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Re: Export, Import, Convert Types Again?
*To*: mathgroup at smc.vnet.net
*Subject*: [mg57245] Re: Export, Import, Convert Types Again?
*From*: Bill Rowe <readnewsciv at earthlink.net>
*Date*: Sat, 21 May 2005 02:41:22 -0400 (EDT)
*Sender*: owner-wri-mathgroup at wolfram.com
On 5/20/05 at 4:43 AM, leenewm at umich.edu (Lee Newman) wrote:
>Q: What is the (best) way to save a big expression of data to be
>used later in the same notebook without having to do type
>conversion processing?
>Situation:
>1) I have a notebook that imports data from various Excel
>spreadsheets and converts the data to the desired types. When
>complete, I "save" the expression containing the data using Export,
>i.e, Export["featureDB.tsv", "TSV"]. I do this so I don't have to
>re-process the data everytime I do analysis on it in this notebook.
>2) When I open the noteobook again, I import the data using the
>statement: featureDB = Import["featureDB.tsv"]
>Problem: Many of type conversions that I took the time to process
>are lost. Using Head /@ featureDB[[1]] before and after the
>export/re-import, the types are as follows:
>Before: {Integer, Integer, List, List, List, Integer, List, List,
>Symbol, List} After : {Integer, Integer, String, String, String,
>Integer, String, String, String, String}
>So it seems that lists become Strings as Symbols (e.g. True,
>False, Null). I use Export so I can view the data outside
>Mathematica using a text editor -- should I use Save instead? Is
>there another Export format that will preserve mathematica
>expressions like lists upon Import?
A combination of Import/Export is quite useful when data is in a format not native to Mathematica or when you want the data saved in a format easier to read in a different application. But once you have imported the data into Mathematica and plan only to use the data in Mathematica using either Put or DumpSave is much more efficient.
Put[expr, "filename"] or equivalently, expr >> "filename", writes all of the values associated with expr to a text file in a format Mathematica understands. Then in a future notebook you can restore things using Get as in expr = Get["filename"] or equivalently expr = << "filename".
An alternative which is more efficient, would be DumpSave. DumpSave["file.nx", symbol] writes everything associated with symbol including the name to a binary file. Then Get["file.mx"] or <<"file.mx" restores things including symbol names. In fact you can save the entire Global` context and restore it using DumpSave and Get.
The disadvantage of DumpSave is the binary format is not portable accross platforms. That is if I used DumpSave to create a file on my Mac and send it to someone using Windows, they will not be able to read the file correctly.
For myself, I typically use Export{"filename", expr, "CSV"] and modify the file with a text editor so that I can restor things using expr = << "filename". This is somewhat more effort than simply using Put. I prefer my method over Put becuase the file Put creates is not neatly formated and is consequently more difficult to read in something other than Mathematica.
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