# HDF5: varying size dataset

**URL:** <https://discourse.julialang.org/t/hdf5-varying-size-dataset/47126>\
**Category:** New to Julia\
**Tags:** question, hdf5\
**Created:** [September 23, 2020, 10:01am UTC](https://discourse.julialang.org/t/hdf5-varying-size-dataset/47126 "2020-09-23T10:01:13Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![iHany](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ihany/32/18151_2.png) [@iHany](https://discourse.julialang.org/u/iHany)\
**Post date:** [September 23, 2020, 10:01am UTC](https://discourse.julialang.org/t/hdf5-varying-size-dataset/47126/1 "2020-09-23T10:01:13Z")

</div>

I’m struggle with using `HDF5`.

I’m trying to append data in `.h5` file.  
So, I wanna make a dataset whose prescribed size is unlimited (on the first axis, specifically).  
For example, receiving data with dims `(5,6)`, then append the existing array with size `(n, 5, 6)` to be `(n+1, 5, 6)`.

I’ve read this [docs](https://juliaio.github.io/HDF5.jl/stable/#Supported-data-types), but I don’t understand what I have to do for that.

Here is an example code:

```julia
function test_hdf5()
    # using HDF5 (done at the beginning of this file)

    path = "data/tmp.h5"
    name = "asdf"
    val = zeros(1, 5, 6)

    h5open(path, "w") do dummy
    end # generate `data/tmp.h5`
    h5open(path, "r+") do h5file
        dset = d_create(
                        h5file,
                        name,
                        Float64,
                        (size(val), (-1, size(val)[2:end]...)), # (1)
                        "chuck", # (2)
                        size(val), # (3)
                       )
    end
end

```

1. do I have to write dims and max\_dims (see (1))? Can’t I just write information like `(typemax(Int64), 5, 6)`?
2. Do I have to add “chuck”? and `size(val)`?
3. To do what I need, what should be changed in the example code?

Sorry for my poor understand in use of `HDF5`.

---

<div class="post-metadata">

**Author:** ![iHany](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ihany/32/18151_2.png) [@iHany](https://discourse.julialang.org/u/iHany)\
**Post date:** [September 24, 2020, 6:35am UTC](https://discourse.julialang.org/t/hdf5-varying-size-dataset/47126/2 "2020-09-24T06:35:21Z")

</div>

I have NO idea but with some modifications it works (codes seems exactly the same to me though).

Anyway, this would help others to create a HDF5 dataset with multi-dimensional arrray data (very slightly different from [example](https://juliaio.github.io/HDF5.jl/stable/#Mid-level-routines), which is of one-dimensional array).  
Perhaps this post is helpful only for beginners like me ☹