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Apply a rolling min based on another column

Description

[Experimental]

Given a by column <t_0, t_1, …, t_n>, then closed = “right” (the default) means the windows will be:

  • (t_0 - window_size, t_0\]
  • (t_1 - window_size, t_1\]
  • …
  • (t_n - window_size, t_n\]

Usage

<Expr>$rolling_min_by(
  by,
  window_size,
  ...,
  min_periods = 1,
  closed = c("right", "both", "left", "none")
)

Arguments

by Should be DateTime, Date, UInt64, UInt32, Int64, or Int32 data type after conversion by as_polars_expr(). Note that the integer ones require using “i” in window_size. Accepts expression input. Strings are parsed as column names.
window_size The length of the window. Can be a dynamic temporal size indicated by a timedelta or the following string language:
  • 1ns (1 nanosecond)
  • 1us (1 microsecond)
  • 1ms (1 millisecond)
  • 1s (1 second)
  • 1m (1 minute)
  • 1h (1 hour)
  • 1d (1 calendar day)
  • 1w (1 calendar week)
  • 1mo (1 calendar month)
  • 1q (1 calendar quarter)
  • 1y (1 calendar year)
Or combine them: “3d12h4m25s” \# 3 days, 12 hours, 4 minutes, and 25 seconds By "calendar day", we mean the corresponding time on the next day (which may not be 24 hours, due to daylight savings). Similarly for "calendar week", "calendar month", "calendar quarter", and "calendar year".
… These dots are for future extensions and must be empty.
min_periods The number of values in the window that should be non-null before computing a result. If NULL (default), it will be set equal to window_size.
closed Define which sides of the interval are closed (inclusive). Default is “right”.

Details

If you want to compute multiple aggregation statistics over the same dynamic window, consider using $rolling() - this method can cache the window size computation.

Value

A polars expression

Examples

library("polars")

df_temporal <- pl$select(
  index = 0:24,
  date = pl$datetime_range(
    as.POSIXct("2001-01-01"),
    as.POSIXct("2001-01-02"),
    "1h"
  )
)

# Compute the rolling min with the temporal windows closed on the right
# (default)
df_temporal$with_columns(
  rolling_row_min = pl$col("index")$rolling_min_by(
    "date",
    window_size = "2h"
  )
)

#> shape: (25, 3)
#> ┌───────┬─────────────────────┬─────────────────┐
#> │ index ┆ date                ┆ rolling_row_min │
#> │ ---   ┆ ---                 ┆ ---             │
#> │ i32   ┆ datetime[ms]        ┆ i32             │
#> ╞═══════╪═════════════════════╪═════════════════╡
#> │ 0     ┆ 2001-01-01 00:00:00 ┆ 0               │
#> │ 1     ┆ 2001-01-01 01:00:00 ┆ 0               │
#> │ 2     ┆ 2001-01-01 02:00:00 ┆ 1               │
#> │ 3     ┆ 2001-01-01 03:00:00 ┆ 2               │
#> │ 4     ┆ 2001-01-01 04:00:00 ┆ 3               │
#> │ …     ┆ …                   ┆ …               │
#> │ 20    ┆ 2001-01-01 20:00:00 ┆ 19              │
#> │ 21    ┆ 2001-01-01 21:00:00 ┆ 20              │
#> │ 22    ┆ 2001-01-01 22:00:00 ┆ 21              │
#> │ 23    ┆ 2001-01-01 23:00:00 ┆ 22              │
#> │ 24    ┆ 2001-01-02 00:00:00 ┆ 23              │
#> └───────┴─────────────────────┴─────────────────┘

# Compute the rolling min with the closure of windows on both sides
df_temporal$with_columns(
  rolling_row_min = pl$col("index")$rolling_min_by(
    "date",
    window_size = "2h",
    closed = "both"
  )
)

#> shape: (25, 3)
#> ┌───────┬─────────────────────┬─────────────────┐
#> │ index ┆ date                ┆ rolling_row_min │
#> │ ---   ┆ ---                 ┆ ---             │
#> │ i32   ┆ datetime[ms]        ┆ i32             │
#> ╞═══════╪═════════════════════╪═════════════════╡
#> │ 0     ┆ 2001-01-01 00:00:00 ┆ 0               │
#> │ 1     ┆ 2001-01-01 01:00:00 ┆ 0               │
#> │ 2     ┆ 2001-01-01 02:00:00 ┆ 0               │
#> │ 3     ┆ 2001-01-01 03:00:00 ┆ 1               │
#> │ 4     ┆ 2001-01-01 04:00:00 ┆ 2               │
#> │ …     ┆ …                   ┆ …               │
#> │ 20    ┆ 2001-01-01 20:00:00 ┆ 18              │
#> │ 21    ┆ 2001-01-01 21:00:00 ┆ 19              │
#> │ 22    ┆ 2001-01-01 22:00:00 ┆ 20              │
#> │ 23    ┆ 2001-01-01 23:00:00 ┆ 21              │
#> │ 24    ┆ 2001-01-02 00:00:00 ┆ 22              │
#> └───────┴─────────────────────┴─────────────────┘