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Re: runs slowly?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg122313] Re: runs slowly?
  • From: DrMajorBob <btreat1 at austin.rr.com>
  • Date: Mon, 24 Oct 2011 05:16:03 -0400 (EDT)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <201110231025.GAA10587@smc.vnet.net>
  • Reply-to: drmajorbob at yahoo.com

t = {"14-May-10 09:58:05", {"Day", "-", "MonthNameShort", "-",
     "YearShort", " ", "Hour", ":", "Minute", ":", "Second"}};
tList = Table[t, {i, 100000}];
a = Timing[AbsoluteTime[#] & /@ tList;]

{63.1612, Null}

AbsoluteTime is taking about 6 ten thousandths of a second for each call,  
which doesn't seem unreasonable... but this is twenty times faster:

t = {2010, 5, 14, 9, 58, 5};
tList = Table[t, {i, 100000}];
a = Timing[AbsoluteTime[#] & /@ tList;]

{3.05313, Null}

...if you could have gotten dates in that format beforehand.

This is ten times faster than the original:

DateString[#, "MonthNameShort"] & /@ Table[{2010, i}, {i, 12}];
twelve = Dispatch@Thread[% -> Range@12];
convert =
   ToExpression[
     StringSplit[#, {"-", " ", ":"}][[{3, 2, 1, 4, 5, 6}]] /. twelve] &;
t = {"14-May-10 09:58:05", {"Day", "-", "MonthNameShort", "-",
     "YearShort", " ", "Hour", ":", "Minute", ":", "Second"}};
tList = Table[t, {i, 100000}];
a = Timing[AbsoluteTime@convert@First@# & /@ tList;]

{6.24957, Null}

You can use "convert" to change all your dates to list format, and then  
use AbsoluteTime only when it's needed.

You REALLY don't want to store 100,000 identical copies of {"Day", "-",  
"MonthNameShort", "-",
     "YearShort", " ", "Hour", ":", "Minute", ":", "Second"}.

Bobby

On Sun, 23 Oct 2011 05:25:28 -0500, Robert McHugh  
<bob_mchugh_2000 at yahoo.com> wrote:

> The following code takes over a minute to run on my machine.  Is this  
> expected behavior?
>
> t = {"14-May-10 09:58:05", {"Day", "-", "MonthNameShort", "-",  
> "YearShort", " ", "Hour", ":", "Minute", ":", "Second"}};
> tList = Table[t, {i, 100000}];
> a = Timing[AbsoluteTime[#] & /@ tList ;]
>
> For reference, an operation like the following takes less than a tenth  
> of a second. (Of course this second example needs quite a bit of  
> modification to provide a correct answer, but it does show how fast the  
> program can operate on a large list.)
> t = {14, 5, 10, 9, 58, 05};
> tList = Table[t, {i, 100000}];
> a = Timing[( ((#[[3]] 0 + #[[2]] 30 + #[[1]]24) + #[[4]]) 60 + #[[5]])  
> 60 + #[[6]] & /@ tList;]
>
> Some background: am analyzing some historical data (about 500 000  
> records, one data point a minute for about a year) and am making a few  
> utilities to retrieve the data for any given time interval.   My  
> original plan was to change the time stamp to absolute time and then use  
> a select statement.  This step in the above example, changing the time  
> stamps to absolute time, is the rate limiting step in the code  
> (everything else runs in about 5 seconds).
>
> Was wondering if someone could explain why AbsoluteTime[] is relatively  
> slow operation and perhaps some faster operations for date and time  
> comparisons.
>
> Thanks
>


-- 
DrMajorBob at yahoo.com



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