docs/source/cpp/tutorials/io_tutorial.rst
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.. default-domain:: cpp .. highlight:: cpp
.. cpp:namespace:: arrow
Apache Arrow provides file I/O functions to facilitate use of Arrow from the start to end of an application. In this article, you will:
Read an Arrow file into a :class:RecordBatch and write it back out afterwards
Read a CSV file into a :class:Table and write it back out afterwards
Read a Parquet file into a :class:Table and write it back out afterwards
Before continuing, make sure you have:
An Arrow installation, which you can set up here: :doc:/cpp/build_system
An understanding of basic Arrow data structures from :doc:/cpp/tutorials/basic_arrow
A directory to run the final application in – this program will generate some files, so be prepared for that.
Before writing out some file I/O, we need to fill in a couple gaps:
We need to include necessary headers.
A main() is needed to glue things together.
We need files to play with.
Includes ^^^^^^^^
Before writing C++ code, we need some includes. We'll get iostream for output, then import Arrow's
I/O functionality for each file type we'll work with in this article:
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: Includes) :end-before: (Doc section: Includes)
Main() ^^^^^^
For our glue, we’ll use the main() pattern from the previous tutorial on
data structures:
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: Main) :end-before: (Doc section: Main)
Which, like when we used it before, is paired with a RunMain():
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: RunMain) :end-before: (Doc section: RunMain)
Generating Files for Reading ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
We need some files to actually play with. In practice, you’ll likely have some input for your own application. Here, however, we want to explore doing I/O for the sake of it, so let’s generate some files to make this easy to follow. To create those, we’ll define a helper function that we’ll run first. Feel free to read through this, but the concepts used will be explained later in this article. Note that we’re using the day/month/year data from the previous tutorial. For now, just copy the function in:
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: GenInitialFile) :end-before: (Doc section: GenInitialFile)
To get the files for the rest of your code to function, make sure to
call GenInitialFile() as the very first line in RunMain() to initialize
the environment:
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: Gen Files) :end-before: (Doc section: Gen Files)
We’re going to go through this step by step, reading then writing, as follows:
Reading a file
a. Open the file
b. Bind file to :class:ipc::RecordBatchFileReader
c. Read file to :class:RecordBatch
Writing a file
a. Get a :class:io::FileOutputStream
b. Write to file from :class:RecordBatch
Opening a File ^^^^^^^^^^^^^^
To actually read a file, we need to get some sort of way to point to it.
In Arrow, that means we’re going to get a :class:io::ReadableFile object – much
like an :class:ArrayBuilder can clear and make new arrays, we can reassign this
to new files, so we’ll use this instance throughout the examples:
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: ReadableFile Definition) :end-before: (Doc section: ReadableFile Definition)
A :class:io::ReadableFile does little alone – we actually have it bind to a file
with :func:io::ReadableFile::Open. For
our purposes here, the default arguments suffice:
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: Arrow ReadableFile Open) :end-before: (Doc section: Arrow ReadableFile Open)
Opening an Arrow file Reader ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
An :class:io::ReadableFile is too generic to offer all functionality to read an Arrow file.
We need to use it to get an :class:ipc::RecordBatchFileReader object. This object implements
all the logic needed to read an Arrow file with correct formatting. We get one through
:func:ipc::RecordBatchFileReader::Open:
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: Arrow Read Open) :end-before: (Doc section: Arrow Read Open)
Reading an Open Arrow File to RecordBatch ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
We have to use a :class:RecordBatch to read an Arrow file, so we’ll get a
:class:RecordBatch. Once we have that, we can actually read the file. Arrow
files can have multiple :class:RecordBatches <RecordBatch>, so we must pass an index. This
file only has one, so pass 0:
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: Arrow Read) :end-before: (Doc section: Arrow Read)
Prepare a FileOutputStream ^^^^^^^^^^^^^^^^^^^^^^^^^^
For output, we need a :class:io::FileOutputStream. Just like our :class:io::ReadableFile,
we’ll be reusing this, so be ready for that. We open files the same way
as when reading:
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: Arrow Write Open) :end-before: (Doc section: Arrow Write Open)
Write Arrow File from RecordBatch ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Now, we grab our :class:RecordBatch we read into previously, and use it, along
with our target file, to create a :class:ipc::RecordBatchWriter. The
:class:ipc::RecordBatchWriter needs two things:
the target file
the :class:Schema for our :class:RecordBatch (in case we need to write more :class:RecordBatches <RecordBatch> of the same format.)
The :class:Schema comes from our existing :class:RecordBatch and the target file is
the output stream we just created.
