# Expressions

`Polars`

has a powerful concept called expressions that is central to its very fast performance.

Expressions are at the core of many data science operations:

- taking a sample of rows from a column
- multiplying values in a column
- extracting a column of years from dates
- convert a column of strings to lowercase
- and so on!

However, expressions are also used within other operations:

- taking the mean of a group in a
`groupby`

operation - calculating the size of groups in a
`groupby`

operation - taking the sum horizontally across columns

`Polars`

performs these core data transformations very quickly by:

- automatic query optimization on each expression
- automatic parallelization of expressions on many columns

Polars expressions are a mapping from a series to a series (or mathematically `Fn(Series) -> Series`

). As expressions have a `Series`

as an input and a `Series`

as an output then it is straightforward to do a sequence of expressions (similar to method chaining in `Pandas`

).

## Examples

The following is an expression:

The snippet above says:

- Select column "foo"
- Then sort the column (not in reversed order)
- Then take the first two values of the sorted output

The power of expressions is that every expression produces a new expression, and that they
can be *piped* together. You can run an expression by passing them to one of `Polars`

execution contexts.

Here we run two expressions by running `df.select`

:

```
df.select(
[
pl.col("foo").sort().head(2),
pl.col("bar").filter(pl.col("foo") == 1).sum(),
]
)
```

```
df.clone().lazy().select([
col("foo").sort(Default::default()).head(Some(2)),
col("bar").filter(col("foo").eq(lit(1))).sum(),
]).collect()?;
```

```
df.select(
pl.col("foo").sort().head(2),
pl.col("bar").filter(pl.col("foo").eq(1)).sum(),
);
```

All expressions are run in parallel, meaning that separate `Polars`

expressions are **embarrassingly parallel**. Note that within an expression there may be more parallelization going on.

## Conclusion

This is the tip of the iceberg in terms of possible expressions. There are a ton more, and they can be combined in a variety ways. This page is intended to get you familiar with the concept of expressions, in the section on expressions we will dive deeper.