# \`filter()\` and Boolean masks

**URL:** <https://discourse.julialang.org/t/filter-and-boolean-masks/10005>\
**Category:** General Usage\
**Created:** [March 27, 2018, 1:06pm UTC](https://discourse.julialang.org/t/filter-and-boolean-masks/10005 "2018-03-27T13:06:50Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![ValdarT](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/valdart/32/24146_2.png) [@ValdarT](https://discourse.julialang.org/u/ValdarT)\
**Post date:** [March 27, 2018, 1:06pm UTC](https://discourse.julialang.org/t/filter-and-boolean-masks/10005/1 "2018-03-27T13:06:50Z")

</div>

It often comes up that one wants to filter/select rows of a Matrix based on the values of some column. I believe the best way of doing this is with Boolean masks? But these can get a bit weird… For example:

```julia
a = ["a" "x"; "b" "y"]
a[in.(a[:, 1], [["a", "c"]]), :]

```

A bit hard to read and it is probably confusing at first that you need to wrap the iterable in another iterable.

In this case I would prefer something like `filter!()`. For example with `DataFrames` I can do it like this (although it is currently not at all efficient; but it could be):

```julia
using DataFrames
b = DataFrame(["a" "x"; "b" "y"])
filter!(row -> row[1] in ["a", "c"], b)

```

Which got me thinking that perhaps it makes sense to add another version of `filter()` / `filter!()` that also has a dimension argument (`1` for iterating rows as in `DataFrame`)? Or am I missing something and it is already easy to achieve somehow?

And also, why is `filter!()` so much slower than using a Boolean mask?

```julia
fun1(arr) = filter!(x -> x in ["a", "c"], arr)
fun2(arr) = arr[in.(arr, [["a","c"]]), :]
c = rand(["a","b","c","d"], 10000000);
@time fun1(c); @time fun2(c);
@time fun1(c); @time fun2(c);

```