:orphan:
:html_theme.sidebar_secondary.remove:

.. This document is auto-generated by the altair-gallery extension. Do not modify directly.

.. _gallery_top_k_letters:


Top K Letters
-------------
This example shows how to use a window transform in order to display only the
top K categories by number of entries. In this case, we rank the characters in
the first paragraph of Dickens' *A Tale of Two Cities* by number of occurances.


.. altair-plot::
    :remove-code:
    


    import altair as alt
    import pandas as pd
    import numpy as np

    # Excerpt from A Tale of Two Cities; public domain text
    text = """
    It was the best of times, it was the worst of times, it was the age of wisdom,
    it was the age of foolishness, it was the epoch of belief, it was the epoch of
    incredulity, it was the season of Light, it was the season of Darkness, it was
    the spring of hope, it was the winter of despair, we had everything before us,
    we had nothing before us, we were all going direct to Heaven, we were all going
    direct the other way - in short, the period was so far like the present period,
    that some of its noisiest authorities insisted on its being received, for good
    or for evil, in the superlative degree of comparison only.
    """

    source = pd.DataFrame(
        {'letters': np.array([c for c in text if c.isalpha()])}
    )

    alt.Chart(source).transform_aggregate(
        count='count()',
        groupby=['letters']
    ).transform_window(
        rank='rank(count)',
        sort=[alt.SortField('count', order='descending')]
    ).transform_filter(
        alt.datum.rank < 10
    ).mark_bar().encode(
        y=alt.Y('letters:N', sort='-x'),
        x='count:Q',
    )


.. tab-set::

    .. tab-item:: Method syntax
        :sync: method

        .. code:: python


            import altair as alt
            import pandas as pd
            import numpy as np

            # Excerpt from A Tale of Two Cities; public domain text
            text = """
            It was the best of times, it was the worst of times, it was the age of wisdom,
            it was the age of foolishness, it was the epoch of belief, it was the epoch of
            incredulity, it was the season of Light, it was the season of Darkness, it was
            the spring of hope, it was the winter of despair, we had everything before us,
            we had nothing before us, we were all going direct to Heaven, we were all going
            direct the other way - in short, the period was so far like the present period,
            that some of its noisiest authorities insisted on its being received, for good
            or for evil, in the superlative degree of comparison only.
            """

            source = pd.DataFrame(
                {'letters': np.array([c for c in text if c.isalpha()])}
            )

            alt.Chart(source).transform_aggregate(
                count='count()',
                groupby=['letters']
            ).transform_window(
                rank='rank(count)',
                sort=[alt.SortField('count', order='descending')]
            ).transform_filter(
                alt.datum.rank < 10
            ).mark_bar().encode(
                y=alt.Y('letters:N').sort('-x'),
                x='count:Q',
            )


    .. tab-item:: Attribute syntax
        :sync: attribute

        .. code:: python


            import altair as alt
            import pandas as pd
            import numpy as np

            # Excerpt from A Tale of Two Cities; public domain text
            text = """
            It was the best of times, it was the worst of times, it was the age of wisdom,
            it was the age of foolishness, it was the epoch of belief, it was the epoch of
            incredulity, it was the season of Light, it was the season of Darkness, it was
            the spring of hope, it was the winter of despair, we had everything before us,
            we had nothing before us, we were all going direct to Heaven, we were all going
            direct the other way - in short, the period was so far like the present period,
            that some of its noisiest authorities insisted on its being received, for good
            or for evil, in the superlative degree of comparison only.
            """

            source = pd.DataFrame(
                {'letters': np.array([c for c in text if c.isalpha()])}
            )

            alt.Chart(source).transform_aggregate(
                count='count()',
                groupby=['letters']
            ).transform_window(
                rank='rank(count)',
                sort=[alt.SortField('count', order='descending')]
            ).transform_filter(
                alt.datum.rank < 10
            ).mark_bar().encode(
                y=alt.Y('letters:N', sort='-x'),
                x='count:Q',
            )