# Cell array or 3d array that enable its sub-matrix (2d array) freely renew themself without size constraint
**URL:** https://discourse.julialang.org/t/cell-array-or-3d-array-that-enable-its-sub-matrix-2d-array-freely-renew-themself-without-size-constraint/34211
**Category:** General Usage
**Created:** [February 5, 2020, 4:30pm UTC](https://discourse.julialang.org/t/cell-array-or-3d-array-that-enable-its-sub-matrix-2d-array-freely-renew-themself-without-size-constraint/34211 "2020-02-05T16:30:46Z")
**Posts on this page:** 10
**Page:** 1
### Author:  [@1634](https://discourse.julialang.org/u/1634)
#### Post date: [February 5, 2020, 4:30pm UTC](https://discourse.julialang.org/t/cell-array-or-3d-array-that-enable-its-sub-matrix-2d-array-freely-renew-themself-without-size-constraint/34211/1 "2020-02-05T16:30:46Z")
Hi community,
What I am looking for is described in the title (as the row numbers of my 2d sub-matrixs are changing within a for loop).
The only expression of cell array I found, as below, doesn’t work any more.
` A = Array(Array{Float64,2},3)`
And this one don’t allow its sub-matrix to have different size with the initial definition e.g. (2,2).
`A = [zeros(2,2) for i=1:3]`
#Not also uninitialized array
```julia
A = Array{Float64,2}(undef, 2, 3, 5)
A[:, :, 5]=rand(3,3) #error as row and column has to match the original
```
I checked available manual about multidimensional arrays and posts as much as possible but still stay in darkness. Do anyone have a better suggestion?
---
### Author:  [@feanor12](https://discourse.julialang.org/u/feanor12)
#### Post date: [February 5, 2020, 4:37pm UTC](https://discourse.julialang.org/t/cell-array-or-3d-array-that-enable-its-sub-matrix-2d-array-freely-renew-themself-without-size-constraint/34211/3 "2020-02-05T16:37:18Z")
Maybe this can help:
```julia
A = Array{Array{Float64,2},3}(undef,10,10,10)
```
---
### Author:  [@1634](https://discourse.julialang.org/u/1634)
#### Post date: [February 5, 2020, 4:41pm UTC](https://discourse.julialang.org/t/cell-array-or-3d-array-that-enable-its-sub-matrix-2d-array-freely-renew-themself-without-size-constraint/34211/4 "2020-02-05T16:41:15Z")
Thanks I tried but still:
```julia
julia> A = Array{Array{Float64,2},3}(undef,3,3,3)
julia> A[:,:,3]=rand(4,3)
ERROR: DimensionMismatch("tried to assign 4×3 array to 3×3×1 destination")
```
---
### Author:  [@feanor12](https://discourse.julialang.org/u/feanor12)
#### Post date: [February 5, 2020, 4:42pm UTC](https://discourse.julialang.org/t/cell-array-or-3d-array-that-enable-its-sub-matrix-2d-array-freely-renew-themself-without-size-constraint/34211/5 "2020-02-05T16:42:31Z")
Do you want a 3D array with 2D array elements (= 5D data)?
> [@1634](#):
>
> A[:,:,3]=rand(4,3)
```julia
julia> A = Array{Array{Float64,2},3}(undef,3,3,3)
3×3×3 Array{Array{Float64,2},3}:
[:, :, 1] =
#undef #undef #undef
#undef #undef #undef
#undef #undef #undef
[:, :, 2] =
#undef #undef #undef
#undef #undef #undef
#undef #undef #undef
[:, :, 3] =
#undef #undef #undef
#undef #undef #undef
#undef #undef #undef
julia> A[1,1,1]=rand(4,3)
4×3 Array{Float64,2}:
0.573546 0.32481 0.890951
0.7106 0.557957 0.107592
0.471512 0.439862 0.580529
0.743509 0.0780317 0.95439
julia> A
3×3×3 Array{Array{Float64,2},3}:
[:, :, 1] =
[0.573546 0.32481 0.890951; 0.7106 0.557957 0.107592; 0.471512 0.439862 0.580529; 0.743509 0.0780317 0.95439] … #undef #undef
#undef #undef #undef
#undef #undef #undef
[:, :, 2] =
#undef #undef #undef
#undef #undef #undef
#undef #undef #undef
[:, :, 3] =
#undef #undef #undef
#undef #undef #undef
#undef #undef #undef
```
For 3D data you could use a 1D array with 2d elements :
```julia
julia> B = Array{Float32,2}[]
0-element Array{Array{Float32,2},1}
julia> push!(B,rand(2,2))
1-element Array{Array{Float32,2},1}:
[0.9932752 0.4904164; 0.2926822 0.00089553307]
julia> push!(B,rand(3,3))
2-element Array{Array{Float32,2},1}:
[0.9932752 0.4904164; 0.2926822 0.00089553307]
[0.44206983 0.33984074 0.8440299; 0.4682441 0.65651804 0.30746987; 0.6282512 0.76050675 0.48171687]
julia> B[1]
2×2 Array{Float32,2}:
0.993275 0.490416
0.292682 0.000895533
```
---
### Author:  [@1634](https://discourse.julialang.org/u/1634)
#### Post date: [February 5, 2020, 4:57pm UTC](https://discourse.julialang.org/t/cell-array-or-3d-array-that-enable-its-sub-matrix-2d-array-freely-renew-themself-without-size-constraint/34211/6 "2020-02-05T16:57:31Z")
I want a 1d array containing 2d arrays. It works. Your instruction makes my day!
