sklearn DecisionTree 原始碼分析

sklearn.tree._classes.basedecisiontree#fit

y至少為1維（意思是可以處理multilabels資料）

y = np.atleast_1d(y)

if is_classifier(self)
: self.tree_ = tree(self.n_features_,
self.n_classes_, self.n_outputs_)
else
: self.tree_ = tree(self.n_features_,
# todo: tree should't need this in this case
np.array([1
]* self.n_outputs_, dtype=np.intp)
, self.n_outputs_)

self.n_outputs_ = y.shape[
1]

self.n_classes_ = self.n_classes_[
0]

self.n_classes_ =
for k in
range
(self.n_outputs_)
: classes_k, y_encoded[
:, k]
= np.unique(y[
:, k]
, return_inverse=
true)0
])

np.unique([3
,2,2
,3,3
,4], return_inverse=
true
)out[4]
:(array([2
,3,4
]), array([1
,0,0
,1,1
,2])
)

return_inverse類似於labelencode

sklearn.tree._tree.tree

def
__cinit__
(self,
int n_features, np.ndarray[size_t, ndim=
1] n_classes,
int n_outputs)
:

特徵數

類別數label維度

# use bestfirst if max_leaf_nodes given; use depthfirst otherwise if max_leaf_nodes <0: builder = depthfirsttreebuilder(splitter, min_samples_split, min_samples_leaf, min_weight_leaf, max_depth, self.min_impurity_decrease, min_impurity_split) else : builder = bestfirsttreebuilder(splitter, min_samples_split, min_samples_leaf, min_weight_leaf, max_depth, max_leaf_nodes, self.min_impurity_decrease, min_impurity_split)

scikit-learn決策樹演算法類庫介紹

最大葉子節點數max_leaf_nodes

通過限制最大葉子節點數，可以防止過擬合，預設是"none」，即不限制最大的葉子節點數。如果加了限制，演算法會建立在最大葉子節點數內最優的決策樹。如果特徵不多，可以不考慮這個值，但是如果特徵分成多的話，可以加以限制，具體的值可以通過交叉驗證得到。

sklearn.tree._tree.depthfirsttreebuilder#build

builder.build(self.tree_, x, y, sample_weight, x_idx_sorted)

cpdef build(self, tree tree,
object x, np.ndarray y,
np.ndarray sample_weight=
none
, np.ndarray x_idx_sorted=
none
):

注意到乙個現象，這裡該有的引數都有，但是class_weight去哪了呢？懷疑是轉化了sample_weight了

if self.class_weight is
notnone
: expanded_class_weight = compute_sample_weight(
self.class_weight, y_original)

if expanded_class_weight is notnone :if sample_weight is notnone : sample_weight = sample_weight * expanded_class_weight else : sample_weight = expanded_class_weight

sklearn/tree/_tree.pyx:203

splitter.init(x, y, sample_weight_ptr, x_idx_sorted)

cdef size_t n_node_samples = splitter.n_samples

rc = stack.push(
0, n_node_samples,
0, _tree_undefined,
0, infinity,
0)

rc是根節點，在**前含有所有的樣本

stack和stackrecord都是sklearn自己寫的資料結構

is_leaf =
(depth >= max_depth or
n_node_samples < min_samples_split or
n_node_samples <
2* min_samples_leaf or
weighted_n_node_samples <
2* min_weight_leaf)

is_leaf =
(is_leaf or
(impurity <= min_impurity_split)
)

滿足以上條件直接停止**

sklearn.tree._splitter.bestsplitter

sklearn.tree._splitter.bestsplitter#node_split

scikit-learn uses an optimised version of the cart algorithm; however, scikit-learn implementation does not support categorical variables for now.

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