Re: why DateListPlot is so slow?

*To*: mathgroup at smc.vnet.net*Subject*: [mg79948] Re: why DateListPlot is so slow?*From*: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>*Date*: Thu, 9 Aug 2007 05:24:41 -0400 (EDT)*Organization*: The Open University, Milton Keynes, UK*References*: <f9c0ae$5o4$1@smc.vnet.net>

Arkadiusz.Majka at gmail.com wrote: > Hi, > > I have a list (a time series): > > data={{00:00:30,x1},{00:01:00,x2},{00:01:30,x3},....} > > at equally spaced time intervals (30 seconds). The length od data is > 8100 (24 h) > > DateListPlot[data] works fine but takes 28 sec, since > ListPlot[data[[All,2]]] does it in less than 0.1 sec.... > > Can you explain me why DateListPlot is so time consuming? Can we > improve it? Hi Arek, It seems that the slowness of *DateListPlot* is due to the conversion process that occurs to set up the date/time in canonical form. First we generate some random data and compare *DateListPlot* against *ListPlot*. In[1]:= data = Module[{t = 0}, Table[{DateString[t += 10 + RandomInteger[{-1, 1}], "Time"], 40000. + 10000*Sin[t/20000] + RandomReal[{-100, 100}]}, {8100}]]; Timing[DateListPlot[data]][[1]] Timing[ListPlot[data[[All, 2]], Axes -> None, Frame -> True, GridLines -> {Automatic, None}]][[1]] Out[2]= 33.172 Out[3]= 0.016 My understanding of how *DateListPlot* works is that the function first converts the date/time data in canonical form, canonical form that must be in the form {yy, mm, dd, hh, mn, ss}. (The function in charge of the conversation is set up with the option *DateFunction*.) This seem to be very time consuming. In[4]:= Timing[DateList /@ data[[All, 1]]; ][[1]] Out[4]= 32.11 Now, if we massage first the data and then use *DateListPlot*, the speed improvement is dramatic (though still not as fast as *ListPlot*). In[5]:= Timing[data2 = Transpose[{Apply[Join, Transpose[ {Table[{2007, 1, 1}, {Length[data]}], (ToExpression[StringSplit[#1, ":"]] & ) /@ data[[All, 1]]}], {1}], data[[All, 2]]}]; ][[1]] Timing[DateListPlot[data2]][[1]] %% + % Timing[ListPlot[data2[[All, 2]], Axes -> None, Frame -> True, GridLines -> {Automatic, None}]][[1]] Out[5]= 0.265 Out[6]= 0.672 Out[7]= 0.937 Out[8]= 0.016 In[8]:= $Version Out[8]= "6.0 for Microsoft Windows (32-bit) (June 19, 2007)" -- Jean-Marc