readr 0.2.0

Source: ../../vignettes/releases/readr-0.2.0.Rmd

readr 0.2.0 is now available on CRAN. readr makes it easy to read many types of tabular data, including csv, tsv and fixed width. Compared to base equivalents like read.csv(), readr is much faster and gives more convenient output: it never converts strings to factors, can parse date/times, and it doesn’t munge the column names.

This is a big release, so below I describe the new features divided into four main categories:

  • Improved support for international data.
  • Column parsing improvements.
  • File parsing improvements, including support for comments.
  • Improved writers.

There were too many minor improvements and bug fixes to describe in detail here. See the release notes for a complete list.

Internationalisation

readr now has a strategy for dealing with settings that vary across languages and localities: locales. A locale, created with locale(), includes:

  • The names of months and days, used when parsing dates.
  • The default time zone, used when parsing datetimes.
  • The character encoding, used when reading non-ASCII strings.
  • Default date format, used when guessing column types.
  • The decimal and grouping marks, used when reading numbers.

I’ll cover the most important of these parameters below. For more details, see vignette("locales").

To override the default US-centric locale, you pass a custom locale to read_csv(), read_tsv(), or read_fwf(). Rather than showing those funtions here, I’ll use the parse_*() functions because they work with character vectors instead of a files, but are otherwise identical.

Timezones

readr assumes that times are in Coordinated Universal Time, aka UTC. UTC is the best timezone for data because it doesn’t have daylight savings. If your data isn’t already in UTC, you’ll need to supply a tz in the locale:

parse_datetime("2001-10-10 20:10")
#> [1] "2001-10-10 20:10:00 UTC"
parse_datetime("2001-10-10 20:10", 
  locale = locale(tz = "Pacific/Auckland"))
#> [1] "2001-10-10 20:10:00 NZDT"
parse_datetime("2001-10-10 20:10", 
  locale = locale(tz = "Europe/Dublin"))
#> [1] "2001-10-10 20:10:00 IST"

List all available times zones with OlsonNames(). If you’re American, note that “EST” is not Eastern Standard Time – it’s a Canadian time zone that doesn’t have DST! Instead of relying on ambiguous abbreivations, use:

  • PST/PDT = “US/Pacific”
  • CST/CDT = “US/Central”
  • MST/MDT = “US/Mountain”
  • EST/EDT = “US/Eastern”

Default formats

Locales also provide default date and time formats. The time format isn’t currently used for anything, but the date format is used when guessing column types. The default date format is %Y-%m-%d because that’s unambiguous:

If you’re an American, you might want you use your illogical date sytem::

Character encoding

All readr functions yield strings encoded in UTF-8. This encoding is the most likely to give good results in the widest variety of settings. By default, readr assumes that your input is also in UTF-8, which is less likely to be the case, especially when you’re working with older datasets. To parse a dataset that’s not in UTF-8, you need to a supply an encoding.

The following code creates a string encoded with latin1 (aka ISO-8859-1), and shows how it’s different from the string encoded as UTF-8, and how to parse it with readr:

If you don’t know what encoding the file uses, try guess_encoding(). It’s not 100% perfect (as it’s fundamentally a heuristic), but should at least get you pointed in the right direction:

Numbers

Some countries use the decimal point, while others use the decimal comma. The decimal_mark option controls which readr uses when parsing doubles:

parse_double("1,23", locale = locale(decimal_mark = ","))
#> [1] 1.23

The big_mark option describes which character is used to space groups of digits. Do you write 1,000,000, 1.000.000, 1 000 000, or 1'000'000? Specifying the grouping mark allows parse_number() to parse large number as they’re commonly written:

Column parsing improvements

One of the most useful parts of readr are the column parsers: the tools that turns character input into usefully typed data frame columns. This process is now described more fully in a new vignette: vignette("column-types").

By default, column types are guessed by looking at the data. I’ve made a number of tweaks to make it more likely that your code will load correctly the first time:

  • readr now looks at the first 1000 rows (instead of just the first 100) when guessing column types: this only takes a fraction more time, but should hopefully yield better guesses for more inputs.

  • col_date() and col_datetime() no longer recognise partial dates like 19, 1900, 1900-01. These triggered many false positives and after re-reading the ISO8601 spec, I believe they actually refer to periods of time, so should not be parsed into a specific instant.

  • col_integer() no longer recognises values started with zeros (e.g. 0001) as these are often used as identifiers.

  • col_number() will automatically recognise numbers containing the grouping mark (see below for more details).

But you can override these defaults with the col_types() argument. In this version, col_types gains some much needed flexibility:

Many of the individual parsers have also been improved:

File parsing improvements

read_csv(), read_tsv(), and read_delim() gain extra arguments that allow you to parse more files:

Specifying the wrong number of column names, or having rows with an unexpected number of columns, now gives a warning, rather than an error:

Note that the warning message now also shows you the first five problems. I hope this will often allow you to iterate immediately, rather than having to look at the full problems().

Writers

Despite the name, readr also provides some tools for writing data frames to disk. In this version there are three output functions:

All these functions invisibly return their output so you can use them as part of a pipeline:

You can now control how missing values are written with the na argument, and the quoting algorithm has been further refined to only add quotes when needed: when the string contains a quote, the delimiter, a new line or the same text as missing value.

Output for doubles now uses the same precision as R, and POSIXt vectors are saved in a ISO8601 compatible format.

For testing, you can use format_csv(), format_tsv(), and format_delim() to write csv to a string:

This is particularly useful for generating reprexes.