There are similar questions to this like list search but this question is focused on finding at subset of dates in DateList format within a range of specified dates from the original key list. The actual list of events is large at ~300,000 x 60 table covering 18 months of activities. The dates are indexed but this maybe not the best approach for find the subset items. I have generated a sample set of the same format using

sampledata = Table[DateString[DatePlus[DateList[][[1 ;; 3]],
  RandomInteger[547]], {"Month",",", "Day", ",", "Year"}], {i, 2000}];

I am attempting to use the keys from position index to

dates = PositionIndex[ DateList[{#, {"Month", "Day", "Year"}}][[1 ;; 3]] & /@ sampledata]

but the search method may be too slow to embed in a Manipulate loop with the date limits dynamically selected from the keys to display the other data associated with the dates.

upperlimit = {2015, 1, 30};
lowerlimit = {2015, 1, 15};

datekeys = Flatten[Position[(DateDifference[#, upperlimit ] > Quantity[0, "Days"] 
&& DateDifference[#, lowerlimit] < Quantity[0, "Days"]) & /@ dates, True]]

selecteddates = Sort[Flatten[dateindex[dates[[#]]] & /@ datekeys]]

Gives the index of the original data list


I chose this method to generate the date index outside of the Manipulate call since the other functions are fast. Any suggestions on how to speed up the process?


Try working with DateObjects instead of strings from DateString.

sampleDates = 
  DateObject[(FromDigits /@ StringSplit[#, ","])[[{3, 1, 2}]]] & /@ sampledata;

{upperlimit, lowerlimit} = 
  DateObject/@ {{2017, 1, 30}, {2017, 1, 1}};

With everything as DateObjects the matching does not need to translate from a string into a date for every comparison. Now operators like Between can be used directly.

selecteddates = 
  Flatten[Position[sampleDates, #] & /@ 
    Select[sampleDates, Between[{lowerlimit, upperlimit}]]];


If you want to view dates in that particular format then use the DataFormat option of DateObject.

Hope this helps.

  • $\begingroup$ I tried Parallelize@Select to try and speed it up but it doesn't help as it does not parallel select. $\endgroup$ – Edmund Dec 2 '15 at 9:25

Adapting Edmund's answer to Dataset - given sampleDates:

ds = Dataset[sampleDates][
 Select[Between[DateObject@{{2017, 1, 1}, {2017, 1, 30}}]]]

Then, can easily post process, eg, makeshift calendar:

ds[GroupBy[DateValue[#, "Week"] &] /* KeySort, Sort]

enter image description here

Who knows why Jan 1 is in week 52.


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