Re: Storing an array element as a scalar in a record

["Johnson, Andrew N. via Tech-talk" <tech-talk at aps.anl.gov>]

Hi Andy,

On Mar 5, 2021, at 7:39 PM, Wang, Andrew via Tech-talk <tech-talk at aps.anl.gov> wrote:

So I have a waveform record that allows four numbers to be stored. I tried to use the subArray record to get the ith element from the array. I set the MALM, NELM and INDX fields to be 1, 1, and 2 respectively. However, when I process the subArray record, I get the first element of the array, rather than the third element.

The Array Parameters section of the subArray Reference page says:

The user specifies the maximum number of elements that can be read into the subarray in the MALM field. This number should normally be equal to the number of elements of the Waveform array (found in the Waveform's NELM field). The MALM field is used to allocate memory. The subArray's Number of Elements (NELM) field is where the user specifies the actual number of elements that the subArray will extract.

Because of the link API it uses the subArray record has to fetch the complete array from the waveform record before it can extract the part that you want, so it needs enough memory to hold the whole array. That’s why you need to set MALM to 4, not 1.

If this is not a viable way to go about it, are there other potential ways to store a scalar from an array in a record aside from using an asub record?

If you don’t need the index to change, there is a simpler way to extract a specific element from an array, and the latest EPICS release 7.0.5 makes that even better. The array channel filter lets you specify the subarray as part of a PV name; prior to 7.0.5 you had to explicitly use the CA protocol for it, but in 7.0.5 this syntax now works with DB links as well. 

An ai record retrieving the 2nd element from a waveform record named wf would have have an INP field something like this:

field(INP, "wf.[1] CA")

In 7.0.5 that can become this:

field(INP, "wf.[1]")

HTH,

- Andrew

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Complexity comes for free, simplicity you have to work for.