docs/source/cpp/tutorials/basic_arrow.rst

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.. default-domain:: cpp
.. highlight:: cpp

.. cpp:namespace:: arrow

===========================
Basic Arrow Data Structures
===========================

Apache Arrow provides fundamental data structures for representing data:
:class:`Array`, :class:`ChunkedArray`, :class:`RecordBatch`, and :class:`Table`.
This article shows how to construct these data structures from primitive
data types; specifically, we will work with integers of varying size
representing days, months, and years. We will use them to create the following data structures:

#. Arrow :class:`Arrays <Array>`
#. :class:`ChunkedArrays<ChunkedArray>`
#. :class:`RecordBatch`, from :class:`Arrays <Array>`
#. :class:`Table`, from :class:`ChunkedArrays<ChunkedArray>`

Pre-requisites
--------------
Before continuing, make sure you have:

#. An Arrow installation, which you can set up here: :doc:`/cpp/build_system`
#. Understanding of how to use basic C++ data structures
#. Understanding of basic C++ data types


Setup
-----

Before trying out Arrow, we need to fill in a couple gaps:

1. We need to include necessary headers.

2. ``A main()`` is needed to glue things together.

Includes
^^^^^^^^

First, as ever, we need some includes. We'll get ``iostream`` for output, then import Arrow's basic
functionality from ``api.h``, like so:

.. literalinclude:: ../../../../cpp/examples/tutorial_examples/arrow_example.cc
  :language: cpp
  :start-after: (Doc section: Includes)
  :end-before: (Doc section: Includes)

Main()
^^^^^^

Next, we need a ``main()`` – a common pattern with Arrow looks like the
following:

.. literalinclude:: ../../../../cpp/examples/tutorial_examples/arrow_example.cc
  :language: cpp
  :start-after: (Doc section: Main)
  :end-before: (Doc section: Main)

This allows us to easily use Arrow's error-handling macros, which will
return back to ``main()`` with a :class:`arrow::Status` object if a failure occurs – and
this ``main()`` will report the error. Note that this means Arrow never
raises exceptions, instead relying upon returning :class:`Status`. For more on
that, read here: :doc:`/cpp/conventions`.

To accompany this ``main()``, we have a ``RunMain()`` from which any :class:`Status`
objects can return – this is where we'll write the rest of the program:

.. literalinclude:: ../../../../cpp/examples/tutorial_examples/arrow_example.cc
  :language: cpp
  :start-after: (Doc section: RunMain Start)
  :end-before: (Doc section: RunMain Start)


Making an Arrow Array
---------------------

Building int8 Arrays
^^^^^^^^^^^^^^^^^^^^

Given that we have some data in standard C++ arrays, and want to use Arrow, we need to move
the data from said arrays into Arrow arrays. We still guarantee contiguity of memory in an
:class:`Array`, so no worries about a performance loss when using :class:`Array` vs C++ arrays.
The easiest way to construct an :class:`Array` uses an :class:`ArrayBuilder`.

.. seealso:: :doc:`/cpp/arrays` for more technical details on :class:`Array`

The following code initializes an :class:`ArrayBuilder` for an :class:`Array` that will hold 8 bit
integers. Specifically, it uses the ``AppendValues()`` method, present in concrete
:class:`arrow::ArrayBuilder` subclasses, to fill the :class:`ArrayBuilder` with the
contents of a standard C++ array. Note the use of :c:macro:`ARROW_RETURN_NOT_OK`.
If ``AppendValues()`` fails, this macro will return to ``main()``, which will
print out the meaning of the failure.

.. literalinclude:: ../../../../cpp/examples/tutorial_examples/arrow_example.cc
  :language: cpp
  :start-after: (Doc section: int8builder 1 Append)
  :end-before: (Doc section: int8builder 1 Append)

Given an :class:`ArrayBuilder` has the values we want in our :class:`Array`, we can use
:func:`ArrayBuilder::Finish` to output the final structure to an :class:`Array` – specifically,
we output to a ``std::shared_ptr<arrow::Array>``. Note the use of :c:macro:`ARROW_ASSIGN_OR_RAISE`
in the following code. :func:`~ArrayBuilder::Finish` outputs a :class:`arrow::Result` object, which :c:macro:`ARROW_ASSIGN_OR_RAISE`
can process. If the method fails, it will return to ``main()`` with a :class:`Status`
that will explain what went wrong. If it succeeds, then it will assign
the final output to the left-hand variable.

.. literalinclude:: ../../../../cpp/examples/tutorial_examples/arrow_example.cc
  :language: cpp
  :start-after: (Doc section: int8builder 1 Finish)
  :end-before: (Doc section: int8builder 1 Finish)

As soon as :class:`ArrayBuilder` has had its :func:`Finish <ArrayBuilder::Finish>` method called, its state resets, so
it can be used again, as if it was fresh. Thus, we repeat the process above for our second array:

.. literalinclude:: ../../../../cpp/examples/tutorial_examples/arrow_example.cc
  :language: cpp
  :start-after: (Doc section: int8builder 2)
  :end-before: (Doc section: int8builder 2)

Building int16 Arrays
^^^^^^^^^^^^^^^^^^^^^

An :class:`ArrayBuilder` has its type specified at the time of declaration.
Once this is done, it cannot have its type changed. We have to make a new one when we switch to year data, which
requires a 16-bit integer at the minimum. Of course, there's an :class:`ArrayBuilder` for that.
It uses the exact same methods, but with the new data type:

.. literalinclude:: ../../../../cpp/examples/tutorial_examples/arrow_example.cc
  :language: cpp
  :start-after: (Doc section: int16builder)
  :end-before: (Doc section: int16builder)

Now, we have three Arrow :class:`Arrays <arrow::Array>`, with some variance in type.

