.. _sphx_glr_auto_examples_model_selection_plot_confusion_matrix.py:


================
Confusion matrix
================

Example of confusion matrix usage to evaluate the quality
of the output of a classifier on the iris data set. The
diagonal elements represent the number of points for which
the predicted label is equal to the true label, while
off-diagonal elements are those that are mislabeled by the
classifier. The higher the diagonal values of the confusion
matrix the better, indicating many correct predictions.

The figures show the confusion matrix with and without
normalization by class support size (number of elements
in each class). This kind of normalization can be
interesting in case of class imbalance to have a more
visual interpretation of which class is being misclassified.

Here the results are not as good as they could be as our
choice for the regularization parameter C was not the best.
In real life applications this parameter is usually chosen
using :ref:`grid_search`.




.. rst-class:: sphx-glr-horizontal


    *

      .. image:: /auto_examples/model_selection/images/sphx_glr_plot_confusion_matrix_001.png
            :scale: 47

    *

      .. image:: /auto_examples/model_selection/images/sphx_glr_plot_confusion_matrix_002.png
            :scale: 47


.. rst-class:: sphx-glr-script-out

 Out::

      Confusion matrix, without normalization
    [[13  0  0]
     [ 0 10  6]
     [ 0  0  9]]
    Normalized confusion matrix
    [[ 1.    0.    0.  ]
     [ 0.    0.62  0.38]
     [ 0.    0.    1.  ]]




|


.. code-block:: python


    print(__doc__)

    import itertools
    import numpy as np
    import matplotlib.pyplot as plt

    from sklearn import svm, datasets
    from sklearn.model_selection import train_test_split
    from sklearn.metrics import confusion_matrix

    # import some data to play with
    iris = datasets.load_iris()
    X = iris.data
    y = iris.target
    class_names = iris.target_names

    # Split the data into a training set and a test set
    X_train, X_test, y_train, y_test = train_test_split(X, y, random_state=0)

    # Run classifier, using a model that is too regularized (C too low) to see
    # the impact on the results
    classifier = svm.SVC(kernel='linear', C=0.01)
    y_pred = classifier.fit(X_train, y_train).predict(X_test)


    def plot_confusion_matrix(cm, classes,
                              normalize=False,
                              title='Confusion matrix',
                              cmap=plt.cm.Blues):
        """
        This function prints and plots the confusion matrix.
        Normalization can be applied by setting `normalize=True`.
        """
        plt.imshow(cm, interpolation='nearest', cmap=cmap)
        plt.title(title)
        plt.colorbar()
        tick_marks = np.arange(len(classes))
        plt.xticks(tick_marks, classes, rotation=45)
        plt.yticks(tick_marks, classes)

        if normalize:
            cm = cm.astype('float') / cm.sum(axis=1)[:, np.newaxis]
            print("Normalized confusion matrix")
        else:
            print('Confusion matrix, without normalization')

        print(cm)

        thresh = cm.max() / 2.
        for i, j in itertools.product(range(cm.shape[0]), range(cm.shape[1])):
            plt.text(j, i, cm[i, j],
                     horizontalalignment="center",
                     color="white" if cm[i, j] > thresh else "black")

        plt.tight_layout()
        plt.ylabel('True label')
        plt.xlabel('Predicted label')

    # Compute confusion matrix
    cnf_matrix = confusion_matrix(y_test, y_pred)
    np.set_printoptions(precision=2)

    # Plot non-normalized confusion matrix
    plt.figure()
    plot_confusion_matrix(cnf_matrix, classes=class_names,
                          title='Confusion matrix, without normalization')

    # Plot normalized confusion matrix
    plt.figure()
    plot_confusion_matrix(cnf_matrix, classes=class_names, normalize=True,
                          title='Normalized confusion matrix')

    plt.show()

**Total running time of the script:**
(0 minutes 0.302 seconds)



.. container:: sphx-glr-download

    **Download Python source code:** :download:`plot_confusion_matrix.py <plot_confusion_matrix.py>`


.. container:: sphx-glr-download

    **Download IPython notebook:** :download:`plot_confusion_matrix.ipynb <plot_confusion_matrix.ipynb>`