# Array of Plot objects

**URL:** <https://discourse.julialang.org/t/array-of-plot-objects/41679>\
**Category:** General Usage\
**Tags:** plotting\
**Created:** [June 18, 2020, 4:40pm UTC](https://discourse.julialang.org/t/array-of-plot-objects/41679 "2020-06-18T16:40:42Z")\
**Posts on this page:** 12\
**Page:** 1

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**Author:** ![jmurray](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jmurray/32/15806_2.png) [@jmurray](https://discourse.julialang.org/u/jmurray)\
**Post date:** [June 18, 2020, 4:40pm UTC](https://discourse.julialang.org/t/array-of-plot-objects/41679/1 "2020-06-18T16:40:42Z")

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I want to make an array containing plots, but the two approaches I’ve tried both fail.

My first approach was to allocate an empty array and `push!` new plot objects:

```julia
p_arr1 = Array{Plots.Plot{Plots.GRBackend},1}[]
push!(p_arr1, plot(1:10,rand(10))) 
# MethodError: Cannot `convert` an object of type
# Plots.Plot{Plots.GRBackend} to an object of type 
# Array{Plots.Plot{Plots.GRBackend},1}

```

My second approach was to `resize` an existing array and assign by index:

```julia
p_arr2 = [plot(1:10,rand(10))]
resize!(p_arr2,2)
p_arr2[1] = plot(1:10,rand(10))
p_arr2[2] = plot(1:10,rand(10))
display(p_arr2[2]) 
# UndefRefError: access to undefined reference

```

Any ideas?

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**Author:** ![rdeits](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rdeits/32/286_2.png) [@rdeits](https://discourse.julialang.org/u/rdeits)\
**Post date:** [June 18, 2020, 4:41pm UTC](https://discourse.julialang.org/t/array-of-plot-objects/41679/2 "2020-06-18T16:41:52Z")

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> [@jmurray](#):
>
> `Array{Plots.Plot{Plots.GRBackend},1}[]`

That’s just not the right syntax. You want `Array{...}()` not `Array{...}[]`. Your code was creating a new array of arrays of plots.

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**Author:** ![DrPapa](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/drpapa/32/6835_2.png) [@DrPapa](https://discourse.julialang.org/u/DrPapa)\
**Post date:** [June 18, 2020, 4:44pm UTC](https://discourse.julialang.org/t/array-of-plot-objects/41679/3 "2020-06-18T16:44:46Z")

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Alternatively you can do

```julia
p_arr1 = Plots.Plot{Plots.GRBackend}[]

```

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<div class="post-metadata">

**Author:** ![jmurray](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jmurray/32/15806_2.png) [@jmurray](https://discourse.julialang.org/u/jmurray)\
**Post date:** [June 18, 2020, 4:50pm UTC](https://discourse.julialang.org/t/array-of-plot-objects/41679/4 "2020-06-18T16:50:39Z")

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That did it! Thanks. I’m new to Julia and am still trying to figure out the syntax in places.

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<div class="post-metadata">

**Author:** ![jmurray](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jmurray/32/15806_2.png) [@jmurray](https://discourse.julialang.org/u/jmurray)\
**Post date:** [June 18, 2020, 4:58pm UTC](https://discourse.julialang.org/t/array-of-plot-objects/41679/5 "2020-06-18T16:58:02Z")

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This works too! And it’s more concise if you won’t be pre-allocating at a certain size (as e.g., `Array{...}(undef, 10)`). I’m sorry I’m not able to mark both answers as a solution!

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<div class="post-metadata">

**Author:** ![rdeits](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rdeits/32/286_2.png) [@rdeits](https://discourse.julialang.org/u/rdeits)\
**Post date:** [June 18, 2020, 5:00pm UTC](https://discourse.julialang.org/t/array-of-plot-objects/41679/6 "2020-06-18T17:00:00Z")

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In case you’re curious, the syntax `T[]` means “a 1D array of `T`”. You’ll usually see it used as `Int[]` meaning “a 1D array of `Int`s”, but that `T` can be any type at all. Thus, `Array{Plot, 1}[]` means "an array of `Array{Plot, 1}`. Even though that’s not what you want here, it’s a perfectly valid thing to construct, so the language will let you do it.

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<div class="post-metadata">

**Author:** ![jmurray](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jmurray/32/15806_2.png) [@jmurray](https://discourse.julialang.org/u/jmurray)\
**Post date:** [June 18, 2020, 5:10pm UTC](https://discourse.julialang.org/t/array-of-plot-objects/41679/7 "2020-06-18T17:10:12Z")

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Yes, I’m definitely curious – I want to get a proper handle on all this, so thanks.

