Run any polars expression on the sub-lists’ values

Description

Run any polars expression on the sub-lists’ values

Usage

<Expr>$list$eval(expr, ..., parallel = FALSE)

Arguments

expr	Expression to run. Note that you can select an element with pl$element(), pl$first(), and more. See Examples.
…	These dots are for future extensions and must be empty.
parallel	Run all expressions in parallel. Don’t activate this blindly. Parallelism is worth it if there is enough work to do per thread. This likely should not be used in the $group_by() context, because groups are already executed in parallel.

Value

A polars expression

Examples

library("polars")

df <- pl$DataFrame(
  a = list(c(1, 8, 3), c(3, 2), c(NA, NA, 1)),
  b = list(c("R", "is", "amazing"), c("foo", "bar"), "text")
)

df

#> shape: (3, 2)
#> ┌───────────────────┬────────────────────────┐
#> │ a                 ┆ b                      │
#> │ ---               ┆ ---                    │
#> │ list[f64]         ┆ list[str]              │
#> ╞═══════════════════╪════════════════════════╡
#> │ [1.0, 8.0, 3.0]   ┆ ["R", "is", "amazing"] │
#> │ [3.0, 2.0]        ┆ ["foo", "bar"]         │
#> │ [null, null, 1.0] ┆ ["text"]               │
#> └───────────────────┴────────────────────────┘

# standardize each value inside a list, using only the values in this list
df$select(
  a_stand = pl$col("a")$list$eval(
    (pl$element() - pl$element()$mean()) / pl$element()$std()
  )
)

#> shape: (3, 1)
#> ┌──────────────────────────────┐
#> │ a_stand                      │
#> │ ---                          │
#> │ list[f64]                    │
#> ╞══════════════════════════════╡
#> │ [-0.83205, 1.1094, -0.27735] │
#> │ [0.707107, -0.707107]        │
#> │ [null, null, null]           │
#> └──────────────────────────────┘

# count characters for each element in list. Since column "b" is list[str],
# we can apply all `$str` functions on elements in the list:
df$select(
  b_len_chars = pl$col("b")$list$eval(
    pl$element()$str$len_chars()
  )
)

#> shape: (3, 1)
#> ┌─────────────┐
#> │ b_len_chars │
#> │ ---         │
#> │ list[u32]   │
#> ╞═════════════╡
#> │ [1, 2, 7]   │
#> │ [3, 3]      │
#> │ [4]         │
#> └─────────────┘

# concat strings in each list
df$select(
  pl$col("b")$list$eval(pl$element()$str$join(" "))$list$first()
)

#> shape: (3, 1)
#> ┌──────────────┐
#> │ b            │
#> │ ---          │
#> │ str          │
#> ╞══════════════╡
#> │ R is amazing │
#> │ foo bar      │
#> │ text         │
#> └──────────────┘