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MathGroup Archive 2005

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Re: Importing (and dealing with) multiple files

  • To: mathgroup at smc.vnet.net
  • Subject: [mg60799] Re: [mg60778] Importing (and dealing with) multiple files
  • From: "David Annetts" <davidannetts at aapt.net.au>
  • Date: Thu, 29 Sep 2005 05:41:07 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Hi Dan, 

> I have experimental data that is stored in a number of files. 
>  I've been importing the files (as tables) into Mathematica 
> one-by-one, and working on them like so:
> 
> rawdata=Import["C:\\filename.ASC", "Table"];
> 
> croppedcorrelation=Take[rawdata,{75,185}];
> 
> statistics=NonlinearRegress[croppedcorrelation, math_here...]
> 
> then painstakingly picking out the relevant statistical 
> parameter from each file, collecting them all, and later 
> plotting them.  What I'd really like to do is import an 
> entire directory's worth of these files (each file is a 
> different time point), do the same statistical thing to each 
> file, save the relevant parameters from each file in a table 
> form and then plot the whole shebang.
> 
> One of the problems is that I don't know ahead of time how 
> many .ASC files are in the directory, so I need to set up the 
> For loop (or whatever structure) to account for that.  I 
> tried using FileNames[] to get the list of files, assigning 
> them to an array, then using Import[] on each of those array 
> elements.  It didn't work.

Stating exactly how it didn't work (eg posting code) would've been useful.

> Does anyone have any suggestions?

You shouldn't need to use a For loop for this, and things are quite
straightforward if you can use Import[] instead of some customised reading
function.  For example, we can set up some temporary data in a temporary
directory using 

	SetDirectory["c:/Tmpfiles"]

	outf = "test_";
	outf = StringJoin[outf, ToString[PaddedForm[#, 3, NumberPadding ->
{"0", "0"}, NumberSigns -> {"", ""}]], ".dat"] & /@ Range[5]

	(
      test = {#, 10 * Exp[-#/10] + .5 * Random[]} & /@ Range[128];
      Export[outf[[#]], test, "TABLE"]
      ) & /@ Range[Length@outf]

Now, read them using 

	ifil = FileNames["test*.dat"];
	data = Import[ifil[[#]], "TABLE"] & /@ Range[Length@ifil];

We can plot them using 

	(
      	ListPlot[
	      	data[[#]], PlotRange -> {0, 10}, PlotLabel ->
StringJoin["File : 
      	    	", ifil[[#]]]];
      ) & /@ Range[Length@ifil]

Or 

	Needs["Graphics`Graphics3D`"]
	iplt = ListPlot[#, DisplayFunction -> Identity] & /@ data;
	Show[StackGraphics[iplt], DisplayFunction -> $DisplayFunction];

And perform the regressions using

	stat = NonlinearRegress[data[[#]], a Exp[b x], x, {{a, 10}, {b,
-1}}] & /@ Range[Length@ifil];

We can now pick the interesting bits from stat, for example

	best = BestFitParameters /. stat
	evar = EstimatedVariance /. stat

Regards,

Dave.


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