.. image:: ../images/AlphaTwirl_logo_black.png
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:alt: alphatwirl
AlphaTwirl is a python library that loops over event data and summarizes them into multi-dimensional categorical data as data frames. Event data, input to AlphaTwirl, are data with one entry (or row) for one event: for example, data in ROOT TTrees with one entry per collision event of an LHC experiment at CERN. Event data are often large---too large to be loaded in memory---because they have as many entries as events. Multi-dimensional categorical data, the output of AlphaTwirl, have one row for one category. They are usually small---small enough to be loaded in memory---because they only have as many rows as categories. Users can, for example, import them as data frames into R and pandas, which usually load all data in memory, and can perform categorical data analyses with a rich set of data operations available in R and pandas.