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   "cell_type": "markdown",
   "id": "cell-001",
   "metadata": {},
   "source": [
    "# Module 5 — Lab 3: Building a Decision Tree Baseline\n",
    "\n",
    " \n",
    "**Topics:** stratified split, dummy baseline, bounded tree, leakage-safe pipeline\n",
    "\n",
    "> **Solution version:** Run the cells in order, compare the outputs with your work, and read the suggested answers."
   ]
  },
  {
   "cell_type": "markdown",
   "id": "cell-002",
   "metadata": {},
   "source": [
    "## Learning objectives\n",
    "\n",
    "After completing this lab, you will be able to:\n",
    "\n",
    "- create a reproducible stratified train-test split;\n",
    "- fit missing-value imputation on training data only;\n",
    "- compare a bounded Decision Tree with a dummy baseline;\n",
    "- evaluate malware precision, recall, F1, false positives, and false negatives;\n",
    "- explain why trees do not require feature scaling."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "id": "cell-003",
   "metadata": {},
   "outputs": [],
   "source": [
    "from pathlib import Path\n",
    "import numpy as np\n",
    "import pandas as pd\n",
    "import matplotlib.pyplot as plt\n",
    "\n",
    "DATA_PATH = Path(\"data/malware_features.csv\")\n",
    "\n",
    "if not DATA_PATH.exists():\n",
    "    raise FileNotFoundError(\n",
    "        \"Missing data/malware_features.csv. Follow data/README.md, place the three \"\n",
    "        \"official UCI CSV files under data/raw/, and run: python data/prepare_dataset.py\"\n",
    "    )\n",
    "\n",
    "\n",
    "from sklearn.dummy import DummyClassifier\n",
    "from sklearn.impute import SimpleImputer\n",
    "from sklearn.metrics import confusion_matrix, precision_score, recall_score, f1_score, accuracy_score\n",
    "from sklearn.model_selection import train_test_split\n",
    "from sklearn.pipeline import Pipeline\n",
    "from sklearn.tree import DecisionTreeClassifier\n",
    "\n",
    "\n",
    "def security_metrics(y_true, y_pred):\n",
    "    tn, fp, fn, tp = confusion_matrix(y_true, y_pred, labels=[0, 1]).ravel()\n",
    "    return {\n",
    "        \"accuracy\": accuracy_score(y_true, y_pred),\n",
    "        \"malware_precision\": precision_score(y_true, y_pred, zero_division=0),\n",
    "        \"malware_recall\": recall_score(y_true, y_pred, zero_division=0),\n",
    "        \"malware_f1\": f1_score(y_true, y_pred, zero_division=0),\n",
    "        \"false_positives\": int(fp),\n",
    "        \"false_negatives\": int(fn),\n",
    "        \"false_positive_rate\": fp / (fp + tn) if fp + tn else 0.0,\n",
    "    }"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "id": "cell-004",
   "metadata": {},
   "outputs": [],
   "source": [
    "df = pd.read_csv(DATA_PATH, low_memory=False)\n",
    "feature_cols = [column for column in df.columns if column not in {\"source\", \"label\"}]\n",
    "X = df[feature_cols].apply(pd.to_numeric, errors=\"coerce\")\n",
    "y = df[\"label\"].astype(int)\n",
    "\n",
    "X_train, X_test, y_train, y_test = train_test_split(\n",
    "    X, y, test_size=0.20, stratify=y, random_state=42\n",
    ")\n",
    "print(\"Train shape:\", X_train.shape, \"Test shape:\", X_test.shape)\n",
    "print(\"Malware rate — train/test:\", y_train.mean().round(4), y_test.mean().round(4))"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "cell-005",
   "metadata": {},
   "source": [
    "## 1. Establish a non-learning baseline\n",
    "\n",
    "Because malware is the majority class, a most-frequent classifier can achieve high accuracy while providing little operational value."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "id": "cell-006",
   "metadata": {},
   "outputs": [],
   "source": [
    "dummy = DummyClassifier(strategy=\"most_frequent\")\n",
    "dummy.fit(X_train, y_train)\n",
    "dummy_predictions = dummy.predict(X_test)\n",
    "dummy_metrics = security_metrics(y_test, dummy_predictions)\n",
    "pd.Series(dummy_metrics, name=\"dummy\")"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "cell-007",
   "metadata": {},
   "source": [
    "## 2. Train a bounded Decision Tree\n",
    "\n",
    "The imputer is inside the pipeline, so its medians are learned from training rows only. A shallow tree is intentionally chosen for readability and to reduce overfitting."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "id": "cell-008",
   "metadata": {},
   "outputs": [],
   "source": [
    "tree_pipeline = Pipeline([\n",
    "    (\"imputer\", SimpleImputer(strategy=\"median\", add_indicator=True)),\n",
    "    (\"tree\", DecisionTreeClassifier(\n",
    "        max_depth=4,\n",
    "        min_samples_leaf=10,\n",
    "        random_state=42,\n",
    "    )),\n",
    "])\n",
    "tree_pipeline.fit(X_train, y_train)\n",
    "tree_predictions = tree_pipeline.predict(X_test)\n",
    "tree_metrics = security_metrics(y_test, tree_predictions)\n",
    "\n",
    "comparison = pd.DataFrame([dummy_metrics, tree_metrics], index=[\"Dummy\", \"Decision Tree\"])\n",
    "display(comparison.round(4))\n",
    "display(pd.DataFrame(\n",
    "    confusion_matrix(y_test, tree_predictions, labels=[0, 1]),\n",
    "    index=[\"Actual benign\", \"Actual malware\"],\n",
    "    columns=[\"Predicted benign\", \"Predicted malware\"],\n",
    "))"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "cell-009",
   "metadata": {},
   "source": [
    "## Self-check"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "id": "cell-010",
   "metadata": {},
   "outputs": [],
   "source": [
    "assert X_train.shape[0] + X_test.shape[0] == len(df)\n",
    "assert abs(y_train.mean() - y_test.mean()) < 0.01\n",
    "assert set(tree_predictions).issubset({0, 1})\n",
    "assert tree_pipeline.named_steps[\"tree\"].get_depth() <= 4\n",
    "assert set(comparison.index) == {\"Dummy\", \"Decision Tree\"}\n",
    "assert comparison.loc[\"Decision Tree\", \"malware_f1\"] >= comparison.loc[\"Dummy\", \"malware_f1\"]\n",
    "print(\"✓ Lab 3 completed\")"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "cell-011",
   "metadata": {},
   "source": [
    "## Suggested answers\n",
    "\n",
    "1. **Why can dummy accuracy look high?** Malware is the majority class, so always predicting malware is often correct but creates many false positives.\n",
    "2. **Why no scaler?** Decision Trees compare one feature with a threshold; multiplying a feature by a constant changes the threshold but not the ordering of samples.\n",
    "3. **Why bound the depth?** A shallow tree is easier to audit and is less likely to memorize small training regions."
   ]
  }
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