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I've got this CSV file I've imported that has tens of millions of lines in it. It takes around 20 minutes to import. I've been working with it for a while and have the processed data spread out in a bunch of variables.

Now Windows is bugging me that I need to restart the computer.

I thought about gathering all the data up in a table and then export and import it, but that would be a lot of hassle and take ages. I also thought about just saving the notebook and re-evaluate it, but with this amount of data that will also take a long time.

I wonder what is the best way to save all the data so that I can get it back after having restarted the computer? Something fast and with minimum of hassle would be great.

PS. I have no idea how to tag this thing. There is apparently no big-data tag.

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2 Answers 2

up vote 18 down vote accepted

Assuming you haven't placed your variables in a non-standard context you can save them all at once using DumpSave's second syntax form, which saves everything in the indicated context.

Quit[] (* start a fresh kernel *)

x = 1; (* define some symbols *)
y = 2;
z[x_] := x^2

Names["Global`*"] (* Check they're there *)

(* ==> {"x", "y", "z"}  *)

(* Save everything in the context *)
DumpSave["C:\\Users\\Sjoerd\\Desktop\\dump.mx", "Global`"];    

Quit[] (* kill kernel to simulate a new start *)

Names["Global`*"] (* Are we clean? *)
(* ==> {} *)

(* Get the save symbols *)
<< "C:\\Users\\Sjoerd\\Desktop\\dump.mx"

(* Are they there? *)
(* ==> {"x", "y", "z"} *)

(* ==> 4 *)
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Works wonderfully, thanks! –  Mr Alpha May 12 '12 at 21:28
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You can export to the MX format:

Export["out.mx", data, "MX"]

Be aware that this format is not portable between different computer architectures (e.g. 32/64 bit).

Import using

data = Import["out.mx"];

This is the fastest available format in Mathematica. Most likely you can't do better than this, unless you write an interface to a specialized external library.

Related reading:

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+1 for the link collection! –  Yves Klett May 11 '12 at 12:49
@Yves I feel we need a "canonical question" for this, but I wasn't able to find an existing question which would be suitable without a lot of editing. Maybe eventually we can repurpose this one. This question is quite straightforward, without many extra details, but it'll be necessary to write a good answer which explains the caveats of MX, the difference between DumpSave and Export and mentions Export["out.mz", Compress[expr], "String"] ... –  Szabolcs May 11 '12 at 12:52
... as a cross-platform alternative (something like Mr. Wizard's answer, made a bit more beginner friendly). –  Szabolcs May 11 '12 at 12:53
This still means I have to do it separately for each variable or collect all the data up into one table? –  Mr Alpha May 11 '12 at 13:23
@OleksandrR. Of course, it would be very nice if we could have a supported interface (or, rather, a specification) for creation of .mx files (I mean, not from within Mathematica). This would allow one to write very fast converters from a given external format to .mx, which would load way faster (and save tones of memory as well) than whatever we can now write using the Import-Export framework. –  Leonid Shifrin May 11 '12 at 14:41
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