# Size vs length

**URL:** <https://discourse.julialang.org/t/size-vs-length/37101>\
**Category:** Performance\
**Created:** [April 6, 2020, 9:54am UTC](https://discourse.julialang.org/t/size-vs-length/37101 "2020-04-06T09:54:26Z")\
**Posts on this page:** 14\
**Page:** 1

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**Author:** ![isentropic](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/isentropic/32/11894_2.png) [@isentropic](https://discourse.julialang.org/u/isentropic)\
**Post date:** [April 6, 2020, 9:54am UTC](https://discourse.julialang.org/t/size-vs-length/37101/1 "2020-04-06T09:54:26Z")

</div>

I wonder why the performance of `size` is so much faster when it comes to matrices:

```julia
julia> a = rand(100,2)
100×2 Array{Float64,2}:
 0.47076 0.605881
 0.0432887 0.318102
 0.12271 0.121367
 0.0737559 0.885239
 0.389849 0.164043
 0.468373 0.703576
 0.943903 0.189336
 0.9317 0.852962
 0.58266 0.830349
 0.440795 0.436561
 0.504023 0.76458
 ⋮
 0.0640017 0.0812839
 0.985068 0.228459
 0.94799 0.547918
 0.951806 0.6971
 0.848779 0.19903
 0.450803 0.315992
 0.155962 0.0482648
 0.488436 0.954154
 0.163412 0.36779
 0.837305 0.675525
 0.798766 0.257736

julia> @benchmark length(a[:,1])
BenchmarkTools.Trial:
  memory estimate: 912 bytes
  allocs estimate: 2
  --------------
  minimum time: 439.081 ns (0.00% GC)
  median time: 466.611 ns (0.00% GC)
  mean time: 493.373 ns (2.84% GC)
  maximum time: 4.766 μs (89.33% GC)
  --------------
  samples: 10000
  evals/sample: 198

julia> @benchmark size(a)
BenchmarkTools.Trial:
  memory estimate: 32 bytes
  allocs estimate: 1
  --------------
  minimum time: 15.753 ns (0.00% GC)
  median time: 16.885 ns (0.00% GC)
  mean time: 19.617 ns (7.53% GC)
  maximum time: 2.197 μs (99.08% GC)
  --------------
  samples: 10000
  evals/sample: 998

```

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<div class="post-metadata">

**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:** [April 6, 2020, 10:00am UTC](https://discourse.julialang.org/t/size-vs-length/37101/2 "2020-04-06T10:00:17Z")

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It has nothing to do with `size`. It takes extra time because `a[:, 1]` creates a temporary array. If you want the size of only the first dimension you should write

```julia
size(a, 1)

```

`size` and `length` have identical performance for arrays:

```julia
julia> using BenchmarkTools

julia> @btime length(x) setup=(x=rand(10,12));
  1.647 ns (0 allocations: 0 bytes)

julia> @btime size(x, 1) setup=(x=rand(10,12));
  1.647 ns (0 allocations: 0 bytes)

julia> @btime size(x) setup=(x=rand(10,12));
  1.647 ns (0 allocations: 0 bytes)

```

BTW: in case you brought this habit with you from another programming language, you should probably not do this in that other language either. In matlab you should write `size(a, 1)`, exactly like in Julia, and in Python, it should be `numpy.size(a, 0)`, or `a.shape[0]`.

---

<div class="post-metadata">

**Author:** ![isentropic](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/isentropic/32/11894_2.png) [@isentropic](https://discourse.julialang.org/u/isentropic)\
**Post date:** [April 6, 2020, 10:14am UTC](https://discourse.julialang.org/t/size-vs-length/37101/3 "2020-04-06T10:14:38Z")

</div>

Wouldn’t creating a view fix this?

```julia
julia> @benchmark length(@view a[:,1])
BenchmarkTools.Trial:
  memory estimate: 64 bytes
  allocs estimate: 2
  --------------
  minimum time: 279.348 ns (0.00% GC)
  median time: 284.824 ns (0.00% GC)
  mean time: 293.145 ns (0.86% GC)
  maximum time: 6.973 μs (95.24% GC)
  --------------
  samples: 10000
  evals/sample: 296

```

as views do not allocate?

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<div class="post-metadata">

**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:** [April 6, 2020, 10:15am UTC](https://discourse.julialang.org/t/size-vs-length/37101/4 "2020-04-06T10:15:55Z")

</div>

It still takes time, creating the view itself also has a cost. And anyway it is non-idiomatic, and a bit pointless, when there is already a correct way to do it.

---

<div class="post-metadata">

**Author:** ![isentropic](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/isentropic/32/11894_2.png) [@isentropic](https://discourse.julialang.org/u/isentropic)\
**Post date:** [April 6, 2020, 10:17am UTC](https://discourse.julialang.org/t/size-vs-length/37101/5 "2020-04-06T10:17:04Z")

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Do you then suggest always using `size(a, dim)` over slicing the array and then taking `length`?

---

<div class="post-metadata">

**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:** [April 6, 2020, 10:20am UTC](https://discourse.julialang.org/t/size-vs-length/37101/6 "2020-04-06T10:20:01Z")

</div>

Oh, yes, absolutely.

Sometimes the performance can be the same, if the compiler realizes that it doesn’t need to actually create the view, and skips straight to getting the result. But you cannot rely on it always, and besides it is un-idiomatic and confusing to readers.