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: Arrow Writer) :end-before: (Doc section: Arrow Writer)
We can just call :func:ipc::RecordBatchWriter::WriteRecordBatch with our :class:RecordBatch to fill up our
file:
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: Arrow Write) :end-before: (Doc section: Arrow Write)
For IPC in particular, the writer has to be closed since it anticipates more than one batch may be written. To do that:
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: Arrow Close) :end-before: (Doc section: Arrow Close)
Now we’ve read and written an IPC file!
We’re going to go through this step by step, reading then writing, as follows:
Reading a file
a. Open the file
b. Prepare Table
c. Read File using :class:csv::TableReader
Writing a file
a. Get a :class:io::FileOutputStream
b. Write to file from :class:Table
Opening a CSV File ^^^^^^^^^^^^^^^^^^
For a CSV file, we need to open a :class:io::ReadableFile, just like an Arrow file,
and reuse our :class:io::ReadableFile object from before to do so:
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: CSV Read Open) :end-before: (Doc section: CSV Read Open)
Preparing a Table ^^^^^^^^^^^^^^^^^
CSV can be read into a :class:Table, so declare a pointer to a :class:Table:
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: CSV Table Declare) :end-before: (Doc section: CSV Table Declare)
Read a CSV File to Table ^^^^^^^^^^^^^^^^^^^^^^^^
The CSV reader has option structs which need to be passed – luckily,
there are defaults for these which we can pass directly. For reference
on the other options, go here: :doc:/cpp/api/formats.
without any special delimiters and is small, so we can make our reader
with defaults:
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: CSV Reader Make) :end-before: (Doc section: CSV Reader Make)
With the CSV reader primed, we can use its :func:csv::TableReader::Read method to fill our
:class:Table:
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: CSV Read) :end-before: (Doc section: CSV Read)
Write a CSV File from Table ^^^^^^^^^^^^^^^^^^^^^^^^^^^
CSV writing to :class:Table looks exactly like IPC writing to :class:RecordBatch,
except with our :class:Table, and using :func:ipc::RecordBatchWriter::WriteTable instead of
:func:ipc::RecordBatchWriter::WriteRecordBatch. Note that the same writer class is used --
we're writing with :func:ipc::RecordBatchWriter::WriteTable because we have a :class:Table. We’ll target
a file, use our :class:Table’s <Table> :class:Schema, and then write the :class:Table:
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: CSV Write) :end-before: (Doc section: CSV Write)
Now, we’ve read and written a CSV file!
We’re going to go through this step by step, reading then writing, as follows:
Reading a file
a. Open the file
b. Prepare :class:parquet::arrow::FileReader
c. Read file to :class:Table
Writing a file
a. Write :class:Table to file
Opening a Parquet File ^^^^^^^^^^^^^^^^^^^^^^
Once more, this file format, Parquet, needs a :class:io::ReadableFile, which we
already have, and for the :func:io::ReadableFile::Open method to be called on a file:
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: Parquet Read Open) :end-before: (Doc section: Parquet Read Open)
Setting up a Parquet Reader ^^^^^^^^^^^^^^^^^^^^^^^^^^^
As always, we need a Reader to actually read the file. We’ve been
getting Readers for each file format from the Arrow namespace. This
time, we enter the Parquet namespace to get the :class:parquet::arrow::FileReader:
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: Parquet FileReader) :end-before: (Doc section: Parquet FileReader)
Now, to set up our reader, we call :func:parquet::arrow::OpenFile. Yes, this is necessary
even though we used :func:io::ReadableFile::Open. Note that we pass our
:class:parquet::arrow::FileReader by reference, instead of assigning to it in output:
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: Parquet OpenFile) :end-before: (Doc section: Parquet OpenFile)
Reading a Parquet File to Table ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
With a prepared :class:parquet::arrow::FileReader in hand, we can read to a
:class:Table, except we must pass the :class:Table by reference instead of outputting to it:
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: Parquet Read) :end-before: (Doc section: Parquet Read)
Writing a Parquet File from Table ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
For single-shot writes, writing a Parquet file does not need a writer object. Instead, we give it our table, point to the memory pool it will use for any necessary memory consumption, tell it where to write, and the chunk size if it needs to break up the file at all:
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: Parquet Write) :end-before: (Doc section: Parquet Write)
At the end, we just return :func:Status::OK, so the main() knows that
we’re done, and that everything’s okay. Just like in the first tutorial.
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: Return) :end-before: (Doc section: Return)
With that, you’ve read and written IPC, CSV, and Parquet in Arrow, and can properly load data and write output! Now, we can move into processing data with compute functions in the next article.
Refer to the below for a copy of the complete code:
.. literalinclude:: ../../../../cpp/examples/tutorial_examples/file_access_example.cc :language: cpp :start-after: (Doc section: File I/O) :end-before: (Doc section: File I/O) :linenos: :lineno-match: