# Convenient creation of an OffsetArray?

**URL:** <https://discourse.julialang.org/t/convenient-creation-of-an-offsetarray/113047>\
**Category:** General Usage\
**Tags:** offsetarrays, creation\
**Created:** [April 17, 2024, 5:53am UTC](https://discourse.julialang.org/t/convenient-creation-of-an-offsetarray/113047 "2024-04-17T05:53:25Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![ryofurue](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ryofurue/32/24531_2.png) [@ryofurue](https://discourse.julialang.org/u/ryofurue)\
**Post date:** [April 17, 2024, 5:53am UTC](https://discourse.julialang.org/t/convenient-creation-of-an-offsetarray/113047/1 "2024-04-17T05:53:25Z")

</div>

I’ve been using OffsetArrays and is a bit frustrated with its cumbersome creation:

```julia
# xax[0:imax], zax[0:kmax] are OffsetArrays.
# I want to create an OffsetArray [0:imax, 0:kmax]
oa = OffsetArray(Array{Float64,2}(undef, length(xax), length(zax)), axes(xax,1), axes(zax,1))
# calculate values and assign them to oa . . . 

```

Is there a standard, easy solution?

If the `eltype` of the new array is the same as those of `xax` and `zax` and the new array is always 2D, then I could use a hack like

```julia
oa = xax * zax'

```

But, for other cases, it’s getting more and more cumbersome:

```julia
oa = Float64.(xax * reshape(yax, 1, axes(yax)) * reshape(zax, 1, 1, axes(zax)))

```

I do this a lot and so I wish I would be able to do something like

```julia
oa = OffsetArray{Float64,2}(undef, axes(xax,1), axes(zax,1))
# calculate values and assign them to oa . . .

```

It’s entirely possible (from my past experience with Julia) that I haven’t read the right documentation. If so, I apologize. I’m trying but I have found only this:

[https://juliaarrays.github.io/OffsetArrays.jl/stable/](https://juliaarrays.github.io/OffsetArrays.jl/stable/)

which seems to me to suggest that OffsetArray is always a wrapper to an existing “real” array.

---

<div class="post-metadata">

**Author:** ![jishnub](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jishnub/32/33620_2.png) [@jishnub](https://discourse.julialang.org/u/jishnub)\
**Post date:** [April 17, 2024, 7:14am UTC](https://discourse.julialang.org/t/convenient-creation-of-an-offsetarray/113047/2 "2024-04-17T07:14:19Z")

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> [@ryofurue](#):
>
> ```julia
> oa = OffsetArray{Float64,2}(undef, axes(xax,1), axes(zax,1))
> 
> ```

This is already possible:

```julia
julia> oa = OffsetArray{Float64,2}(undef, 2:3, 2:3)
2×2 OffsetArray(::Matrix{Float64}, 2:3, 2:3) with eltype Float64 with indices 2:3×2:3:
 8.80086e199 2.64131e233
 2.58742e161 5.89012e-310

```

---

<div class="post-metadata">

**Author:** ![ryofurue](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ryofurue/32/24531_2.png) [@ryofurue](https://discourse.julialang.org/u/ryofurue)\
**Post date:** [April 19, 2024, 7:52am UTC](https://discourse.julialang.org/t/convenient-creation-of-an-offsetarray/113047/3 "2024-04-19T07:52:12Z")

</div>

Thanks!!!

Is this a new feature? or is the documentation I looked at (see my initial message) is obsolete? or did I miss this feature in the documentation?

Again and again I fail to find the right documentation . . .

* * *

A tangent: I’m now trying to create a triangular or symmetric matrix but all examples in the official documentation indicate that you first create a square matrix and then the `Symmetric` object acts as a wrapper.

This time, I’ve experimented:

```julia
using LinearAlgebra
a = Symmetric{Float64,2}(undef, 3, 3) # or...(undef, 3)

```

which fails.

---

<div class="post-metadata">

**Author:** ![jishnub](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jishnub/32/33620_2.png) [@jishnub](https://discourse.julialang.org/u/jishnub)\
**Post date:** [April 19, 2024, 9:44am UTC](https://discourse.julialang.org/t/convenient-creation-of-an-offsetarray/113047/4 "2024-04-19T09:44:08Z")

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The constructor of this form is usually defined when you can create the array and then populate the elements. In `Symmetric`, you can’t set off-diagonal indices, so the constructor is not defined. In this case, the wrapper is the only route.

---

<div class="post-metadata">

**Author:** ![jishnub](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jishnub/32/33620_2.png) [@jishnub](https://discourse.julialang.org/u/jishnub)\
**Post date:** [April 19, 2024, 9:46am UTC](https://discourse.julialang.org/t/convenient-creation-of-an-offsetarray/113047/5 "2024-04-19T09:46:28Z")

</div>

> [@ryofurue](#):
>
> Is this a new feature? or is the documentation I looked at (see my initial message) is obsolete? or did I miss this feature in the documentation?

Perhaps the documentation needs to be improved to include such examples. This isn’t a new feature, but this doesn’t seem documented.

---

<div class="post-metadata">

**Author:** ![ryofurue](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ryofurue/32/24531_2.png) [@ryofurue](https://discourse.julialang.org/u/ryofurue)\
**Post date:** [April 19, 2024, 11:01am UTC](https://discourse.julialang.org/t/convenient-creation-of-an-offsetarray/113047/6 "2024-04-19T11:01:04Z")

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> [@jishnub](#):
>
> In `Symmetric`, you can’t set off-diagonal indices

Would you care to elaborate? I don’t understand “set off-diagonal indices”.