# Why is BitArray so slow?

**URL:** <https://discourse.julialang.org/t/why-is-bitarray-so-slow/14383>\
**Category:** Performance\
**Created:** [August 31, 2018, 7:25pm UTC](https://discourse.julialang.org/t/why-is-bitarray-so-slow/14383 "2018-08-31T19:25:05Z")\
**Posts on this page:** 9\
**Page:** 2

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**Author:** ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)\
**Post date:** [January 13, 2022, 9:39am UTC](https://discourse.julialang.org/t/why-is-bitarray-so-slow/14383/22 "2022-01-13T09:39:44Z")

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That may be, but the claim that BitArrays are generally slow and only to be used when one is memory constrained is inaccurate. BitArrays are very fast for some use cases, and less so for others.

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**Author:** ![mbauman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mbauman/32/31082_2.png) [@mbauman](https://discourse.julialang.org/u/mbauman)\
**Post date:** [January 13, 2022, 2:34pm UTC](https://discourse.julialang.org/t/why-is-bitarray-so-slow/14383/23 "2022-01-13T14:34:38Z")

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This would be a fun project — I’d bet you could get a really efficient BitArray\*Array matmul implementation.

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**Author:** ![Oscar\_Smith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oscar_smith/32/25343_2.png) [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)\
**Post date:** [January 13, 2022, 2:44pm UTC](https://discourse.julialang.org/t/why-is-bitarray-so-slow/14383/24 "2022-01-13T14:44:16Z")

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Especially for `BitMatrix` this seems like it should be extremely convenient. since you can pack 8x more data into registers.

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**Author:** ![e3c6](https://avatars.discourse-cdn.com/v4/letter/e/e79b87/32.png) [@e3c6](https://discourse.julialang.org/u/e3c6)\
**Post date:** [January 13, 2022, 11:38pm UTC](https://discourse.julialang.org/t/why-is-bitarray-so-slow/14383/25 "2022-01-13T23:38:22Z")

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It is also practically useful. E.g. for one-hot encoding categorical data.

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**Author:** ![jling](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jling/32/212909_2.png) [@jling](https://discourse.julialang.org/u/jling)\
**Post date:** [January 14, 2022, 12:11am UTC](https://discourse.julialang.org/t/why-is-bitarray-so-slow/14383/26 "2022-01-14T00:11:15Z")

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[![](https://global.discourse-cdn.com/julialang/original/3X/c/5/c5a2f02c81c376da005f8610ce9cdd4ad47cfdf4.jpeg "JuliaCon 2019 | The Unreasonable Effectiveness of Multiple Dispatch | Stefan Karpinski") ](https://www.youtube.com/watch?v=kc9HwsxE1OY&t=874)

but we have better one-hot…

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**Author:** ![e3c6](https://avatars.discourse-cdn.com/v4/letter/e/e79b87/32.png) [@e3c6](https://discourse.julialang.org/u/e3c6)\
**Post date:** [January 14, 2022, 8:07pm UTC](https://discourse.julialang.org/t/why-is-bitarray-so-slow/14383/27 "2022-01-14T20:07:27Z")

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such as?

In any case `BitArray` is nice for purely binary data.

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**Author:** ![jzr](https://avatars.discourse-cdn.com/v4/letter/j/eb9ed0/32.png) [@jzr](https://discourse.julialang.org/u/jzr)\
**Post date:** [January 14, 2022, 8:41pm UTC](https://discourse.julialang.org/t/why-is-bitarray-so-slow/14383/28 "2022-01-14T20:41:24Z")

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[https://fluxml.ai/Flux.jl/stable/data/onehot/](https://fluxml.ai/Flux.jl/stable/data/onehot/)

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**Author:** ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)\
**Post date:** [January 14, 2022, 10:06pm UTC](https://discourse.julialang.org/t/why-is-bitarray-so-slow/14383/29 "2022-01-14T22:06:03Z")

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> [@e3c6](#):
>
> such as?

It is described in the video, if I remember correctly.

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**Author:** ![jling](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jling/32/212909_2.png) [@jling](https://discourse.julialang.org/u/jling)\
**Post date:** [January 14, 2022, 10:43pm UTC](https://discourse.julialang.org/t/why-is-bitarray-so-slow/14383/30 "2022-01-14T22:43:46Z")

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it is described in the video at the time stamp I linked to

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