docs/source/cpp/csv.rst

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.. to you under the Apache License, Version 2.0 (the
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..   http://www.apache.org/licenses/LICENSE-2.0

.. Unless required by applicable law or agreed to in writing,
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.. default-domain:: cpp
.. highlight:: cpp

.. cpp:namespace:: arrow::csv

=================
Reading CSV files
=================

Arrow provides a fast CSV reader allowing ingestion of external data
as Arrow tables.

Basic usage
===========

A CSV file is read from a :class:`~arrow::io::InputStream`.

.. code-block:: cpp

   #include "arrow/csv/api.h"

   {
      // ...
      arrow::Status st;
      arrow::MemoryPool* pool = default_memory_pool();
      std::shared_ptr<arrow::io::InputStream> input = ...;

      auto read_options = arrow::csv::ReadOptions::Defaults();
      auto parse_options = arrow::csv::ParseOptions::Defaults();
      auto convert_options = arrow::csv::ConvertOptions::Defaults();

      // Instantiate TableReader from input stream and options
      std::shared_ptr<arrow::csv::TableReader> reader;
      st = arrow::csv::TableReader::Make(pool, input, read_options,
                                         parse_options, convert_options,
                                         &reader);
      if (!st.ok()) {
         // Handle TableReader instantiation error...
      }

      std::shared_ptr<arrow::Table> table;
      // Read table from CSV file
      st = reader->Read(&table);
      if (!st.ok()) {
         // Handle CSV read error
         // (for example a CSV syntax error or failed type conversion)
      }
   }

Column names
============

There are three possible ways to infer column names from the CSV file:

* By default, the column names are read from the first row in the CSV file
* If :member:`ReadOptions::column_names` is set, it forces the column
  names in the table to these values (the first row in the CSV file is
  read as data)
* If :member:`ReadOptions::autogenerate_column_names` is true, column names
  will be autogenerated with the pattern "f0", "f1"... (the first row in the
  CSV file is read as data)

Column selection
================

By default, Arrow reads all columns in the CSV file.  You can narrow the
selection of columns with the :member:`ConvertOptions::include_columns`
option.  If some columns in :member:`ConvertOptions::include_columns`
are missing from the CSV file, an error will be emitted unless
:member:`ConvertOptions::include_missing_columns` is true, in which case
the missing columns are assumed to contain all-null values.

Interaction with column names
-----------------------------

If both :member:`ReadOptions::column_names` and
:member:`ConvertOptions::include_columns` are specified,
the :member:`ReadOptions::column_names` are assumed to map to CSV columns,
and :member:`ConvertOptions::include_columns` is a subset of those column
names that will part of the Arrow Table.

Data types
==========

By default, the CSV reader infers the most appropriate data type for each
column.  Type inference considers the following data types, in order:

* Null
* Int64
* Boolean
* Timestamp (with seconds unit)
* Float64
* String
* Binary

It is possible to override type inference for select columns by setting
the :member:`ConvertOptions::column_types` option.  Explicit data types
can be chosen from the following list:

* Null
* All Integer types
* Float32 and Float64
* Decimal128
* Boolean
* Timestamp
* Binary and Large Binary
* String and Large String (with optional UTF8 input validation)
* Fixed-Size Binary

Other data types do not support conversion from CSV values and will error out.

Nulls
-----

Null values are recognized from the spellings stored in
:member:`ConvertOptions::null_values`.  The :func:`ConvertOptions::Defaults`
factory method will initialize a number of conventional null spellings such
as ``N/A``.

Performance
===========

By default, the CSV reader will parallelize reads in order to exploit all
CPU cores on your machine.  You can change this setting in
:member:`ReadOptions::use_threads`.  A reasonable expectation is at least
100 MB/s per core on a modern desktop machine (measured in source CSV bytes,
not target Arrow data bytes).