FHIR Chat · Maintaining State During Iteration

Stream: cql

Topic: Maintaining State During Iteration

Pascal Brandt (Apr 14 2021 at 17:44):

@Bryn Rhodes, or anyone, is there any good way to maintain state during iteration? In my case, I'm trying to calculate a running total over a list of non-unique values. So, I want to turn something that looks kind of like:

L = { 1, 2, 2, 3, 4, 5 }

Into something that looks like:

T = { 1, 3, 5, 8, 12, 17 }

My elements are actually Tuples with a date and a value, but the above example illustrates the problem.

At first I tried recursion, but it seems like that isn't supported. Then I thought I could use IndexOf, Take, and Sum, but my elements are not unique. Is there a way to do this that I'm not thinking of? I'm using cql-to-elm to generate my ELM, and cql-execution to run it.

Chris Moesel (Apr 14 2021 at 18:00):

Hi @Pascal Brandt! Are you able to use the soon-to-be-published CQL 1.5 version? If so, you might want to take a look at Aggregate Queries. If I understand what you're trying to do correctly, I think this would provide a pretty straight-forward approach.

The CQL-to-ELM 1.5.2 translator supports it already and we've also added support in cql-execution (thanks to @Rob Dingwell) -- but note that it is not yet released in cql-execution. We can probably cut a new release of cql-execution soon if you think this will be helpful.

Pascal Brandt (Apr 14 2021 at 18:07):

I'm using all kinds of custom builds, so I don't think pulling in new features is going to be a problem. I'm already on 1.5.2 of cql-to-elm. I'm using the CQL Testing Framework for execution, but I'm on a custom build of that as well, so let me update cql-execution there. Any gitsha you can recommend?

Thanks @Chris Moesel!

Chris Moesel (Apr 14 2021 at 18:47):

I haven't tried loading the cql-execution package using a reference to it on GH. Hopefully that works OK. You can use the HEAD of master right now, which is sha 160b69e29a2d88e0891276986a551661772df4b5.

Pascal Brandt (Apr 14 2021 at 20:04):

Just as a follow-up, everything seems to be working as expected. Here are the steps I followed in case anyone wants to do the same.

First I added a resolution in my package.json to grab the gitsha I wanted:

"resolutions": {
  "cql-execution": "cqframework/cql-execution#160b69e"
}

Then, I had to add a postinstall script to build the (required) lib directory:

"scripts": {
  "postinstall": "yarn --cwd ./node_modules/cql-execution install"
}

Chris Moesel (Apr 14 2021 at 20:16):

Ah, yes. It was that postinstall step that had me concerned. Thanks for sharing. And... let us know how the aggregate query approach works for you!

Pascal Brandt (Apr 14 2021 at 22:02):

It worked perfectly! Here's my expression:

define "Sorted Dates With Totals":
    (Tail("Sorted Dates With Counts")) DC
        aggregate all DT starting ({
            Tuple {
                date: First("Sorted Dates With Counts").date,
                total: First("Sorted Dates With Counts").count
            }
        }): flatten {
            DT,
            Tuple {
                date: DC.date,
                total: Last(DT).total + DC.count
            }
        }

"Sorted Dates With Totals" is of type List<Tuple { date System.DateTime, count Integer }>. The only thing that tripped me up initially is the default to set semantics. Adding all made sure duplicates weren't ignored.

Last updated: Apr 12 2022 at 19:14 UTC

Main menu

FHIR Chat · Maintaining State During Iteration · cql