# Datatype when save matrices in HDF5 file

**URL:** <https://discourse.julialang.org/t/datatype-when-save-matrices-in-hdf5-file/95364>\
**Category:** General Usage\
**Tags:** hdf5\
**Created:** [March 1, 2023, 2:03am UTC](https://discourse.julialang.org/t/datatype-when-save-matrices-in-hdf5-file/95364 "2023-03-01T02:03:06Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![jisutich](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jisutich/32/33342_2.png) [@jisutich](https://discourse.julialang.org/u/jisutich)\
**Post date:** [March 1, 2023, 2:03am UTC](https://discourse.julialang.org/t/datatype-when-save-matrices-in-hdf5-file/95364/1 "2023-03-01T02:03:06Z")

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Hi,

I am trying to write a HDF5 file in Julia. Now suppose I want to create a dataset `mydataset` which contains a matrix `A` in the group `mygroup`, what datatype should I use in the function create\_dataset? (In the document, the function looks like create\_dataset(parent, path, datatype, dataspace; properties…)).

Also, suppose I want to save an object which is a list of matrices `[A1,A2,A3,A4]`, what datatype should I use?

Finally, if `A` is a static array. Should I transform it to a normal array before saving it to the HDF5 file? I want to make sure that I can read the dataset in python.

Thanks

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**Author:** ![HanD](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/hand/32/213908_2.png) [@HanD](https://discourse.julialang.org/u/HanD)\
**Post date:** [March 1, 2023, 7:56am UTC](https://discourse.julialang.org/t/datatype-when-save-matrices-in-hdf5-file/95364/2 "2023-03-01T07:56:22Z")

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Hello!

Usually you don’t need to call `create_dataset` directly, you can simply dump your matrix into the `HDF5.File` using indexing:

```julia
julia> using HDF5

julia> h5 = h5open("/tmp/x.h5", "w")
🗂️ HDF5.File: (read-write) /tmp/x.h5

julia> h5["m"] = rand(2,3,4)
2×3×4 Array{Float64, 3}:
[:, :, 1] =
 0.636573 0.539704 0.134521
 0.705768 0.795797 0.186137

[:, :, 2] =
 0.359359 0.0768516 0.542245
 0.0529602 0.882116 0.475718

[:, :, 3] =
 0.698961 0.992394 0.198544
 0.235275 0.455456 0.88217

[:, :, 4] =
 0.515136 0.495796 0.0160039
 0.692038 0.356454 0.915089

julia> size(h5["m"])
(2, 3, 4)

```

This also answers your second question: you cannot serialize a vector of matrices directly, but if those matrices have the same size, then you can merge them all together in the third dimension, and store them as a single 3D tensor. Or if that doesn’t work, you need to manually assign subkeys to your arrays/matrices:

```julia
for (i, A) in enumerate([A1, A2, A3, A4])
    h5["A/$i"] = A
end

```

As for the third question, I have no experience with static arrays, but I would assume whatever conversion is necessary, HDF5 takes care of it for you. That being said, it should be fairly straightworward to try it out.

HTH.

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**Author:** ![jisutich](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jisutich/32/33342_2.png) [@jisutich](https://discourse.julialang.org/u/jisutich)\
**Post date:** [March 1, 2023, 8:18pm UTC](https://discourse.julialang.org/t/datatype-when-save-matrices-in-hdf5-file/95364/3 "2023-03-01T20:18:28Z")

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Hi David. Thanks a lot for the help. I will play with what you said.