README.Rmd

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# tidyr <a href="https://tidyr.tidyverse.org"><img src="man/figures/logo.png" align="right" height="138" alt="tidyr website" /></a>

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## Overview

The goal of tidyr is to help you create __tidy data__. Tidy data is data where:

1. Each variable is a column; each column is a variable.
1. Each observation is a row; each row is an observation.
1. Each value is a cell; each cell is a single value.

Tidy data describes a standard way of storing data that is used wherever possible throughout the [tidyverse](https://www.tidyverse.org/). If you ensure that your data is tidy, you'll spend less time fighting with the tools and more time working on your analysis. Learn more about tidy data in `vignette("tidy-data")`.

## Installation

```{r, eval = FALSE}
# The easiest way to get tidyr is to install the whole tidyverse:
install.packages("tidyverse")

# Alternatively, install just tidyr:
install.packages("tidyr")

# Or the development version from GitHub:
# install.packages("pak")
pak::pak("tidyverse/tidyr")
```

## Cheatsheet

<a href="https://github.com/rstudio/cheatsheets/blob/master/tidyr.pdf"><img src="https://raw.githubusercontent.com/rstudio/cheatsheets/master/pngs/thumbnails/tidyr-thumbs.png" width="630" height="252"/></a>  

## Getting started

```{r}
library(tidyr)
```

tidyr functions fall into five main categories:

* "Pivoting" which converts between long and wide forms. tidyr 1.0.0 
  introduces `pivot_longer()` and `pivot_wider()`, replacing the older 
  `spread()` and `gather()` functions. See `vignette("pivot")` for more 
  details.
  
* "Rectangling", which turns deeply nested lists (as from JSON) into tidy
  tibbles. See `unnest_longer()`, `unnest_wider()`, `hoist()`, and 
  `vignette("rectangle")` for more details.
  
* Nesting converts grouped data to a form where each group becomes a
  single row containing a nested data frame, and unnesting does the opposite.
  See `nest()`, `unnest()`, and  `vignette("nest")` for more details.

* Splitting and combining character columns. Use `separate_wider_delim()`,
  `separate_wider_position()`, and `separate_wider_regex()` to pull a single
  character column into multiple columns; use `unite()` to combine multiple
  columns into a single character column.

* Make implicit missing values explicit with `complete()`; make explicit 
  missing values implicit with `drop_na()`; replace missing values with
  next/previous value with `fill()`, or a known value with `replace_na()`.

## Related work

tidyr [supersedes](https://lifecycle.r-lib.org/articles/stages.html#superseded) reshape2 (2010-2014) and reshape (2005-2010). Somewhat counterintuitively, each iteration of the package has done less. tidyr is designed specifically for tidying data, not general reshaping (reshape2), or the general aggregation (reshape). 

[data.table](https://rdatatable.gitlab.io/data.table) provides high-performance implementations of `melt()` and `dcast()`

If you'd like to read more about data reshaping from a CS perspective, I'd recommend the following three papers:

* [Wrangler: Interactive visual specification of data transformation scripts](http://vis.stanford.edu/papers/wrangler)

* [An interactive framework for data cleaning](https://www2.eecs.berkeley.edu/Pubs/TechRpts/2000/CSD-00-1110.pdf) (Potter's wheel)

* [On efficiently implementing SchemaSQL on a SQL database system](https://www.vldb.org/conf/1999/P45.pdf)

To guide your reading, here's a translation between the terminology used in different places:

| tidyr 1.0.0       | pivot longer | pivot wider |
|-------------------|--------------|-------------|
| tidyr < 1.0.0     | gather       | spread      |
| reshape(2)        | melt         | cast        |
| spreadsheets      | unpivot      | pivot       | 
| databases         | fold         | unfold      |

## Getting help

If you encounter a clear bug, please file a minimal reproducible example on [github](https://github.com/tidyverse/tidyr/issues). For questions and other discussion, please use [forum.posit.co](https://forum.posit.co/).

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Please note that the tidyr project is released with a [Contributor Code of Conduct](https://tidyr.tidyverse.org/CODE_OF_CONDUCT.html). By contributing to this project, you agree to abide by its terms.