data_to_long: Reshape (pivot) data from wide to long

Description

This function "lengthens" data, increasing the number of rows and decreasing the number of columns. This is a dependency-free base-R equivalent of tidyr::pivot_longer().

Usage

data_to_long(
  data,
  select = "all",
  names_to = "name",
  names_prefix = NULL,
  names_sep = NULL,
  names_pattern = NULL,
  values_to = "value",
  values_drop_na = FALSE,
  rows_to = NULL,
  ignore_case = FALSE,
  regex = FALSE,
  ...,
  cols
)
reshape_longer(
  data,
  select = "all",
  names_to = "name",
  names_prefix = NULL,
  names_sep = NULL,
  names_pattern = NULL,
  values_to = "value",
  values_drop_na = FALSE,
  rows_to = NULL,
  ignore_case = FALSE,
  regex = FALSE,
  ...,
  cols
)

Value

If a tibble was provided as input, reshape_longer() also returns a tibble. Otherwise, it returns a data frame.

Arguments

data

A data frame to convert to long format, so that it has more rows and fewer columns after the operation.

select

Variables that will be included when performing the required tasks. Can be either

a variable specified as a literal variable name (e.g., column_name),
a string with the variable name (e.g., "column_name"), or a character vector of variable names (e.g., c("col1", "col2", "col3")),
a formula with variable names (e.g., ~column_1 + column_2),
a vector of positive integers, giving the positions counting from the left (e.g. 1 or c(1, 3, 5)),
a vector of negative integers, giving the positions counting from the right (e.g., -1 or -1:-3),
one of the following select-helpers: starts_with(), ends_with(), contains(), a range using : or regex(""). starts_with(), ends_with(), and contains() accept several patterns, e.g starts_with("Sep", "Petal").
or a function testing for logical conditions, e.g. is.numeric() (or is.numeric), or any user-defined function that selects the variables for which the function returns TRUE (like: foo <- function(x) mean(x) > 3),
ranges specified via literal variable names, select-helpers (except regex()) and (user-defined) functions can be negated, i.e. return non-matching elements, when prefixed with a -, e.g. -ends_with(""), -is.numeric or -(Sepal.Width:Petal.Length). Note: Negation means that matches are excluded, and thus, the exclude argument can be used alternatively. For instance, select=-ends_with("Length") (with -) is equivalent to exclude=ends_with("Length") (no -). In case negation should not work as expected, use the exclude argument instead.

If NULL, selects all columns. Patterns that found no matches are silently ignored, e.g. extract_column_names(iris, select = c("Species", "Test")) will just return "Species".

names_to

The name of the new column (variable) that will contain the names from columns in select as values, to identify the source of the values. names_to can be a character vector with more than one column name, in which case names_sep or names_pattern must be provided in order to identify which parts of the column names go into newly created columns. See also 'Examples'.

names_prefix

A regular expression used to remove matching text from the start of each variable name.

names_sep, names_pattern

If names_to contains multiple values, this argument controls how the column name is broken up. names_pattern takes a regular expression containing matching groups, i.e. "()".

values_to

The name of the new column that will contain the values of the columns in select.

values_drop_na

If TRUE, will drop rows that contain only NA in the values_to column. This effectively converts explicit missing values to implicit missing values, and should generally be used only when missing values in data were created by its structure.

rows_to

The name of the column that will contain the row names or row numbers from the original data. If NULL, will be removed.

ignore_case

Logical, if TRUE and when one of the select-helpers or a regular expression is used in select, ignores lower/upper case in the search pattern when matching against variable names.

regex

Logical, if TRUE, the search pattern from select will be treated as regular expression. When regex = TRUE, select must be a character string (or a variable containing a character string) and is not allowed to be one of the supported select-helpers or a character vector of length > 1. regex = TRUE is comparable to using one of the two select-helpers, select = contains("") or select = regex(""), however, since the select-helpers may not work when called from inside other functions (see 'Details'), this argument may be used as workaround.

...

Currently not used.

cols

Identical to select. This argument is here to ensure compatibility with tidyr::pivot_longer(). If both select and cols are provided, cols is used.

Details

Reshaping data into long format usually means that the input data frame is in wide format, where multiple measurements taken on the same subject are stored in multiple columns (variables). The long format stores the same information in a single column, with each measurement per subject stored in a separate row. The values of all variables that are not in select will be repeated.

The necessary information for data_to_long() is:

The columns that contain the repeated measurements (select).
The name of the newly created column that will contain the names of the columns in select (names_to), to identify the source of the values. names_to can also be a character vector with more than one column name, in which case names_sep or names_pattern must be provided to specify which parts of the column names go into the newly created columns.
The name of the newly created column that contains the values of the columns in select (values_to).

In other words: repeated measurements that are spread across several columns will be gathered into a single column (values_to), with the original column names, that identify the source of the gathered values, stored in one or more new columns (names_to).

Examples

Run this code

if (FALSE) { # requireNamespace("psych") && requireNamespace("tidyr")
wide_data <- setNames(
  data.frame(replicate(2, rnorm(8))),
  c("Time1", "Time2")
)
wide_data$ID <- 1:8
wide_data

# Default behaviour (equivalent to tidyr::pivot_longer(wide_data, cols = 1:3))
# probably doesn't make much sense to mix "time" and "id"
data_to_long(wide_data)

# Customizing the names
data_to_long(
  wide_data,
  select = c("Time1", "Time2"),
  names_to = "Timepoint",
  values_to = "Score"
)

# Reshape multiple columns into long format.
mydat <- data.frame(
  age = c(20, 30, 40),
  sex = c("Female", "Male", "Male"),
  score_t1 = c(30, 35, 32),
  score_t2 = c(33, 34, 37),
  score_t3 = c(36, 35, 38),
  speed_t1 = c(2, 3, 1),
  speed_t2 = c(3, 4, 5),
  speed_t3 = c(1, 8, 6)
)
# The column names are split into two columns: "type" and "time". The
# pattern for splitting column names is provided in `names_pattern`. Values
# of all "score_*" and "speed_*" columns are gathered into a single column
# named "count".
data_to_long(
  mydat,
  select = 3:8,
  names_to = c("type", "time"),
  names_pattern = "(score|speed)_t(\\d+)",
  values_to = "count"
)

# Full example
# ------------------
data <- psych::bfi # Wide format with one row per participant's personality test

# Pivot long format
very_long_data <- data_to_long(data,
  select = regex("\\d"), # Select all columns that contain a digit
  names_to = "Item",
  values_to = "Score",
  rows_to = "Participant"
)
head(very_long_data)

even_longer_data <- data_to_long(
  tidyr::who,
  select = new_sp_m014:newrel_f65,
  names_to = c("diagnosis", "gender", "age"),
  names_pattern = "new_?(.*)_(.)(.*)",
  values_to = "count"
)
head(even_longer_data)
}