Code sample to concatenate JSON arrays

ObjectScript doesn't include any built-in method for appending one JSON dynamic array to another. Here's a code snippet I use that's equivalent to the JavaScript concat() method.

Call it with any number of arguments to concatenate them into a new array. If an argument is a dynamic array, its elements will be added. Otherwise the argument itself will be added.

ClassMethod ConcatArrays(pArgs...) As %DynamicArray
{
    set outArray = ##class(%DynamicArray).%New()
    for i=1:1:pArgs {
        set arg = pArgs(i)
        if ($IsObject(arg) && arg.%IsA("%DynamicArray")) {
            set iter = arg.%GetIterator()
            while iter.%GetNext(.key, .value) {
                do outArray.%Push(value)
            }
        } else {
            do outArray.%Push(arg)
        }
    }
    return outArray
}

Feel free to let me know if there's a better way to do this!

Julius Kavay · May 2, 2020

In case, you want some speed, use

set outArray = []

instead of

set outArray = ##class(%DynamicArray).%New()

Your speed gain: ca. 50%

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Ashok Kumar T · Nov 4

Hi @Julius Kavay
The literal array syntax set outArray = [] internally creates a %DynamicArray object. If both approaches ultimately produce a %DynamicArray, why is using the literal syntax reportedly about 50% faster than explicitly calling set outArray = ##class(%DynamicArray).%New()?

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Julius Kavay · Nov 4

Well, I'm neither part of the ObjectScript nor the Objects developer team, hence I can't answer the "why" part of your question but fact is, timing mesuremenst show a significat higher speeds for the literal versions:

ClassMethod DynObject()
{
	while $zh#1 {} set t1=$zh for i=1:1:1E6 { if ##class(%DynamicArray).%New()  } set t1=$zh-t1
	while $zh#1 {} set t2=$zh for i=1:1:1E6 { if ##class(%DynamicObject).%New() } set t2=$zh-t2
	while $zh#1 {} set t3=$zh for i=1:1:1E6 { if [] } set t3=$zh-t3
	while $zh#1 {} set t4=$zh for i=1:1:1E6 { if {} } set t4=$zh-t4
	
	write "Times for :    Class  Literal     Diff",!
	write "DynArray  :", $j(t1,9,3), $j(t3,9,3), $j(t1/t3-1*100,9,2),"%",!
	write "DynObject :", $j(t2,9,3), $j(t4,9,3), $j(t2/t4-1*100,9,2),"%",!
}

The output will depend on
- IRIS/Cache version in use and
- on the underlying hardware
My values are

USER>d ##class(DC.Times).DynObject()
Times for :    Class  Literal     Diff
DynArray  :    0.665    0.401    65.90%
DynObject :    0.649    0.401    61.87%

Maybe someone else or the WRC has an explanation...

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