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performance tuning - Speed up Flatten[] of a large nested list


I have a large jagged list, that is each sub-list has a different length. I would like to Flatten this list for Histogram purposes, but it seems to be taking an inordinate amount of time and memory


jaggedList=Table[RandomReal[1,RandomSample[Range[400000,800000],1]],{n,100}];

Just to illustrate, length of each of elements of the main list



ListPlot[Length/@jaggedList]

list lengths


Full Flatten takes a long time, my real data is several times larger, it gets painfully slow


fullFlatten=Flatten@jaggedList;//AbsoluteTiming
{10.0055,Null}

I noticed flattening non-jagged sub-lists is not a problem


partialFlatten=Flatten/@jaggedList;//AbsoluteTiming
{0.289219,Null}


Memory usage is huge on the final result of the full list, even though number of elements is the same:


ByteCount/@{fullFlatten,partialFlatten,jaggedList}
{1460378864,486808224,486808224}

Would super appreciate any tips on what I can change to make this faster / more memory compact !



Answer



Applying Join is much faster than Flatten:


SeedRandom[1]
jaggedList = Table[RandomReal[1, RandomSample[Range[400000, 800000], 1]], {n, 100}];


fullFlatten = Flatten@jaggedList; // AbsoluteTiming // First


8.2375848



fullFlatten2 = Join @@ jaggedList; // AbsoluteTiming // First


0.29729




fullFlatten2 == fullFlatten


True



ByteCount /@ {fullFlatten, fullFlatten2, jaggedList}


{1462957016, 487652456, 487667608}




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