.. _fwflag_add_char_ngrams:
==================
--add_char_ngrams
==================
Switch
======

--add_char_ngrams

Description
===========

Extracts character n-grams (rather than word n-grams) from a message table.

Argument and Default Value
==========================

No argument. Boolean switch, default: ``False``. Uses :doc:`fwflag_n` for the n-gram size(s), and can be combined with :doc:`fwflag_sqrt`/:doc:`fwflag_gzip_csv`, with or without :doc:`fwflag_use_collocs`.

Details
=======

Verified table naming: ``-n 3`` creates ``feat$3Cgram$<corptable>$<correl_field>`` (the ``C`` marks it as a character n-gram table, distinguishing it from the word-level ``feat$3gram$...``).

**Known issue — broken in SQLite mode.** ``FeatureExtractor.addCharNGramTable()`` calls ``mm.disableTableKeys(...)`` directly — a raw ``mysqlmethods`` call issuing MySQL's ``ALTER TABLE ... DISABLE KEYS`` syntax — instead of going through the SQLite-aware ``self.data_engine.disable_table_keys()`` that :doc:`fwflag_add_ngrams`'s own ``addNGramTable()`` uses (which has no such call at all). Verified: running ``--add_char_ngrams`` against a SQLite corpus creates the (empty) destination table, then fails with ``sqlite3.OperationalError: near "DISABLE": syntax error`` before any rows are inserted. Works against MySQL.

Other Switches
==============

Required Switches:

* :doc:`fwflag_d`, :doc:`fwflag_c`, :doc:`fwflag_t`

Optional Switches:

* :doc:`fwflag_n`
* :doc:`fwflag_sqrt`, :doc:`fwflag_gzip_csv`
* :doc:`fwflag_use_collocs`

Example Commands
================

Using the packaged ``dla_tutorial`` dataset against MySQL (see :doc:`../../tutorials/tut_dla`) — broken against SQLite, see Known issue above:

.. code-block:: bash

	dlatkInterface.py -d dla_tutorial -t msgs -c user_id --add_char_ngrams -n 3