python-polars is there a np.where equivalent? - python-polars

Question

Polars is there a np.where equivalent? trying to replicate the following code in polars.
If the value is above a certain threshold column called Is_Acceptable is 1 or if it is below it is 0
import pandas as pd
import numpy as np
df = pd.DataFrame({"fruit":["orange","apple","mango","kiwi"], "value":[1,0.8,0.7,1.2]})
df["Is_Acceptable?"] = np.where(df["value"].lt(0.9), 1, 0)
print(df)

Answer 1

Yes, there is pl.when().then().otherwise() expression
import polars as pl
from polars import col
df = pl.DataFrame({
"fruit": ["orange","apple","mango","kiwi"],
"value": [1, 0.8, 0.7, 1.2]
})
df = df.with_column(
pl.when(col('value') < 0.9).then(1).otherwise(0).alias('Is_Acceptable?')
)
print(df)
┌────────┬───────┬────────────────┐
│ fruit ┆ value ┆ Is_Acceptable? │
│ --- ┆ --- ┆ --- │
│ str ┆ f64 ┆ i64 │
╞════════╪═══════╪════════════════╡
│ orange ┆ 1.0 ┆ 0 │
├╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┤
│ apple ┆ 0.8 ┆ 1 │
├╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┤
│ mango ┆ 0.7 ┆ 1 │
├╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┤
│ kiwi ┆ 1.2 ┆ 0 │
└────────┴───────┴────────────────┘

Answer 2

The when/then/otherwise expression is a good general-purpose answer. However, in this case, one shortcut is to simply create a boolean expression.
(
df
.with_column(
(pl.col('value') < 0.9).alias('Is_Acceptable')
)
)
shape: (4, 3)
┌────────┬───────┬───────────────┐
│ fruit ┆ value ┆ Is_Acceptable │
│ --- ┆ --- ┆ --- │
│ str ┆ f64 ┆ bool │
╞════════╪═══════╪═══════════════╡
│ orange ┆ 1.0 ┆ false │
├╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┤
│ apple ┆ 0.8 ┆ true │
├╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┤
│ mango ┆ 0.7 ┆ true │
├╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┤
│ kiwi ┆ 1.2 ┆ false │
└────────┴───────┴───────────────┘
In numeric computations, False will be upcast to 0, and True will be upcast to 1. Or, if you prefer, you can upcast them explicitly to a different type.
(
df
.with_column(
(pl.col('value') < 0.9).cast(pl.Int64).alias('Is_Acceptable')
)
)
shape: (4, 3)
┌────────┬───────┬───────────────┐
│ fruit ┆ value ┆ Is_Acceptable │
│ --- ┆ --- ┆ --- │
│ str ┆ f64 ┆ i64 │
╞════════╪═══════╪═══════════════╡
│ orange ┆ 1.0 ┆ 0 │
├╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┤
│ apple ┆ 0.8 ┆ 1 │
├╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┤
│ mango ┆ 0.7 ┆ 1 │
├╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┤
│ kiwi ┆ 1.2 ┆ 0 │
└────────┴───────┴───────────────┘