df1 has ['a', 'b', 'c'] 3 cols,
I want to get a df2 with 4 cols of ['a', 'b', 'c', 'd']. And d is computed like this:
if a>5 {
d = b + c
} else if a<-5 {
d = c - b + a
} else {
d = 3.0 * a
}
How can I do this with polars in rust? maybe for both eager and lazy.
You can use a when -> then -> when -> then -> otherwise expression. Note that you can extend the when, then indefinitely, just like an else if branch.
Below is an example:
use polars::df;
use polars::prelude::*;
fn main() -> Result<()> {
let df = df![
"a" => [2, 9, 2, 5],
"b" => [1, 2, 3, 4],
"c" => [4, 4, 8, 4],
]?;
let out = df
.lazy()
.select([
col("*"),
when(col("a").gt(lit(5)))
.then(col("b") + col("c"))
.when(col("a").lt(lit(5)))
.then(col("c") - col("b") + col("a"))
.otherwise(lit(3) * col("a"))
.alias("d"),
])
.collect()?;
println!("{}", out);
Ok(())
}
This outputs:
shape: (4, 4)
┌─────┬─────┬─────┬─────┐
│ a ┆ b ┆ c ┆ d │
│ --- ┆ --- ┆ --- ┆ --- │
│ i32 ┆ i32 ┆ i32 ┆ i32 │
╞═════╪═════╪═════╪═════╡
│ 2 ┆ 1 ┆ 4 ┆ 5 │
├╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌┤
│ 9 ┆ 2 ┆ 4 ┆ 6 │
├╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌┤
│ 2 ┆ 3 ┆ 8 ┆ 7 │
├╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌┤
│ 5 ┆ 4 ┆ 4 ┆ 15 │
└─────┴─────┴─────┴─────┘
