Python Polars find the length of a string in a dataframe

February 8, 2023

I am trying to count the number of letters in a string in Polars.
I could probably just use an apply method and get the len(Name).
However, I was wondering if there is a polars specific method?

import polars as pl

mydf = pl.DataFrame(
    {"start_date": ["2020-01-02", "2020-01-03", "2020-01-04"],
     "Name": ["John", "Joe", "James"]})

print(mydf)

│start_date  ┆ Name  │
│ ---        ┆ ---   │
│ str        ┆ str   │
╞════════════╪═══════╡
│ 2020-01-02 ┆ John  │
│ 2020-01-03 ┆ Joe   │
│ 2020-01-04 ┆ James │

In the end John would have 5, Joe would be 3 and James would be 5

I thought something like below might work based on the Pandas equivalent

# Assume that its a Pandas Dataframe
mydf['count'] = mydf ['Name'].str.len()

# Polars equivalent - ERRORs
mydf = mydf.with_columns(
        pl.col('Name').str.len().alias('count')
)

>Solution :

You can use

.str.lengths() that counts number of bytes in the UTF8 string (doc) – faster
.str.n_chars() that counts number of characters (doc)

mydf.with_columns([
    pl.col("Name").str.lengths().alias("len")
])

┌────────────┬───────┬─────┐
│ start_date ┆ Name  ┆ len │
│ ---        ┆ ---   ┆ --- │
│ str        ┆ str   ┆ u32 │
╞════════════╪═══════╪═════╡
│ 2020-01-02 ┆ John  ┆ 4   │
│ 2020-01-03 ┆ Joe   ┆ 3   │
│ 2020-01-04 ┆ James ┆ 5   │
└────────────┴───────┴─────┘