# Array manipulation

**URL:** <https://discourse.julialang.org/t/array-manipulation/22473>\
**Category:** New to Julia\
**Created:** [March 28, 2019, 6:23pm UTC](https://discourse.julialang.org/t/array-manipulation/22473 "2019-03-28T18:23:52Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![BLI](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bli/32/37206_2.png) [@BLI](https://discourse.julialang.org/u/BLI)\
**Post date:** [March 28, 2019, 6:23pm UTC](https://discourse.julialang.org/t/array-manipulation/22473/1 "2019-03-28T18:23:52Z")

</div>

OK… this is probably a silly question… I have a vector of vectors, say:

```nohighlight
> a = [[1,2], [2,3], [3,4]]

```

and I want to “reshape” it into a matrix of dimension either (in this case) 2\times 3 or 3\times 2… One way to do this is the following (2\times 3):

```nohighlight
> m = zeros(2,0)
> for e in a
       global m = hcat(m,e)
   end

```

But there must be a simpler way to do this??

---

<div class="post-metadata">

**Author:** ![kristoffer.carlsson](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kristoffer.carlsson/32/22_2.png) [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)\
**Post date:** [March 28, 2019, 6:29pm UTC](https://discourse.julialang.org/t/array-manipulation/22473/2 "2019-03-28T18:29:31Z")

</div>

`reduce(hcat, a)` should work, I think.

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<div class="post-metadata">

**Author:** ![Adrien](https://avatars.discourse-cdn.com/v4/letter/a/ea666f/32.png) [@Adrien](https://discourse.julialang.org/u/Adrien)\
**Post date:** [March 28, 2019, 8:31pm UTC](https://discourse.julialang.org/t/array-manipulation/22473/3 "2019-03-28T20:31:10Z")

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Or

```julia
hcat(a...)

```

---

<div class="post-metadata">

**Author:** ![BLI](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bli/32/37206_2.png) [@BLI](https://discourse.julialang.org/u/BLI)\
**Post date:** [March 28, 2019, 9:17pm UTC](https://discourse.julialang.org/t/array-manipulation/22473/4 "2019-03-28T21:17:59Z")

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Thanks to Kristoffer and Adrien! Both methods work!

My problem was unrelated to Flux, but I also found that `Flux.batch` provides the same functionality… almost. In fact, the above suggestions seem to be more general than `Flux.batch`:

```nohighlight
julia> a = [rand([0,1,2,3],3) for i in 1:4]
4-element Array{Array{Int64,1},1}:
 [2, 0, 2]
 [2, 0, 3]
 [1, 0, 0]
 [1, 1, 0]

julia> hcat(a...)
3×4 Array{Int64,2}:
 2 2 1 1
 0 0 0 1
 2 3 0 0

julia> using Flux

julia> Flux.batch(a)
3×4 Array{Int64,2}:
 2 2 1 1
 0 0 0 1
 2 3 0 0

julia> b = [rand([0,1,2,3],1) for i in 1:4]
4-element Array{Array{Int64,1},1}:
 [3]
 [0]
 [3]
 [3]

julia> hcat(b...)
1×4 Array{Int64,2}:
 3 0 3 3

julia> Flux.batch(b)
1×4 Array{Int64,2}:
 3 0 3 3

julia> c = rand([0,1,2,3],4)
4-element Array{Int64,1}:
 3
 2
 2
 1

julia> hcat(c...)
1×4 Array{Int64,2}:
 3 2 2 1

julia> Flux.batch(c)
4-element Array{Int64,1}:
 3
 2
 2
 1

```

My “problem” arouse out of calling a function, say, `f(x,y)`, which returns `[v,w]` where `v` and `w` are scalars. Then, with `X` a vector, `f.(X,y)` returns a vector of two element vectors of type `[[v1,w1], [v2,w2],...]` where I wanted to plot `V=[v1,v2,...]` as a function of `X`…

Anyway, thanks!

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<div class="post-metadata">

**Author:** ![BLI](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bli/32/37206_2.png) [@BLI](https://discourse.julialang.org/u/BLI)\
**Post date:** [March 29, 2019, 9:38am UTC](https://discourse.julialang.org/t/array-manipulation/22473/5 "2019-03-29T09:38:25Z")

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Minor follow-up: I benchmarked the three mentioned methods:

```julia
julia> using BenchmarkTools, Flux
julia> a = [rand(4) for i in 1:20];
julia> @benchmark Flux.batch(a)
BenchmarkTools.Trial:
  memory estimate: 736 bytes
  allocs estimate: 1
  --------------
  minimum time: 343.684 ns (0.00% GC)
  median time: 490.051 ns (0.00% GC)
  mean time: 594.442 ns (3.81% GC)
  maximum time: 6.372 μs (78.94% GC)
  --------------
  samples: 10000
  evals/sample: 206

julia> @benchmark hcat(a...)
BenchmarkTools.Trial:
  memory estimate: 1.00 KiB
  allocs estimate: 5
  --------------
  minimum time: 1.140 μs (0.00% GC)
  median time: 1.220 μs (0.00% GC)
  mean time: 1.460 μs (4.27% GC)
  maximum time: 165.370 μs (97.58% GC)
  --------------
  samples: 10000
  evals/sample: 10

julia> @benchmark reduce(hcat,a)
BenchmarkTools.Trial:
  memory estimate: 736 bytes
  allocs estimate: 1
  --------------
  minimum time: 376.617 ns (0.00% GC)
  median time: 400.498 ns (0.00% GC)
  mean time: 474.698 ns (4.64% GC)
  maximum time: 6.445 μs (86.40% GC)
  --------------
  samples: 10000
  evals/sample: 201

```

So… for casual use, `hcat(a...)` seems simplest, but `reduce(hcat,a)` is considerably more efficient. And it seems like `Flux.batch()` uses `reduce(hcat,a)` or a similarly efficient command.

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<div class="post-metadata">

**Author:** ![improbable22](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/improbable22/32/5464_2.png) [@improbable22](https://discourse.julialang.org/u/improbable22)\
**Post date:** [March 29, 2019, 10:26am UTC](https://discourse.julialang.org/t/array-manipulation/22473/6 "2019-03-29T10:26:58Z")

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I think Flux.batch is explicitly copying into a new array, [here’s the source](https://github.com/FluxML/Flux.jl/blob/master/src/utils.jl#L70). But indeed `reduce(hcat,a)` got nicely optimised at some point for this purpose. Splatting a vector not a tuple as in `hcat(a...)` will tend to be slow.