Making a RecordBatch
--------------------

A columnar data format only really comes into play when you have a table.
So, let's make one. The first kind we'll make is the :class:`RecordBatch` – this
uses :class:`Arrays <Array>` internally, which means all data will be contiguous within each
column, but any appending or concatenating will require copying. Making a :class:`RecordBatch`
has two steps, given existing :class:`Arrays <Array>`:

#. Defining a :class:`Schema`
#. Loading the :class:`Schema` and Arrays into the constructor

Defining a Schema
^^^^^^^^^^^^^^^^^

To get started making a :class:`RecordBatch`, we first need to define
characteristics of the columns, each represented by a :class:`Field` instance.
Each :class:`Field` contains a name and datatype for its associated column; then,
a :class:`Schema` groups them together and sets the order of the columns, like
so:

.. literalinclude:: ../../../../cpp/examples/tutorial_examples/arrow_example.cc
  :language: cpp
  :start-after: (Doc section: Schema)
  :end-before: (Doc section: Schema)

Building a RecordBatch
^^^^^^^^^^^^^^^^^^^^^^

With data in :class:`Arrays <Array>` from the previous section, and column descriptions in our
:class:`Schema` from the previous step, we can make the :class:`RecordBatch`. Note that the
length of the columns is necessary, and the length is shared by all columns.

.. literalinclude:: ../../../../cpp/examples/tutorial_examples/arrow_example.cc
  :language: cpp
  :start-after: (Doc section: RBatch)
  :end-before: (Doc section: RBatch)

Now, we have our data in a nice tabular form, safely within the :class:`RecordBatch`.
What we can do with this will be discussed in the later tutorials.

Making a ChunkedArray
---------------------

Let's say that we want an array made up of sub-arrays, because it
can be useful for avoiding data copies when concatenating, for parallelizing work, for fitting each chunk
into cache, or for exceeding the 2,147,483,647 row limit in a
standard Arrow :class:`Array`. For this, Arrow offers :class:`ChunkedArray`, which can be
made up of individual Arrow :class:`Arrays <Array>`. In this example, we can reuse the arrays
we made earlier in part of our chunked array, allowing us to extend them without having to copy
data. So, let's build a few more :class:`Arrays <Array>`,
using the same builders for ease of use:

.. literalinclude:: ../../../../cpp/examples/tutorial_examples/arrow_example.cc
  :language: cpp
  :start-after: (Doc section: More Arrays)
  :end-before: (Doc section: More Arrays)

In order to support an arbitrary amount of :class:`Arrays <Array>` in the construction of the
:class:`ChunkedArray`, Arrow supplies :class:`ArrayVector`. This provides a vector for :class:`Arrays <Array>`,
and we'll use it here to prepare to make a :class:`ChunkedArray`:

.. literalinclude:: ../../../../cpp/examples/tutorial_examples/arrow_example.cc
  :language: cpp
  :start-after: (Doc section: ArrayVector)
  :end-before: (Doc section: ArrayVector)

In order to leverage Arrow, we do need to take that last step, and move into a :class:`ChunkedArray`:

.. literalinclude:: ../../../../cpp/examples/tutorial_examples/arrow_example.cc
  :language: cpp
  :start-after: (Doc section: ChunkedArray Day)
  :end-before: (Doc section: ChunkedArray Day)

With a :class:`ChunkedArray` for our day values, we now just need to repeat the process
for the month and year data:

.. literalinclude:: ../../../../cpp/examples/tutorial_examples/arrow_example.cc
  :language: cpp
  :start-after: (Doc section: ChunkedArray Month Year)
  :end-before: (Doc section: ChunkedArray Month Year)

With that, we are left with three :class:`ChunkedArrays <ChunkedArray>`, varying in type.

Making a Table
--------------

One particularly useful thing we can do with the :class:`ChunkedArrays <ChunkedArray>` from the previous section is creating
:class:`Tables <Table>`. Much like a :class:`RecordBatch`, a :class:`Table` stores tabular data. However, a
:class:`Table` does not guarantee contiguity, due to being made up of :class:`ChunkedArrays <ChunkedArray>`.
This can be useful for logic, parallelizing work, for fitting chunks into cache, or exceeding the 2,147,483,647 row limit
present in :class:`Array` and, thus, :class:`RecordBatch`.

If you read up to :class:`RecordBatch`, you may note that the :class:`Table` constructor in the following code is
effectively identical, it just happens to put the length of the columns
in position 3, and makes a :class:`Table`. We re-use the :class:`Schema` from before, and
make our :class:`Table`:

.. literalinclude:: ../../../../cpp/examples/tutorial_examples/arrow_example.cc
  :language: cpp
  :start-after: (Doc section: Table)
  :end-before: (Doc section: Table)

Now, we have our data in a nice tabular form, safely within the :class:`Table`.
What we can do with this will be discussed in the later tutorials.

Ending Program
--------------

At the end, we just return :func:`Status::OK()`, so the ``main()`` knows that
we're done, and that everything's okay.

.. literalinclude:: ../../../../cpp/examples/tutorial_examples/arrow_example.cc
  :language: cpp
  :start-after: (Doc section: Ret)
  :end-before: (Doc section: Ret)

Wrapping Up
-----------

With that, you've created the fundamental data structures in Arrow, and
can proceed to getting them in and out of a program with file I/O in the next article.

Refer to the below for a copy of the complete code:

.. literalinclude:: ../../../../cpp/examples/tutorial_examples/arrow_example.cc
  :language: cpp
  :start-after: (Doc section: Basic Example)
  :end-before: (Doc section: Basic Example)
  :linenos:
  :lineno-match: