# CSV.jl Documentation

GitHub Repo: https://github.com/JuliaData/CSV.jl

Welcome to CSV.jl! A pure-Julia package for handling delimited text data, be it comma-delimited (csv), tab-delimited (tsv), or otherwise.

## Installation

You can install CSV by typing the following in the Julia REPL:

] add CSV 

followed by

using CSV

to load the package.

## Overview

To start out, let's discuss the high-level functionality provided by the package, which hopefully will help direct you to more specific documentation for your use-case:

• CSV.File: the most commonly used function for ingesting delimited data; will read an entire data input or vector of data inputs, detecting number of columns and rows, along with the type of data for each column. Returns a CSV.File object, which is like a lightweight table/DataFrame. Assuming file is a variable of a CSV.File object, individual columns can be accessed like file.col1, file[:col1], or file["col"]. You can see parsed column names via file.names. A CSV.File can also be iterated, where a CSV.Row is produced on each iteration, which allows access to each value in the row via row.col1, row[:col1], or row[1]. You can also index a CSV.File directly, like file[1] to return the entire CSV.Row at the provided index/row number. Multiple threads will be used while parsing the input data if the input is large enough, and full return column buffers to hold the parsed data will be allocated. CSV.File satisfies the Tables.jl "source" interface, and so can be passed to valid sink functions like DataFrame, SQLite.load!, Arrow.write, etc. Supports a number of keyword arguments to control parsing, column type, and other file metadata options.
• CSV.read: a convenience function identical to CSV.File, but used when a CSV.File will be passed direclty to a sink function, like a DataFrame. In some cases, sinks may make copies of incoming data for their own safety; by calling CSV.read(file, DataFrame), no copies of the parsed CSV.File will be made, and the DataFrame will take direct ownership of the CSV.File's columns, which is more efficient than doing CSV.File(file) |> DataFrame which will result in an extra copy of each column being made. Keyword arguments are identical to CSV.File. Any valid Tables.jl sink function/table type can be passed as the 2nd argument. Like CSV.File, a vector of data inputs can be passed as the 1st argument, which will result in a single "long" table of all the inputs vertically concatenanted. Each input must have identical schemas (column names and types).
• CSV.Rows: an alternative approach for consuming delimited data, where the input is only consumed one row at a time, which allows "streaming" the data with a lower memory footrpint than CSV.File. Supports many of the same options as CSV.File, except column type handling is a little different. By default, every column type will be essentially Union{Missing, String}, i.e. no automatic type detection is done, but column types can be provided manually. Multithreading is not used while parsing. After constructing a CSV.Rows object, rows can be "streamed" by iterating, where each iteration produces a CSV.Row2 object, which operates similar to CSV.File's CSV.Row type where individual row values can be accessed via row.col1, row[:col1], or row[1]. If each row is processed individually, additional memory can be saved by passing reusebuffer=true, which means a single buffer will be allocated to hold the values of only the currently iterated row. CSV.Rows also supports the Tables.jl interface and can also be passed to valid sink functions.
• CSV.Chunks: similar to CSV.File, but allows passing a ntasks::Integer keyword argument which will cause the input file to be "chunked" up into ntasks number of chunks. After constructing a CSV.Chunks object, each iteration of the object will return a CSV.File of the next parsed chunk. Useful for processing extremely large files in "chunks". Because each iterated element is a valid Tables.jl "source", CSV.Chunks satisfies the Tables.partitions interface, so sinks that can process input partitions can operate by passing CSV.Chunks as the "source".
• CSV.write: A valid Tables.jl "sink" function for writing any valid input table out in a delimited text format. Supports many options for controlling the output like delimiter, quote characters, etc. Writes data to an internal buffer, which is flushed out when full, buffer size is configurable. Also supports writing out partitioned inputs as separate output files, one file per input partition. To write out a DataFrame, for example, it's simply CSV.write("data.csv", df), or to write out a matrix, it's using Tables; CSV.write("data.csv", Tables.table(mat))
• CSV.RowWriter: An alternative way to produce csv output; takes any valid Tables.jl input, and on each iteration, produces a single csv-formatted string from the input table's row.

That's quite a bit! Let's boil down a TL;DR:

For the rest of the manual, we're going to have two big sections, Reading and Writing where we'll walk through the various options to CSV.File/CSV.read/CSV.Rows/CSV.Chunks and CSV.write/CSV.RowWriter.