ksuarez1423 commented on code in PR #13859: URL: https://github.com/apache/arrow/pull/13859#discussion_r968883053
########## 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 <the preceding article> + +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`. You can read more on that and its arguments at <link>. 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: + +1. the target file + +2. 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 +just a name – in this case, test_out.arrow. + +.. 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, so 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! + +I/O with CSV +------------ + +We’re going to go through this step by step, reading then writing, as +follows: + +1. Reading a file + + a. Open the file + + b. Prepare Table + + c. Read File using :class:`csv::TableReader` + +2. 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: <link>. This CSV is a standard CSV +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 Review Comment: RecordBatchWriter should really be a generic class, but instead it's in the IPC namelist. ########## 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 <the preceding article> + +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`. You can read more on that and its arguments at <link>. 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: + +1. the target file + +2. 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 +just a name – in this case, test_out.arrow. + +.. 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, so 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! + +I/O with CSV +------------ + +We’re going to go through this step by step, reading then writing, as +follows: + +1. Reading a file + + a. Open the file + + b. Prepare Table + + c. Read File using :class:`csv::TableReader` + +2. 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: <link>. This CSV is a standard CSV +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 Review Comment: RecordBatchWriter should really be a generic class, but instead it's in the IPC namespace. -- This is an automated message from the Apache Git Service. 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