How to select the longest string from a list of strings in polars?

Question

How do I select the longest string from a list of strings in polars?

Example and expected output:

import polars as pl

df = pl.DataFrame({
    "values": [
        ["the", "quickest", "brown", "fox"],
        ["jumps", "over", "the", "lazy", "dog"],
        []
    ]
})

┌──────────────────────────────┬────────────────┐
│ values                       ┆ longest_string │
│ ---                          ┆ ---            │
│ list[str]                    ┆ str            │
╞══════════════════════════════╪════════════════╡
│ ["the", "quickest", … "fox"] ┆ quickest       │
│ ["jumps", "over", … "dog"]   ┆ jumps          │
│ []                           ┆ null           │
└──────────────────────────────┴────────────────┘

My use case is to select the longest overlapping match.

Edit: elaborating on the longest overlapping match, this is the output for the example provided by polars:

┌────────────┬───────────┬─────────────────────────────────┐
│ values     ┆ matches   ┆ matches_overlapping             │
│ ---        ┆ ---       ┆ ---                             │
│ str        ┆ list[str] ┆ list[str]                       │
╞════════════╪═══════════╪═════════════════════════════════╡
│ discontent ┆ ["disco"] ┆ ["disco", "onte", "discontent"] │
└────────────┴───────────┴─────────────────────────────────┘

I desire a way to select the longest match in matches_overlapping.

Can you clarify what you mean by the longest overlapping match? That is completely different to extracting just the longest string from the list. — jqurious
– jqurious, Commented Jun 5 at 18:24

juanpa.arrivillaga · Accepted Answer · 2025-06-06 02:54:56Z

3

You can do something like:

df.with_columns(
    pl.col('values').list.get(
        pl.col('values')
        .list.eval(pl.element().str.len_chars())
        .list.arg_max()
    )
    .alias('longest_string')
)

This expression:

pl.col('values')
.list.eval(pl.element().str.len_chars())
.list.arg_max()

first maps len_chars to each string in each of the lists with .list.eval, then it finds the arg_max (the index of the max element, so in this case, the index of the max length).

The result of that is passed to list.get to retrieve those values.

edited Jun 6 at 2:54

answered Jun 5 at 18:50

juanpa.arrivillaga

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Comments

Valentin Georgiev · Accepted Answer · 2025-06-05 19:02:43Z

0

You can achieve this in Polars using .list.eval() along with .str.len_chars() to determine the longest string in each list.

edited Jun 5 at 19:02

answered Jun 5 at 18:46

Valentin Georgiev

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