How to change a list element by index in a list column

Question

I have a column of lists in my polars dataframe. I would like to access and change a value by list index.

Example input

df = pl.DataFrame({
    "values": [
        [10, 20, 30, 40], 
        [50, 60, 70, 80], 
        [90, 100, 110, 120],
    ],
})

Pseudocode

df = df.with_columns(
    pl.col("values").list.eval(pl.element(3) = 1).alias("values2")
)

Expected outcome

df = pl.DataFrame({
    "values": [
        [10, 20, 30, 1], 
        [50, 60, 70, 1], 
        [90, 100, 110, 1],
    ],
})

Answer 1

• take first 3 elements with list.head().
• add 1
• add remaining elements of list with list.tail() using -4 to get all elements except first 4.
• concat_list() to concat elements together.

df.with_columns(
    pl.concat_list(
        pl.col.values.list.head(3), 1,
        pl.col.values.list.tail(-4)
    )
)

shape: (3, 1)
┌───────────────────┐
│ values            │
│ ---               │
│ list[i64]         │
╞═══════════════════╡
│ [10, 20, 30, 1]   │
│ [50, 60, 70, 1]   │
│ [90, 100, 110, 1] │
└───────────────────┘