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Statistical learning: the setting and the estimator object in the scikit-learn
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Datasets
=========

The `scikit-learn` deals with learning information from one or more
datasets that are represented as 2D arrays. They can be understood as a
list of multi-dimensional observations. We say that the first axis of
these arrays is the **samples** axis, while the second is the
**features** axis.

.. topic:: A simple example shipped with the scikit: iris dataset

    ::

        >>> from sklearn import datasets
        >>> iris = datasets.load_iris()
        >>> data = iris.data
        >>> data.shape
        (150, 4)

    It is made of 150 observations of irises, each described by 4
    features: their sepal and petal length and width, as detailed in
    `iris.DESCR`.

When the data is not initially in the `(n_samples, n_features)` shape, it
needs to be preprocessed in order to be used by scikit.

.. topic:: An example of reshaping data would be the digits dataset 

    .. image:: ../../auto_examples/datasets/images/plot_digits_last_image_1.png
        :target: ../../auto_examples/datasets/plot_digits_last_image.html
        :align: right
        :scale: 60

    The digits dataset is made of 1797 8x8 images of hand-written
    digits ::

        >>> digits = datasets.load_digits()
        >>> digits.images.shape
        (1797, 8, 8)
        >>> import pylab as pl #doctest: +SKIP
        >>> pl.imshow(digits.images[-1], cmap=pl.cm.gray_r) #doctest: +SKIP
        <matplotlib.image.AxesImage object at ...>

    To use this dataset with the scikit, we transform each 8x8 image into a
    feature vector of length 64 ::

        >>> data = digits.images.reshape((digits.images.shape[0], -1))


Estimators objects
===================

.. Some code to make the doctests run

   >>> from sklearn.base import BaseEstimator
   >>> class Estimator(BaseEstimator):
   ...      def __init__(self, param1=0, param2=0):
   ...          self.param1 = param1
   ...          self.param2 = param2
   ...      def fit(self, data):
   ...          pass
   >>> estimator = Estimator()

**Fitting data**: the main API implemented by scikit-learn is that of the
`estimator`. An estimator is any object that learns from data;
it may be a classification, regression or clustering algorithm or
a `transformer` that extracts/filters useful features from raw data.

All estimator objects expose a `fit` method that takes a dataset
(usually a 2-d array):

    >>> estimator.fit(data)

**Estimator parameters**: All the parameters of an estimator can be set
when it is instantiated or by modifying the corresponding attribute::

    >>> estimator = Estimator(param1=1, param2=2)
    >>> estimator.param1
    1

**Estimated parameters**: When data is fitted with an estimator,
parameters are estimated from the data at hand. All the estimated
parameters are attributes of the estimator object ending by an
underscore::

    >>> estimator.estimated_param_ #doctest: +SKIP

