Chop and unchop

Chopping and unchopping preserve the width of a data frame, changing its length. chop() makes df shorter by converting rows within each group into list-columns. unchop() makes df longer by expanding list-columns so that each element of the list-column gets its own row in the output. chop() and unchop() are building blocks for more complicated functions (like unnest(), unnest_longer(), and unnest_wider()) and are generally more suitable for programming than interactive data analysis.

chop(data, cols)

unchop(data, cols, keep_empty = FALSE, ptype = NULL)

Arguments

data	A data frame.
cols	<tidy-select> Columns to chop or unchop (automatically quoted). For unchop(), each column should be a list-column containing generalised vectors (e.g. any mix of NULLs, atomic vector, S3 vectors, a lists, or data frames).
keep_empty	By default, you get one row of output for each element of the list your unchopping/unnesting. This means that if there's a size-0 element (like NULL or an empty data frame), that entire row will be dropped from the output. If you want to preserve all rows, use keep_empty = TRUE to replace size-0 elements with a single row of missing values.
ptype	Optionally, a named list of column name-prototype pairs to coerce cols to, overriding the default that will be guessed from combining the individual values.

Details

Generally, unchopping is more useful than chopping because it simplifies a complex data structure, and nest()ing is usually more appropriate that chop()ing` since it better preserves the connections between observations.

chop() creates list-columns of class vctrs::list_of() to ensure consistent behaviour when the chopped data frame is emptied. For instance this helps getting back the original column types after the roundtrip chop and unchop. Because <list_of> keeps tracks of the type of its elements, unchop() is able to reconstitute the correct vector type even for empty list-columns.

Examples

# Chop ==============================================================
df <- tibble(x = c(1, 1, 1, 2, 2, 3), y = 1:6, z = 6:1)
# Note that we get one row of output for each unique combination of
# non-chopped variables
df %>% chop(c(y, z))
#> # A tibble: 3 × 3
#>       x           y           z
#>   <dbl> <list<int>> <list<int>>
#> 1     1         [3]         [3]
#> 2     2         [2]         [2]
#> 3     3         [1]         [1]
# cf nest
df %>% nest(data = c(y, z))
#> # A tibble: 3 × 2
#>       x data            
#>   <dbl> <list>          
#> 1     1 <tibble [3 × 2]>
#> 2     2 <tibble [2 × 2]>
#> 3     3 <tibble [1 × 2]>

# Unchop ============================================================
df <- tibble(x = 1:4, y = list(integer(), 1L, 1:2, 1:3))
df %>% unchop(y)
#> # A tibble: 6 × 2
#>       x     y
#>   <int> <int>
#> 1     2     1
#> 2     3     1
#> 3     3     2
#> 4     4     1
#> 5     4     2
#> 6     4     3
df %>% unchop(y, keep_empty = TRUE)
#> # A tibble: 7 × 2
#>       x     y
#>   <int> <int>
#> 1     1    NA
#> 2     2     1
#> 3     3     1
#> 4     3     2
#> 5     4     1
#> 6     4     2
#> 7     4     3

# Incompatible types -------------------------------------------------
# If the list-col contains types that can not be natively
df <- tibble(x = 1:2, y = list("1", 1:3))
try(df %>% unchop(y))
#> Error : Can't combine `..1` <character> and `..2` <integer>.

# Unchopping data frames -----------------------------------------------------
# Unchopping a list-col of data frames must generate a df-col because
# unchop leaves the column names unchanged
df <- tibble(x = 1:3, y = list(NULL, tibble(x = 1), tibble(y = 1:2)))
df %>% unchop(y)
#> # A tibble: 3 × 2
#>       x   y$x    $y
#>   <int> <dbl> <int>
#> 1     2     1    NA
#> 2     3    NA     1
#> 3     3    NA     2
df %>% unchop(y, keep_empty = TRUE)
#> # A tibble: 4 × 2
#>       x   y$x    $y
#>   <int> <dbl> <int>
#> 1     1    NA    NA
#> 2     2     1    NA
#> 3     3    NA     1
#> 4     3    NA     2