# Save Array of Arrays in HDF5

**URL:** https://discourse.julialang.org/t/save-array-of-arrays-in-hdf5/31359
**Category:** General Usage
**Tags:** question, hdf5
**Created:** [November 21, 2019, 9:13pm UTC](https://discourse.julialang.org/t/save-array-of-arrays-in-hdf5/31359 "2019-11-21T21:13:27Z")
**Posts on this page:** 2
**Page:** 1

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### Author: ![jacob-roth](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jacob-roth/32/1862_2.png) [@jacob-roth](https://discourse.julialang.org/u/jacob-roth)
#### Post date: [November 21, 2019, 9:13pm UTC](https://discourse.julialang.org/t/save-array-of-arrays-in-hdf5/31359/1 "2019-11-21T21:13:28Z")

</div>

Continuing the discussion from [here](https://discourse.julialang.org/t/write-structure-to-hdf5-file-and-read-a-compound-type-from-hdf5-into-julia-structure/20358/4) and [here](https://stackoverflow.com/questions/58983994/save-array-of-arrays-hdf5-julia), how can I best save a large number of arrays using HDF5? And more generally, how can I save an array of arrays?

I know that HDF5 doesn’t support array of arrays, so trying the below doesn’t work:

```julia
julia> using HDF5

julia> x = [[1]]
1-element Array{Array{Int64,1},1}:
 [1]

julia> fid = h5open("test.h5", "w")
HDF5 data file: test.h5

julia> g = g_create(fid, "array-of-arrays")
HDF5 group: /array-of-arrays (file: test.h5)

julia> write(g, x)
ERROR: MethodError: no method matching write(::HDF5Group, ::Array{Array{Int64,1},1})

```

In my particular case I have a single array containing 10,000 arrays of varying lengths. I’d like the 10,000 arrays to be part of a “group”, but creating new datasets / groups for each array makes reading the file very slow, so I am seeking an alternative.

Thanks!

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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: [November 22, 2019, 9:26am UTC](https://discourse.julialang.org/t/save-array-of-arrays-in-hdf5/31359/2 "2019-11-22T09:26:43Z")

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What did you try, and how did it fail? Please provide an MWE.

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