Sample n rows from a table — sample

sample_n() and sample_frac() have been superseded in favour of slice_sample(). While they will not be deprecated in the near future, retirement means that we will only perform critical bug fixes, so we recommend moving to the newer alternative.

These functions were superseded because we realised it was more convenient to have two mutually exclusive arguments to one function, rather than two separate functions. This also made it to clean up a few other smaller design issues with sample_n()/sample_frac:

The connection to slice() was not obvious.
The name of the first argument, tbl, is inconsistent with other single table verbs which use .data.
The size argument uses tidy evaluation, which is surprising and undocumented.
It was easier to remove the deprecated .env argument.
... was in a suboptimal position.

Usage

sample_n(tbl, size, replace = FALSE, weight = NULL, .env = NULL, ...)

sample_frac(tbl, size = 1, replace = FALSE, weight = NULL, .env = NULL, ...)

Arguments

tbl: A data.frame.
size: <tidy-select> For sample_n(), the number of rows to select. For sample_frac(), the fraction of rows to select. If tbl is grouped, size applies to each group.
replace: Sample with or without replacement?
weight: <tidy-select> Sampling weights. This must evaluate to a vector of non-negative numbers the same length as the input. Weights are automatically standardised to sum to 1.
.env: DEPRECATED.
...: ignored

Examples

df <- tibble(x = 1:5, w = c(0.1, 0.1, 0.1, 2, 2))

# sample_n() -> slice_sample() ----------------------------------------------
# Was:
sample_n(df, 3)
#> # A tibble: 3 × 2
#>       x     w
#>   <int> <dbl>
#> 1     3   0.1
#> 2     4   2  
#> 3     2   0.1
sample_n(df, 10, replace = TRUE)
#> # A tibble: 10 × 2
#>        x     w
#>    <int> <dbl>
#>  1     3   0.1
#>  2     2   0.1
#>  3     2   0.1
#>  4     5   2  
#>  5     1   0.1
#>  6     2   0.1
#>  7     1   0.1
#>  8     2   0.1
#>  9     4   2  
#> 10     4   2  
sample_n(df, 3, weight = w)
#> # A tibble: 3 × 2
#>       x     w
#>   <int> <dbl>
#> 1     5   2  
#> 2     4   2  
#> 3     1   0.1

# Now:
slice_sample(df, n = 3)
#> # A tibble: 3 × 2
#>       x     w
#>   <int> <dbl>
#> 1     5   2  
#> 2     3   0.1
#> 3     1   0.1
slice_sample(df, n = 10, replace = TRUE)
#> # A tibble: 10 × 2
#>        x     w
#>    <int> <dbl>
#>  1     3   0.1
#>  2     4   2  
#>  3     2   0.1
#>  4     1   0.1
#>  5     1   0.1
#>  6     3   0.1
#>  7     4   2  
#>  8     1   0.1
#>  9     2   0.1
#> 10     5   2  
slice_sample(df, n = 3, weight_by = w)
#> # A tibble: 3 × 2
#>       x     w
#>   <int> <dbl>
#> 1     4   2  
#> 2     5   2  
#> 3     2   0.1

# Note that sample_n() would error if n was bigger than the group size
# slice_sample() will just use the available rows for consistency with
# the other slice helpers like slice_head()
try(sample_n(df, 10))
#> Error in sample_n(df, 10) : Can't compute indices.
#> Caused by error:
#> ! `size` must be less than or equal to 5 (size of data).
#> ℹ set `replace = TRUE` to use sampling with replacement.
slice_sample(df, n = 10)
#> # A tibble: 5 × 2
#>       x     w
#>   <int> <dbl>
#> 1     2   0.1
#> 2     3   0.1
#> 3     5   2  
#> 4     1   0.1
#> 5     4   2  

# sample_frac() -> slice_sample() -------------------------------------------
# Was:
sample_frac(df, 0.25)
#> # A tibble: 1 × 2
#>       x     w
#>   <int> <dbl>
#> 1     1   0.1
sample_frac(df, 2, replace = TRUE)
#> # A tibble: 10 × 2
#>        x     w
#>    <int> <dbl>
#>  1     3   0.1
#>  2     1   0.1
#>  3     5   2  
#>  4     4   2  
#>  5     1   0.1
#>  6     5   2  
#>  7     3   0.1
#>  8     1   0.1
#>  9     2   0.1
#> 10     3   0.1

# Now:
slice_sample(df, prop = 0.25)
#> # A tibble: 1 × 2
#>       x     w
#>   <int> <dbl>
#> 1     3   0.1
slice_sample(df, prop = 2, replace = TRUE)
#> # A tibble: 10 × 2
#>        x     w
#>    <int> <dbl>
#>  1     2   0.1
#>  2     3   0.1
#>  3     5   2  
#>  4     2   0.1
#>  5     2   0.1
#>  6     5   2  
#>  7     1   0.1
#>  8     2   0.1
#>  9     1   0.1
#> 10     4   2