```julia
A = Array{Array{Float64,2},1}(undef,10)
A[1]=rand(4,3)
A[2]=rand(7,6)
```
---
### Author:  [@1634](https://discourse.julialang.org/u/1634)
#### Post date: [February 18, 2020, 1:46am UTC](https://discourse.julialang.org/t/cell-array-or-3d-array-that-enable-its-sub-matrix-2d-array-freely-renew-themself-without-size-constraint/34211/7 "2020-02-18T01:46:57Z")
Indexing 1d(outer)^2d(inside) by a[i] and 3d^2d by a[1,1,1] as you mentioned, while a[1,1] for 2d^2d is an exception. Could you please help me 😊?
---
### Author:  [@stillyslalom](https://discourse.julialang.org/u/stillyslalom)
#### Post date: [February 18, 2020, 2:23am UTC](https://discourse.julialang.org/t/cell-array-or-3d-array-that-enable-its-sub-matrix-2d-array-freely-renew-themself-without-size-constraint/34211/8 "2020-02-18T02:23:47Z")
The array type syntax is `Array{eltype,dimensionality}`. The eltype of your 2x2 container array is `Array{Float64,1}`, which is a 1D array. Try
```julia
Array{Array{Float64,2},2}(undef,2,2)
```
A side-note: instead of a screenshot, could you please paste the terminal output as text between triple backticks (```) in the future? It makes it easier for others to help you.
---
### Author:  [@1634](https://discourse.julialang.org/u/1634)
#### Post date: [February 18, 2020, 9:55am UTC](https://discourse.julialang.org/t/cell-array-or-3d-array-that-enable-its-sub-matrix-2d-array-freely-renew-themself-without-size-constraint/34211/9 "2020-02-18T09:55:54Z")
It seems that indexing the element of 2x2 container array could be problematic regardless of the dimension of contained array.
```julia
julia> a=Array{Array{Float64,2},2}(undef,2,2)
2×2 Array{Array{Float64,2},2}:
#undef #undef
#undef #undef
julia> a[1,1]
ERROR: UndefRefError: access to undefined reference
Stacktrace:
[1] getindex(::Array{Array{Float64,2},2}, ::Int64, ::Int64) at .\array.jl:729
[2] top-level scope at none:0
julia> a=Array{Array{Float64,1},2}(undef,2,2)
2×2 Array{Array{Float64,1},2}:
#undef #undef
#undef #undef
julia> a[1,1]
ERROR: UndefRefError: access to undefined reference
Stacktrace:
[1] getindex(::Array{Array{Float64,1},2}, ::Int64, ::Int64) at .\array.jl:729
[2] top-level scope at none:0
```
---
### Author:  [@improbable22](https://discourse.julialang.org/u/improbable22)
#### Post date: [February 18, 2020, 10:18am UTC](https://discourse.julialang.org/t/cell-array-or-3d-array-that-enable-its-sub-matrix-2d-array-freely-renew-themself-without-size-constraint/34211/10 "2020-02-18T10:18:18Z")
What would you like `a[1,1]` to return here? Perhaps it’s simplest to figure out what’s going on without any `undef` stuff:
```julia
julia> [fill(i+10j, 2,3) for i=1:2, j=1:3]
2×3 Array{Array{Int64,2},2}:
[11 11 11; 11 11 11] [21 21 21; 21 21 21] [31 31 31; 31 31 31]
[12 12 12; 12 12 12] [22 22 22; 22 22 22] [32 32 32; 32 32 32]
julia> a[2,3]
2×3 Array{Int64,2}:
32 32 32
32 32 32
julia> a[2,3][1,1]
32
```
Arrays store numbers densly in memory, so an array of `undef` just contains whatever junk happened to be in memory, interpreted as numbers, and you can index it just fine.
But an Array of Arrays stores a list of pointers, and if this is `undef`, there is no object to return when you index.
```julia
julia> Vector{Float64}(undef, 3)
3-element Array{Float64,1}:
2.335323258e-314
2.245962545e-314
2.3353232737e-314
julia> ans[1]
2.335323258e-314
julia> [Vector{Int}(undef, 3) for _=1:3]
3-element Array{Array{Int64,1},1}:
[4944151024, 4551525968, 4975935952]
[0, 4294967296, 0]
[4610370144, 4476805312, 4547078800]
julia> ans[1]
3-element Array{Int64,1}:
4944151024
4551525968
4975935952
julia> v = Vector{Vector{Int}}(undef, 3)
3-element Array{Array{Int64,1},1}:
#undef
#undef
#undef
julia> ans[1]
ERROR: UndefRefError: access to undefined reference
julia> v[2] = [2,3,4,5,6,7,8];
julia> v
3-element Array{Array{Int64,1},1}:
#undef
[2, 3, 4, 5, 6, 7, 8]
#undef
julia> v[3] = rand(3,3)
ERROR: MethodError: no method matching Array{Int64,1}(::Array{Float64,2})
```
---
### Author:  [@1634](https://discourse.julialang.org/u/1634)
#### Post date: [February 18, 2020, 10:26am UTC](https://discourse.julialang.org/t/cell-array-or-3d-array-that-enable-its-sub-matrix-2d-array-freely-renew-themself-without-size-constraint/34211/11 "2020-02-18T10:26:17Z")
Got it. Thank you~