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<div class="post-metadata">

**Author:** ![Henrique\_Becker](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/henrique_becker/32/15443_2.png) [@Henrique\_Becker](https://discourse.julialang.org/u/Henrique_Becker)\
**Post date:** [May 11, 2020, 3:19pm UTC](https://discourse.julialang.org/t/size-vs-length/37101/7 "2020-05-11T15:19:55Z")

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Also, I need to note. [Views do allocate.](https://github.com/JuliaLang/julia/issues/14955) Just sometimes they are elided. If you want to have views that surely do not allocate use [UnsafeArrays.jl](https://github.com/JuliaArrays/UnsafeArrays.jl).

---

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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:** [May 11, 2020, 3:41pm UTC](https://discourse.julialang.org/t/size-vs-length/37101/8 "2020-05-11T15:41:01Z")

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> [@Henrique\_Becker](#):
>
> If you want to have views that surely do not allocate use [UnsafeArrays.jl](https://github.com/JuliaArrays/UnsafeArrays.jl).

Or just wait with great anticipation for Julia 1.5!

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<div class="post-metadata">

**Author:** ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)\
**Post date:** [May 11, 2020, 3:43pm UTC](https://discourse.julialang.org/t/size-vs-length/37101/9 "2020-05-11T15:43:54Z")

</div>

> [@mbauman](#):
>
> wait with great anticipation for Julia 1.5!

Or clone the repo _today_ and just use it 😉

---

<div class="post-metadata">

**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:** [May 11, 2020, 3:44pm UTC](https://discourse.julialang.org/t/size-vs-length/37101/10 "2020-05-11T15:44:24Z")

</div>

When using BenchmarkTools, you want to be careful about how you refer to variables. The key thing to remember is that BenchmarkTools is trying to measure the performance _as that snippet would behave inside a function_. You want to use a `$` to flag the `a` as being a local variable instead of a global (and thus type-unstable) reference:

```nohighlight
julia> @benchmark length(@view a[:,1])
BenchmarkTools.Trial:
  memory estimate: 64 bytes
  allocs estimate: 2
  --------------
  minimum time: 138.676 ns (0.00% GC)
  median time: 142.613 ns (0.00% GC)
  mean time: 148.707 ns (0.66% GC)
  maximum time: 1.140 μs (87.63% GC)
  --------------
  samples: 10000
  evals/sample: 842

julia> @benchmark length(@view $a[:,1])
BenchmarkTools.Trial:
  memory estimate: 0 bytes
  allocs estimate: 0
  --------------
  minimum time: 2.034 ns (0.00% GC)
  median time: 2.048 ns (0.00% GC)
  mean time: 2.074 ns (0.00% GC)
  maximum time: 16.413 ns (0.00% GC)
  --------------
  samples: 10000
  evals/sample: 1000

```

The above is on Julia 1.5 where views are now always non-allocating, but even on Julia 1.4 the compiler was able to see that the view wasn’t acually needed when you’re only computing the length.

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<div class="post-metadata">

**Author:** ![jtravs](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jtravs/32/2010_2.png) [@jtravs](https://discourse.julialang.org/u/jtravs)\
**Post date:** [May 12, 2020, 12:16pm UTC](https://discourse.julialang.org/t/size-vs-length/37101/11 "2020-05-12T12:16:11Z")

</div>

I can’t find anything about this in the NEWS for 1.5. Do you have a reference to the pull request?

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<div class="post-metadata">

**Author:** ![non-Jedi](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/non-jedi/32/3645_2.png) [@non-Jedi](https://discourse.julialang.org/u/non-Jedi)\
**Post date:** [May 12, 2020, 1:06pm UTC](https://discourse.julialang.org/t/size-vs-length/37101/12 "2020-05-12T13:06:13Z")

</div>

The issue for non-allocating array views was [Want non-allocating array views · Issue #14955 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/14955) which was closed by the more general PR below.

[https://github.com/JuliaLang/julia/pull/34126](https://github.com/JuliaLang/julia/pull/34126)

I think adding a note to the NEWS entry for the above PR (that it enables non-allocating array views) would be a helpful thing to create a pull request for. 🙂 Would have to be to the `release-1.5` branch as I understand it; or maybe the `backports-release-1.5` branch?

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<div class="post-metadata">

**Author:** ![jtravs](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jtravs/32/2010_2.png) [@jtravs](https://discourse.julialang.org/u/jtravs)\
**Post date:** [May 12, 2020, 1:24pm UTC](https://discourse.julialang.org/t/size-vs-length/37101/13 "2020-05-12T13:24:36Z")

</div>

Thanks! I probably messed something up, but see here: [Note that views no longer allocate in NEWS by jtravs · Pull Request #35851 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/pull/35851/files)

---

<div class="post-metadata">

**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:** [May 12, 2020, 1:28pm UTC](https://discourse.julialang.org/t/size-vs-length/37101/14 "2020-05-12T13:28:45Z")

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I know Jeff is planning on talking about that and has been working on the release notes and maybe even a blog post… note that this is the direct result of the bullet point you put it below!

> - Immutable structs (including tuples) that contain references can now be allocated on the stack, and allocated inline within arrays and other structs ([#33886](https://github.com/JuliaLang/julia/issues/33886)). This significantly reduces the number of heap allocations in some workloads. Code that requires assumptions about object layout and addresses (usually for interoperability with C or other languages) might need to be updated; for example any object that needs a stable address should be a `mutable struct`.

The most exciting part is that it’s _far_ more general than just array views.