Supposing you used the `A = Int[]` syntax, is it possible to later change it to a 3x2 array? Using `reshape(A, 3, 2)` naturally yields an error: `DimensionMismatch("new dimensions (3, 2) must be consistent with array size 0")`. Does Julia have a way to resize arrays (prior to reshaping), or do you have to allocate a new array? I suppose you could `push!` yourself to the correct length prior to re-shaping…

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<div class="post-metadata">

**Author:** ![rdeits](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rdeits/32/286_2.png) [@rdeits](https://discourse.julialang.org/u/rdeits)\
**Post date:** [June 18, 2020, 5:23pm UTC](https://discourse.julialang.org/t/array-of-plot-objects/41679/8 "2020-06-18T17:23:55Z")

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Yup, you want `resize!`:

```julia
julia> A = Int[]
0-element Array{Int64,1}

julia> resize!(A, 3 * 2)
6-element Array{Int64,1}:
 140500550566576
 140500550566672
 140500550566720
 140500550566768
 140500550566816
 140500550566864

julia> reshape(A, 3, 2)
3×2 Array{Int64,2}:
 140500550566576 140500550566768
 140500550566672 140500550566816
 140500550566720 140500550566864

```

You can also skip resizing by doing:

```julia
A = Array{Int, 1}(undef, 6)

```

or, exactly equivalently:

```julia
A = Vector{Int}(undef, 6)

```

because `Vector{T}` is an alias for `Array{T, 1}`.

Or you can directly construct a `3x2` array and skip all the steps:

```julia
Array{Int, 2}(undef, 3, 2)

```

and again, equivalently:

```julia
Matrix{Int}(undef, 3, 2)

```

because `Matrix{T} === Array{T, 2}`.

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<div class="post-metadata">

**Author:** ![jmurray](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jmurray/32/15806_2.png) [@jmurray](https://discourse.julialang.org/u/jmurray)\
**Post date:** [June 18, 2020, 5:43pm UTC](https://discourse.julialang.org/t/array-of-plot-objects/41679/9 "2020-06-18T17:43:21Z")

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Great – thanks. It will be particularly good to know that `Vector{T} == Array{T, 1}` and `Matrix{T} == Array{T, 2}`.

If I can bug you about one more question… consider:

```julia
julia> A = Array{Int}(6, 3, undef)

6×3 Array{Int64,2}:
 300060112 134827984 131543120
 131543120 131543120 131543120
 300074896 300238224 131543120
 300084816 300239152 131543120
 131543120 151583056 131543120
 151583056 926541680 0

julia> A = Array{Int}(6, 3, undef)
3×6 LinearAlgebra.Transpose{Int64,Array{Int64,2}}:
 300060112 131543120 300074896 300084816 131543120 151583056
 134827984 131543120 300238224 300239152 151583056 926541680
 131543120 131543120 131543120 131543120 131543120 0

```

After transposing, it has a new `LinearAlgebra.Transpose` type. That’s fine… but it also changes the shape to 3x6 from 6x3. I would think it would either retain its type and change its shape, or else it would change its type but retain its shape. Do you have any insights into this?

(As to the array values – these just whatever values were in memory at the array’s address when declared with `undef`, right?)

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<div class="post-metadata">

**Author:** ![rdeits](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rdeits/32/286_2.png) [@rdeits](https://discourse.julialang.org/u/rdeits)\
**Post date:** [June 18, 2020, 5:58pm UTC](https://discourse.julialang.org/t/array-of-plot-objects/41679/10 "2020-06-18T17:58:12Z")

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I see, you want it to be printed as a “transpose of a 6x3 matrix” not a “6x3 transposed matrix”. That’s not unreasonable, but FWIW I don’t agree. The transposed result definitely has 3 rows and 6 columns, so printing it as such seems reasonable to me.

> (As to the array values – these just whatever values were in memory at the array’s address when declared with `undef` , right?)

Yup, exactly.

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<div class="post-metadata">

**Author:** ![jmurray](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jmurray/32/15806_2.png) [@jmurray](https://discourse.julialang.org/u/jmurray)\
**Post date:** [June 18, 2020, 6:03pm UTC](https://discourse.julialang.org/t/array-of-plot-objects/41679/11 "2020-06-18T18:03:28Z")

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Okay, that makes sense… the user wants to see what the _actual_ dimensions are. And Julia defines it as a new `Transpose` type because maybe it makes the math easier later (rather than having to actually shuffle anything around in memory)? By specifying it to be of `Transpose` type, Julia is making itself a note as to how to index the array?

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<div class="post-metadata">

**Author:** ![rdeits](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rdeits/32/286_2.png) [@rdeits](https://discourse.julialang.org/u/rdeits)\
**Post date:** [June 18, 2020, 6:13pm UTC](https://discourse.julialang.org/t/array-of-plot-objects/41679/12 "2020-06-18T18:13:23Z")

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Yeah, that’s exactly right. The `Transpose` is just a pointer to the original data and a clue about how to index it. That means that doing `A' * B` doesn’t have to copy all the data in `A` just to create `A'`. And even cooler is the fact that `A' * B` can be implemented using a special method for multiplication of a `Transpose` and an `Array`, for which there are dedicated BLAS algorithms we can call.