lidavidm commented on code in PR #13859: URL: https://github.com/apache/arrow/pull/13859#discussion_r972208017
########## docs/source/cpp/tutorials/io_tutorial.rst: ########## @@ -0,0 +1,404 @@ +.. Licensed to the Apache Software Foundation (ASF) under one +.. or more contributor license agreements. See the NOTICE file +.. distributed with this work for additional information +.. regarding copyright ownership. The ASF licenses this file +.. to you under the Apache License, Version 2.0 (the +.. "License"); you may not use this file except in compliance +.. with the License. You may obtain a copy of the License at + +.. http://www.apache.org/licenses/LICENSE-2.0 + +.. Unless required by applicable law or agreed to in writing, +.. software distributed under the License is distributed on an +.. "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY +.. KIND, either express or implied. See the License for the +.. specific language governing permissions and limitations +.. under the License. + +.. default-domain:: cpp +.. highlight:: cpp + +.. cpp:namespace:: arrow + +============== +Arrow File I/O +============== + +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: + +1. Read an Arrow file into a :class:`RecordBatch` and write it back out afterwards + +2. Read a CSV file into a :class:`Table` and write it back out afterwards + +3. Read a Parquet file into a :class:`Table` and write it back out afterwards + +Pre-requisites +--------------- + +Before continuing, make sure you have: + +1. An Arrow installation + +2. An understanding of basic Arrow data structures from :doc:`/cpp/tutorials/basic_arrow` + +3. A directory to run the final application in – this program will generate some files, so be prepared for that. + +Setup +----- + +Before writing out some file I/O, we need to fill in a couple gaps: + +1. We need to include necessary headers. + +2. A ``main()`` is needed to glue things together. + +3. 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) + +I/O with Arrow Files +-------------------- + +We’re going to go through this step by step, reading then writing, as +follows: + +1. Reading a file + + a. Open the file + + b. Bind file to :class:`ipc::RecordBatchFileReader` + + c. Read file to :class:`RecordBatch` + +2. 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 Review Comment: Based on this ML discussion we may have to circle back here and make the terminology consistent, but this is OK for now https://lists.apache.org/thread/d2l7179yd41gfdhktrl1h06gh31l8l4l ########## docs/source/cpp/tutorials/basic_arrow.rst: ########## @@ -0,0 +1,284 @@ +.. Licensed to the Apache Software Foundation (ASF) under one +.. or more contributor license agreements. See the NOTICE file +.. distributed with this work for additional information +.. regarding copyright ownership. The ASF licenses this file +.. to you under the Apache License, Version 2.0 (the +.. "License"); you may not use this file except in compliance +.. with the License. You may obtain a copy of the License at + +.. http://www.apache.org/licenses/LICENSE-2.0 + +.. Unless required by applicable law or agreed to in writing, +.. software distributed under the License is distributed on an +.. "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY +.. KIND, either express or implied. See the License for the +.. specific language governing permissions and limitations +.. under the License. + +.. 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 +#. 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: <LINK TO CONCEPTUAL OVERVIEW> Review Comment: this link should probably be filled in? ########## docs/source/cpp/tutorials/basic_arrow.rst: ########## @@ -0,0 +1,284 @@ +.. Licensed to the Apache Software Foundation (ASF) under one +.. or more contributor license agreements. See the NOTICE file +.. distributed with this work for additional information +.. regarding copyright ownership. The ASF licenses this file +.. to you under the Apache License, Version 2.0 (the +.. "License"); you may not use this file except in compliance +.. with the License. You may obtain a copy of the License at + +.. http://www.apache.org/licenses/LICENSE-2.0 + +.. Unless required by applicable law or agreed to in writing, +.. software distributed under the License is distributed on an +.. "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY +.. KIND, either express or implied. See the License for the +.. specific language governing permissions and limitations +.. under the License. + +.. 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 +#. 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: <LINK TO CONCEPTUAL OVERVIEW> + +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 +cutely 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>` Review Comment: ```suggestion :class:`Table` does not guarantee contiguity, due to being made up of :class:`ChunkedArrays <ChunkedArray>`. ``` ########## docs/source/cpp/tutorials/basic_arrow.rst: ########## @@ -0,0 +1,284 @@ +.. Licensed to the Apache Software Foundation (ASF) under one +.. or more contributor license agreements. See the NOTICE file +.. distributed with this work for additional information +.. regarding copyright ownership. The ASF licenses this file +.. to you under the Apache License, Version 2.0 (the +.. "License"); you may not use this file except in compliance +.. with the License. You may obtain a copy of the License at + +.. http://www.apache.org/licenses/LICENSE-2.0 + +.. Unless required by applicable law or agreed to in writing, +.. software distributed under the License is distributed on an +.. "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY +.. KIND, either express or implied. See the License for the +.. specific language governing permissions and limitations +.. under the License. + +.. 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 Review Comment: Do we want to link to the doc page? -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected]
