Reading CSV files¶

Arrow provides a fast CSV reader allowing ingestion of external data as Arrow tables.

Basic usage¶

A CSV file is read from a InputStream.

#include "arrow/csv/api.h"

{
   // ...
   arrow::MemoryPool* pool = default_memory_pool();
   std::shared_ptr<arrow::io::InputStream> input = ...;

   auto read_options = arrow::csv::ReadOptions::Defaults();
   auto parse_options = arrow::csv::ParseOptions::Defaults();
   auto convert_options = arrow::csv::ConvertOptions::Defaults();

   // Instantiate TableReader from input stream and options
   auto maybe_reader =
     arrow::csv::TableReader::Make(pool,
                                   input,
                                   read_options,
                                   parse_options,
                                   convert_options);
   if (!maybe_reader.ok()) {
      // Handle TableReader instantiation error...
   }
   std::shared_ptr<arrow::csv::TableReader> reader = *maybe_reader;

   // Read table from CSV file
   auto maybe_table = reader->Read();
   if (!maybe_table.ok()) {
      // Handle CSV read error
      // (for example a CSV syntax error or failed type conversion)
   }
   std::shared_ptr<arrow::Table> table = *maybe_table;
}

Column names¶

There are three possible ways to infer column names from the CSV file:

By default, the column names are read from the first row in the CSV file
If ReadOptions::column_names is set, it forces the column names in the table to these values (the first row in the CSV file is read as data)
If ReadOptions::autogenerate_column_names is true, column names will be autogenerated with the pattern “f0”, “f1”… (the first row in the CSV file is read as data)

Column selection¶

By default, Arrow reads all columns in the CSV file. You can narrow the selection of columns with the ConvertOptions::include_columns option. If some columns in ConvertOptions::include_columns are missing from the CSV file, an error will be emitted unless ConvertOptions::include_missing_columns is true, in which case the missing columns are assumed to contain all-null values.

Interaction with column names¶

If both ReadOptions::column_names and ConvertOptions::include_columns are specified, the ReadOptions::column_names are assumed to map to CSV columns, and ConvertOptions::include_columns is a subset of those column names that will part of the Arrow Table.

Data types¶

By default, the CSV reader infers the most appropriate data type for each column. Type inference considers the following data types, in order:

Null
Int64
Boolean
Date32
Timestamp (with seconds unit)
Float64
Dictionary<String> (if ConvertOptions::auto_dict_encode is true)
Dictionary<Binary> (if ConvertOptions::auto_dict_encode is true)
String
Binary

It is possible to override type inference for select columns by setting the ConvertOptions::column_types option. Explicit data types can be chosen from the following list:

Null
All Integer types
Float32 and Float64
Decimal128
Boolean
Date32 and Date64
Timestamp
Binary and Large Binary
String and Large String (with optional UTF8 input validation)
Fixed-Size Binary
Dictionary with index type Int32 and value type one of the following: Binary, String, LargeBinary, LargeString, Int32, UInt32, Int64, UInt64, Float32, Float64, Decimal128

Other data types do not support conversion from CSV values and will error out.

Dictionary inference¶

If type inference is enabled and ConvertOptions::auto_dict_encode is true, the CSV reader first tries to convert string-like columns to a dictionary-encoded string-like array. It switches to a plain string-like array when the threshold in ConvertOptions::auto_dict_max_cardinality is reached.

Nulls¶

Null values are recognized from the spellings stored in ConvertOptions::null_values. The ConvertOptions::Defaults() factory method will initialize a number of conventional null spellings such as N/A.

Character encoding¶

CSV files are expected to be encoded in UTF8. However, non-UTF8 data is accepted for Binary columns.

Performance¶

By default, the CSV reader will parallelize reads in order to exploit all CPU cores on your machine. You can change this setting in ReadOptions::use_threads. A reasonable expectation is at least 100 MB/s per core on a performant desktop or laptop computer (measured in source CSV bytes, not target Arrow data bytes).