations

* fixup! from __future__ import annotations

* fixup! from __future__ import annotations

* fixup! Format Python code with psf/black push

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
2020-09-23 13:30:13 +02:00

84 lines
2.0 KiB
Python

from __future__ import annotations
from collections import Counter
from random import random
class MarkovChainGraphUndirectedUnweighted:
"""
Undirected Unweighted Graph for running Markov Chain Algorithm
"""
def __init__(self):
self.connections = {}
def add_node(self, node: str) -> None:
self.connections[node] = {}
def add_transition_probability(
self, node1: str, node2: str, probability: float
) -> None:
if node1 not in self.connections:
self.add_node(node1)
if node2 not in self.connections:
self.add_node(node2)
self.connections[node1][node2] = probability
def get_nodes(self) -> list[str]:
return list(self.connections)
def transition(self, node: str) -> str:
current_probability = 0
random_value = random()
for dest in self.connections[node]:
current_probability += self.connections[node][dest]
if current_probability > random_value:
return dest
def get_transitions(
start: str, transitions: list[tuple[str, str, float]], steps: int
) -> dict[str, int]:
"""
Running Markov Chain algorithm and calculating the number of times each node is
visited
>>> transitions = [
... ('a', 'a', 0.9),
... ('a', 'b', 0.075),
... ('a', 'c', 0.025),
... ('b', 'a', 0.15),
... ('b', 'b', 0.8),
... ('b', 'c', 0.05),
... ('c', 'a', 0.25),
... ('c', 'b', 0.25),
... ('c', 'c', 0.5)
... ]
>>> result = get_transitions('a', transitions, 5000)
>>> result['a'] > result['b'] > result['c']
True
"""
graph = MarkovChainGraphUndirectedUnweighted()
for node1, node2, probability in transitions:
graph.add_transition_probability(node1, node2, probability)
visited = Counter(graph.get_nodes())
node = start
for _ in range(steps):
node = graph.transition(node)
visited[node] += 1
return visited
if __name__ == "__main__":
import doctest
doctest.testmod()