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import sklearn
from sklearn.preprocessing import LabelEncoder
import pandas as pd
import numpy as np
data=pd.read_excel('CART.xlsx')
le_age=LabelEncoder()
le_income=LabelEncoder()
le_student=LabelEncoder()
le_credit_rating=LabelEncoder()
le_buys_computer=LabelEncoder()
data['age_n']=le_age.fit_transform(data['age'])
data['income_n']=le_income.fit_transform(data['income'])
data['student_n']=le_student.fit_transform(data['student'])
data['credit_rating_n']=le_credit_rating.fit_transform(data['credit_rating'])
data['buys_computer_n']=le_credit_rating.fit_transform(data['buys_computer'])

from sklearn.tree import DecisionTreeClassifier
clf=DecisionTreeClassifier()
dt=clf.fit(x,y)
dt
DecisionTreeClassifier(class_weight=None,criterion='gini',max_depth=None,max_features=None,max_leaf_nodes=None,min_impurity_decrease=0.0,
                      min_impurity_split=None,min_samples_leaf=1,min_samples_split=2,min_weight_fraction_leaf=0.0,presort=False,random_state=None,splitter='best')

from sklearn.tree import export_graphviz
from sklearn.externals.six import StringIO
from IPython.display import Image
import pydotplus

dot_data=StringIO()
export_graphviz(dt,out_file=dot_data,filled=True,rounded=True,special_characters=True,feature_names=feature_cols,
                class_names=['0','1'])
graph=pydotplus.graph_from_dot_data(dot_data.getvalue())
graph.write_png('buys_computer.png')

Error:
---------------------------------------------------------------------------
InvocationException                       Traceback (most recent call last)
<ipython-input-17-5ddeb7a2766f> in <module>
      3                 class_names=['0','1'])
      4 graph=pydotplus.graph_from_dot_data(dot_data.getvalue())
----> 5 graph.write_png('buys_computer.png')

C:\ProgramData\Anaconda3\lib\site-packages\pydotplus\graphviz.py in <lambda>(path, f, prog)
   1808                 lambda path,
   1809                 f=frmt,
-> 1810                 prog=self.prog: self.write(path, format=f, prog=prog)
   1811             )
   1812 

C:\ProgramData\Anaconda3\lib\site-packages\pydotplus\graphviz.py in write(self, path, prog, format)
   1916 
   1917             else:
-> 1918                 fobj.write(self.create(prog, format))
   1919         finally:
   1920             if close:

C:\ProgramData\Anaconda3\lib\site-packages\pydotplus\graphviz.py in create(self, prog, format)
   1958             if self.progs is None:
   1959                 raise InvocationException(
-> 1960                     'GraphViz\'s executables not found')
   1961 
   1962         if prog not in self.progs:

InvocationException: GraphViz's executables not found