# Using DataStructures in FunctionCompile?

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 This is a cross post from StackExchange.I am trying to do the equivalent of Select on a "FixedArray" data structure.Here is what I have tried so far: fixedArrayDS = CreateDataStructure["FixedArray", 2^10]; vanillaArray = RandomReal[{0, 1}, fixedArrayDS["Length"]]; Do[fixedArrayDS["SetPart", i, vanillaArray[[i]]], {i, Length[vanillaArray]}] I'd like to do the equivalent of this for a "FixedArray": Select[vanillaArray, # < 0.5 &] without extracting all the elements (i.e, this: Select[fixedArrayDS["Elements"], # < 0.5 &] is considered "cheating")Let's try to compile a function to do this (I am trying to teach myself how to use FunctionCompile with data structures). Sow and Reap seems a plausible way to do this. select = FunctionCompile[ Function[ {Typed[fa, "FixedArray"], Typed[bound, "Real64"]}, Last[ Reap[ Do[ If[fa["Part", i] < bound, Sow[fa["Part", i]]], {i, 1, fa["Length"]} ] ] ] ] ] doesn't work and comes back with a TypeError stating that a version of Reap can't be found that takes one argument. I'll come back to why Reap isn't going to work later and a hypothesis about using KernelFunction later.Setting up some sanity checks. Test and see if this is the correct syntax for FunctionCompile on this data structure. first = FunctionCompile[ Function[ {Typed[fa, "FixedArray"], Typed[multiplier, "Real64"]}, fa["Part", 1] multiplier ] ] this works first[fixedArrayDS, 1] so the FunctionCompile syntax seems to be correct.Another sanity check. See if Compile works with Reap and Sow: cf = Compile[{{l, _Real, 1}, {b, _Real}}, Last[Reap[ Do[ If[l[[i]] < b, Sow[l[[i]]]], {i, 1, Length[l]} ] ] ] ] which works, but is slower than the built-in Select (probably because it is a single thread?): RepeatedTiming[cf[vanillaArray, .5]] Back to the original problem. Getting clues from Tom Wickham Jones' talk in 2020. Reap is not going to work because it isn't in the following list: Needs["CompileUtilitiesSupport"] CompileUtilitiesSupportedSymbols[] Perhaps using KernelFunction in the FunctionCompile would work--but how would I use Type for Reap?Another solution might be to use another data structure that has a rapid Append: select = FunctionCompile[ Function[ {Typed[fa, "FixedArray"], Typed[bound, "Real64"]}, Block[{collection = CreateDataStructure["DynamicArray"]}, Do[ If[fa["Part", i] < bound, collection["Append", fa["Part", i]]], {i, 1, fa["Length"]} ]; collection ] ] ] But, this doesn't compile either.Finally, there is a hint that there is a solution for this task that might be in development. See around minute 51:30 in the TWJ talk linked above; it appears there is a package for using Iterators that would be a natural implementation for select. Needs["IteratorTools"] But it looks like it didn't ship with 12.3.