Appending 1D arrays to a 2D dataset

[Pablo Fernandez Lopez]

Hi all,

I would like to build a 2D dataset by adding 1 extra line in every consecutive save. That is:

First save:
0.2 0.1 0.3 0.5

Second save:
0.2 0.1 0.3 0.5
0.8 0.5 0.6 0.4

and so on. Whenever the user triggers the save action, a new line (trace) is added. However the H5Dcreate_simple.vi seems to interpret every input as a 1D array, and will concatenate the vectors like:

0.2
0.1
0.3
0.5
0.8
0.5
0.6
0.4

If anyone knows a way to do this or a workaround I would be very grateful.

Thank you,
Pablo

[Martijn Jasperse]

Hi Pablo,
The key is how you create and fill the dataset as it must correctly index a 2D array. I've attached an example showing how to achieve this.

Cheers.

[Gerry G.]

Hi Martijn,
I have a follow up question.

I tried your append-dataset-2D.vi example with the 2D strings instead of I32s and it generates a 1D dataset with only first record (0 1 2 3 4) in it. I converted 2D I32 data to 2D strings and wired to the type in terminal of the H5Dcreate_simple function and data terminal of the H5Dwrite.xnode function. As far as I understand, each record must be of a fixed size and strings are variable by default so I wired a "<32>" string constant to the type name terminal of the H5Dwrite.xnode. What I am missing? Is it possible to append 1D string arrays to a 2D string dataset?

Thanks,

Gerry.

[Martijn Jasperse]

Hi Gerry,
Thanks for your question, it raises an interesting point. The "type" of the dataset must be defined at creation time, and cannot be changed after that (not without deleting/recreating the dataset). When using H5Dcreate_simple to initialise the empty dataset, the type is locked in. So whatever you feed into the subsequent H5Dwrite.xnode will be ignored (unless you're writing a compound type, which is a separate issue) because the type was already defined.

This points out a bug in H5Dcreate_simple, because it's currently not possible to create a fixed-length string dataset with this function. I will add this to the bug database and fix it when I have the chance.

There are two immediate solutions: one is to use variable length strings, which is perfectly valid (I'm not sure why you say the records must have a fixed size? Unless that is the spec you must conform to) and should work out of the box. The second is to use H5Dcreate instead of H5Dcreate_simple to create a dataset with the correct fixed-length string type. This is a bit more involved but the example "zipped-dataset.vi" shows how it can be done. If you want to pursue this I could make an example.

But either approach should let you append strings as desired.

Cheers,
Martijn

[Gerry G.]

Hi Martijn,
Thanks for the prompt reply.

I removed the <32> fixed length string constant as per your advice. It still does not append the new data. See modified example in the attachment.

Thanks,
GG.

[Martijn Jasperse]

Hi Gerry,
Can you export your VI to LV2010? I can't open this as it's a LV2013 version. I've attached an example that works for me to append to a 2D dataset of strings. It might not look like it because of how HDFView handles arrays of strings, but does indeed produce a 2D dataset.

Cheers,
Martijn

[Gerry G.]

You are right, Martijn. HDFView deceived me.