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Interpolate null values

Source code

Description

Fill nulls with linear interpolation using non-missing values. Can also be used to regrid data to a new grid - see examples below.

Usage

<Expr>$interpolate(method = "linear")

Arguments

method String, either “linear” (default) or “nearest”.

Value

Expr

Examples

library("polars")

pl$DataFrame(x = c(1, NA, 4, NA, 100, NaN, 150))$
  with_columns(
  interp_lin = pl$col("x")$interpolate(),
  interp_near = pl$col("x")$interpolate("nearest")
)

#> shape: (7, 3)
#> ┌───────┬────────────┬─────────────┐
#> │ x     ┆ interp_lin ┆ interp_near │
#> │ ---   ┆ ---        ┆ ---         │
#> │ f64   ┆ f64        ┆ f64         │
#> ╞═══════╪════════════╪═════════════╡
#> │ 1.0   ┆ 1.0        ┆ 1.0         │
#> │ null  ┆ 2.5        ┆ 4.0         │
#> │ 4.0   ┆ 4.0        ┆ 4.0         │
#> │ null  ┆ 52.0       ┆ 100.0       │
#> │ 100.0 ┆ 100.0      ┆ 100.0       │
#> │ NaN   ┆ NaN        ┆ NaN         │
#> │ 150.0 ┆ 150.0      ┆ 150.0       │
#> └───────┴────────────┴─────────────┘

# x, y interpolation over a grid
df_original_grid = pl$DataFrame(
  grid_points = c(1, 3, 10),
  values = c(2.0, 6.0, 20.0)
)
df_original_grid

#> shape: (3, 2)
#> ┌─────────────┬────────┐
#> │ grid_points ┆ values │
#> │ ---         ┆ ---    │
#> │ f64         ┆ f64    │
#> ╞═════════════╪════════╡
#> │ 1.0         ┆ 2.0    │
#> │ 3.0         ┆ 6.0    │
#> │ 10.0        ┆ 20.0   │
#> └─────────────┴────────┘

df_new_grid = pl$DataFrame(grid_points = (1:10) * 1.0)
df_new_grid

#> shape: (10, 1)
#> ┌─────────────┐
#> │ grid_points │
#> │ ---         │
#> │ f64         │
#> ╞═════════════╡
#> │ 1.0         │
#> │ 2.0         │
#> │ 3.0         │
#> │ 4.0         │
#> │ 5.0         │
#> │ 6.0         │
#> │ 7.0         │
#> │ 8.0         │
#> │ 9.0         │
#> │ 10.0        │
#> └─────────────┘

# Interpolate from this to the new grid
df_new_grid$join(
  df_original_grid,
  on = "grid_points", how = "left"
)$with_columns(pl$col("values")$interpolate())

#> shape: (10, 2)
#> ┌─────────────┬────────┐
#> │ grid_points ┆ values │
#> │ ---         ┆ ---    │
#> │ f64         ┆ f64    │
#> ╞═════════════╪════════╡
#> │ 1.0         ┆ 2.0    │
#> │ 2.0         ┆ 4.0    │
#> │ 3.0         ┆ 6.0    │
#> │ 4.0         ┆ 8.0    │
#> │ 5.0         ┆ 10.0   │
#> │ 6.0         ┆ 12.0   │
#> │ 7.0         ┆ 14.0   │
#> │ 8.0         ┆ 16.0   │
#> │ 9.0         ┆ 18.0   │
#> │ 10.0        ┆ 20.0   │
#> └─────────────┴────────┘