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Given the following text description, write Python code to implement the functionality described below step by step
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Description:
Step1:  Plot learning curves of different classifiers
This example is a small modification of the sciki-learn tutorial test.
Comparison of different linear SVM classifiers on a 2D projection of the iris
dataset. Here I consider only two features of the dataset
Step2:  Pandas
As in previous examples, we use pandas to read a database.
Step3:  Testing sklearn classifiers.
Here I test different classifiers provided by sklearn into my specific test set.
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Python Code:
print(__doc__)
import numpy as np
import matplotlib.pyplot as plt
from sklearn.naive_bayes import GaussianNB
from sklearn.svm import SVC
from sklearn.datasets import load_digits
from sklearn.model_selection import learning_curve
from sklearn.model_selection import ShuffleSplit
def plot_learning_curve(estimator, title, X, y, ylim=None, cv=None,
                        n_jobs=1, train_sizes=np.linspace(.1, 1.0, 5)):
    
    Generate a simple plot of the test and training learning curve.
    Parameters
    ----------
    estimator : object type that implements the "fit" and "predict" methods
        An object of that type which is cloned for each validation.
    title : string
        Title for the chart.
    X : array-like, shape (n_samples, n_features)
        Training vector, where n_samples is the number of samples and
        n_features is the number of features.
    y : array-like, shape (n_samples) or (n_samples, n_features), optional
        Target relative to X for classification or regression;
        None for unsupervised learning.
    ylim : tuple, shape (ymin, ymax), optional
        Defines minimum and maximum yvalues plotted.
    cv : int, cross-validation generator or an iterable, optional
        Determines the cross-validation splitting strategy.
        Possible inputs for cv are:
          - None, to use the default 3-fold cross-validation,
          - integer, to specify the number of folds.
          - An object to be used as a cross-validation generator.
          - An iterable yielding train/test splits.
        For integer/None inputs, if ``y`` is binary or multiclass,
        :class:`StratifiedKFold` used. If the estimator is not a classifier
        or if ``y`` is neither binary nor multiclass, :class:`KFold` is used.
        Refer :ref:`User Guide <cross_validation>` for the various
        cross-validators that can be used here.
    n_jobs : integer, optional
        Number of jobs to run in parallel (default 1).
    
    plt.figure()
    plt.title(title)
    if ylim is not None:
        plt.ylim(*ylim)
    plt.xlabel("Training examples")
    plt.ylabel("Score")
    train_sizes, train_scores, test_scores = learning_curve(
        estimator, X, y, cv=cv, n_jobs=n_jobs, train_sizes=train_sizes)
    train_scores_mean = np.mean(train_scores, axis=1)
    train_scores_std = np.std(train_scores, axis=1)
    test_scores_mean = np.mean(test_scores, axis=1)
    test_scores_std = np.std(test_scores, axis=1)
    plt.grid()
    plt.fill_between(train_sizes, train_scores_mean - train_scores_std,
                     train_scores_mean + train_scores_std, alpha=0.1,
                     color="r")
    plt.fill_between(train_sizes, test_scores_mean - test_scores_std,
                     test_scores_mean + test_scores_std, alpha=0.1, color="g")
    plt.plot(train_sizes, train_scores_mean, 'o-', color="r",
             label="Training score")
    plt.plot(train_sizes, test_scores_mean, 'o-', color="g",
             label="Cross-validation score")
    plt.legend(loc="best")
    return plt
Explanation: Plot learning curves of different classifiers
This example is a small modification of the sciki-learn tutorial test.
Comparison of different linear SVM classifiers on a 2D projection of the iris
dataset. Here I consider only two features of the dataset:
Seed area
Seed asymmetry
Test linear and other models, and use multiple features of the seeds dataset.
End of explanation
print(__doc__)
import numpy as np
import matplotlib.pyplot as plt
from sklearn import svm, datasets
import pandas as pd
#I use this dataset because this has clearly separated cathegories, 
#Read the database using pandas, 
#Note that bad lines are omitted with error_bad_lines=False
df = pd.read_csv('https://archive.ics.uci.edu/ml/'
        'machine-learning-databases/00236/seeds_dataset.txt', header=None, sep="\t", error_bad_lines=False)
#The headers are not given in the dataset, so we give them afterwords:
#1. area A, 
#2. perimeter P, 
#3. compactness C = 4*pi*A/P^2, 
#4. length of kernel, 
#5. width of kernel, 
#6. asymmetry coefficient 
#7. length of kernel groove. 
#8. Class: 1=Kama, 2=Rosa, 3=Canadian
df.columns = ["area","perimeter","compactness","kernel-length","kernel-width",
              "asymmetry","kernel-groove-length","class"]
#This shows the header of the database:
df.head()
Explanation: Pandas
As in previous examples, we use pandas to read a database.
End of explanation
#In the database there are 3 classes of seeds:
#And skilearn can handle multiple classes
import numpy as np
#This sets class=2 to 0 and 3 to 1:
y = df.loc[:,'class']
#Extract all cathegories:
X=df.iloc[:,0:7]
#This is to convert the csv dictionary into a numpy matrix to later standarize:
X=X.as_matrix()
nfeature=X.shape[1]
# standardize features
X_std = np.copy(X)
for ifeat in range(0,nfeature):
    X_std[:,ifeat] = (X[:,ifeat] - X[:,ifeat].mean()) / X[:,ifeat].std()
#Here since we have many features, we just plot the learning curves for the training and cross-validation sets.
title = "Learning Curves (Naive Bayes)"
# Cross validation with 100 iterations to get smoother mean test and train
# score curves, each time with 20% data randomly selected as a validation set.
cv = ShuffleSplit(n_splits=100, test_size=0.2, random_state=0)
estimator = GaussianNB()
plot_learning_curve(estimator, title, X, y, ylim=(0.7, 1.01), cv=cv, n_jobs=4)
title = "Learning Curves (SVC, Poly kernel, $\gamma=0.001$)"
cv = ShuffleSplit(n_splits=100, test_size=0.2, random_state=0)
estimator = SVC(kernel='poly',gamma=0.001)
plot_learning_curve(estimator, title, X, y, (0.7, 1.01), cv=cv, n_jobs=4)
title = "Learning Curves (SVC, RBF kernel, $\gamma=0.001$)"
cv = ShuffleSplit(n_splits=100, test_size=0.2, random_state=0)
estimator = SVC(kernel='rbf',gamma=0.001)
plot_learning_curve(estimator, title, X, y, (0.7, 1.01), cv=cv, n_jobs=4)
title = "Learning Curves (Linear SVC)"
cv = ShuffleSplit(n_splits=100, test_size=0.2, random_state=0)
estimator = svm.LinearSVC(C=1.0)
plot_learning_curve(estimator, title, X, y, (0.7, 1.01), cv=cv, n_jobs=4)
plt.show()
Explanation: Testing sklearn classifiers.
Here I test different classifiers provided by sklearn into my specific test set.
End of explanation
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