Bayesian models：naive bayes and bayesian networks.

Author: luwill

charpter21_Bayesian_models/bayesian_network.ipynb

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+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "### bayesian network"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 1,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "# 导入pgmpy相关模块\n",
+    "from pgmpy.factors.discrete import TabularCPD\n",
+    "from pgmpy.models import BayesianModel\n",
+    "letter_model = BayesianModel([('D', 'G'),\n",
+    "                               ('I', 'G'),\n",
+    "                               ('G', 'L'),\n",
+    "                               ('I', 'S')])"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 4,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "# 学生成绩的条件概率分布\n",
+    "grade_cpd = TabularCPD(\n",
+    "    variable='G', # 节点名称\n",
+    "    variable_card=3, # 节点取值个数\n",
+    "    values=[[0.3, 0.05, 0.9, 0.5], # 该节点的概率表\n",
+    "    [0.4, 0.25, 0.08, 0.3],\n",
+    "    [0.3, 0.7, 0.02, 0.2]],\n",
+    "    evidence=['I', 'D'], # 该节点的依赖节点\n",
+    "    evidence_card=[2, 2] # 依赖节点的取值个数\n",
+    ")\n",
+    "# 考试难度的条件概率分布\n",
+    "difficulty_cpd = TabularCPD(\n",
+    "            variable='D',\n",
+    "            variable_card=2,\n",
+    "            values=[[0.6], [0.4]]\n",
+    ")\n",
+    "# 个人天赋的条件概率分布\n",
+    "intel_cpd = TabularCPD(\n",
+    "            variable='I',\n",
+    "            variable_card=2,\n",
+    "            values=[[0.7], [0.3]]\n",
+    ")\n",
+    "# 推荐信质量的条件概率分布\n",
+    "letter_cpd = TabularCPD(\n",
+    "            variable='L',\n",
+    "            variable_card=2,\n",
+    "            values=[[0.1, 0.4, 0.99],\n",
+    "            [0.9, 0.6, 0.01]],\n",
+    "            evidence=['G'],\n",
+    "            evidence_card=[3]\n",
+    ")\n",
+    "# SAT考试分数的条件概率分布\n",
+    "sat_cpd = TabularCPD(\n",
+    "            variable='S',\n",
+    "            variable_card=2,\n",
+    "            values=[[0.95, 0.2],\n",
+    "            [0.05, 0.8]],\n",
+    "            evidence=['I'],\n",
+    "            evidence_card=[2]\n",
+    ")"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 7,
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stderr",
+     "output_type": "stream",
+     "text": [
+      "WARNING:root:Replacing existing CPD for G\n",
+      "WARNING:root:Replacing existing CPD for D\n",
+      "WARNING:root:Replacing existing CPD for I\n",
+      "WARNING:root:Replacing existing CPD for L\n",
+      "WARNING:root:Replacing existing CPD for S\n",
+      "Finding Elimination Order: : 100%|██████████████████████████████████████████████████████| 2/2 [00:00<00:00, 668.95it/s]\n",
+      "Eliminating: L: 100%|███████████████████████████████████████████████████████████████████| 2/2 [00:00<00:00, 285.72it/s]"
+     ]
+    },
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "+------+----------+\n",
+      "| G    |   phi(G) |\n",
+      "+======+==========+\n",
+      "| G(0) |   0.9000 |\n",
+      "+------+----------+\n",
+      "| G(1) |   0.0800 |\n",
+      "+------+----------+\n",
+      "| G(2) |   0.0200 |\n",
+      "+------+----------+\n"
+     ]
+    },
+    {
+     "name": "stderr",
+     "output_type": "stream",
+     "text": [
+      "\n"
+     ]
+    }
+   ],
+   "source": [
+    "# 将各节点添加到模型中，构建贝叶斯网络\n",
+    "letter_model.add_cpds(\n",
+    "    grade_cpd, \n",
+    "    difficulty_cpd,\n",
+    "    intel_cpd,\n",
+    "    letter_cpd,\n",
+    "    sat_cpd\n",
+    ")\n",
+    "# 导入pgmpy贝叶斯推断模块\n",
+    "from pgmpy.inference import VariableElimination\n",
+    "# 贝叶斯网络推断\n",
+    "letter_infer = VariableElimination(letter_model)\n",
+    "# 天赋较好且考试不难的情况下推断该学生获得推荐信质量的好坏\n",
+    "prob_G = letter_infer.query(\n",
+    "            variables=['G'],\n",
+    "            evidence={'I': 1, 'D': 0})\n",
+    "print(prob_G)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": []
+  }
+ ],
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