How to use cross_val_score with random_state

问题:

I get different values for different runs. What am I doing wrong here?

X=np.random.random((100,5))
y=np.random.randint(0,2,(100,))
clf=RandomForestClassifier()
cv = StratifiedKFold(y, random_state=1)
s = cross_val_score(clf, X,y,scoring='roc_auc', cv=cv)
print(s)
# [ 0.42321429  0.44360902  0.34398496]
s = cross_val_score(clf, X,y,scoring='roc_auc', cv=cv)
print(s)
# [ 0.42678571  0.46804511  0.36090226]

回答1:

The mistake you are making is calling the RandomForestClassifier whose default arg, random_state is None. So, it picks up the seed generated by np.random to produce the random output.

The random_state in both StratifiedKFold and RandomForestClassifier need to be the same inorder to produce equal arrays of scores of cross validation.

Illustration:

X=np.random.random((100,5))
y=np.random.randint(0,2,(100,))

clf = RandomForestClassifier(random_state=1)
cv = StratifiedKFold(y, random_state=1)        # Setting random_state is not necessary here
s = cross_val_score(clf, X,y,scoring='roc_auc', cv=cv)
print(s)
##[ 0.57612457  0.29044118  0.30514706]
print(s)
##[ 0.57612457  0.29044118  0.30514706]

Another way of countering it would be to not provide random_state args for both RFC and SKF. But, simply providing the np.random.seed(value) to create the random integers at the beginning. These would also create equal arrays at the output.