I’ve spotted the following common data access patterns.

Get the Only Element in an Array

You need to get the only element of an array as in

my_val = my_arr[0]

Additionally you want to express the assumption that there should be exactly one element in the array. In NGS it’s simple:

my_val = my_arr.the_one()

the_one() will return the only element or will throw an exception if there is not exactly one element in the given array.

Get the Only Matching Element in an Array

You have an array. Only one element should satisfy some condition. You want to access that element. Again, there is a straightforward way to express this in NGS:

my_instance = instances.the_one({"InstanceId": my_id})

The use of the_one(...) here is again about the assumption that there should be exactly one instance with the given instance id in the instances array. Exception will be thrown by the_one(...) if that is not the case.

Update 2020-08-08: As pointed out, the method is not unique to NGS.

Update 2021-02-14: FHIRPath also has somewhat similar function: single().

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Happy coding and have a nice week!