import - Stream CSV or TSV files

RAM limitations make import impractical for large CSV files. Is it possible to Stream CSV correctly?

str = OpenRead["EU.csv"];
Read[str, "CSV"]

Read::readn: Invalid real number found when reading from EU.csv

Read@str

Read::readt: Invalid input found when reading "lTid, cDealable, CurrencyPair, RateDateTime, RateBid, RateAsk" from EU.csv

Close@str

Here's the CSV file. Though I think it's properly formatted.

Answer

Here is a function which may help:

Clear[readRows];
readRows[stream_, n_] :=
   With[{str = ReadList[stream, "String", n]},
      ImportString[StringJoin[Riffle[str, "\n"]], "Table"] /; str =!= {}];

readRows[__] := $Failed;

I tested on your file and it works all right (it may make sense to read rows in batches, this is much faster):

n=0;
str = OpenRead["C:\\Temp\\EUR_USD_Week1.csv"];
While[readRows[str, 1000] =!= $Failed, n++];
Close[str];
n

(*  82   *)

By the way, speaking of practicality of Import - I agree, but do read this answer - it is based on the same idea as above code, and makes importing whole files quite practical IMO. In particular, using the readTable function from there, I get your file read in its entirety under 3 seconds on my machine:

In[64]:= readTable["C:\\Temp\\EUR_USD_Week1.csv",2000]//Short//AbsoluteTiming
Out[64]= {2.4775391,{{lTid,cDealable,CurrencyPair,RateDateTime,RateBid,RateAsk},
  <<81172>>,{1385715072,D\[Ellipsis] SD,2011-01-07,\[Ellipsis] }}}

with a very decent memory usage:

MaxMemoryUsed[]

(* 71652808 *)

meaning 50 Mb of net usage (when you subtract the startup memory usage) - for this particular example. You can tweak the second parameter to trade run-time for memory efficiency. See the linked post for more details.

Comments

front end - keyboard shortcut to invoke Insert new matrix

I frequently need to type in some matrices, and the menu command Insert > Table/Matrix > New... allows matrices with lines drawn between columns and rows, which is very helpful. I would like to make a keyboard shortcut for it, but cannot find the relevant frontend token command (4209405) for it. Since the FullForm[] and InputForm[] of matrices with lines drawn between rows and columns is the same as those without lines, it's hard to do this via 3rd party system-wide text expanders (e.g. autohotkey or atext on mac). How does one assign a keyboard shortcut for the menu item Insert > Table/Matrix > New... , preferably using only mathematica? Thanks! Answer In the MenuSetup.tr (for linux located in the $InstallationDirectory/SystemFiles/FrontEnd/TextResources/X/ directory), I changed the line MenuItem["&New...", "CreateGridBoxDialog"] to read MenuItem["&New...", "CreateGridBoxDialog", MenuKey["m", Modifiers-...

How to thread a list

I have data in format data = {{a1, a2}, {b1, b2}, {c1, c2}, {d1, d2}} Tableform: I want to thread it to : tdata = {{{a1, b1}, {a2, b2}}, {{a1, c1}, {a2, c2}}, {{a1, d1}, {a2, d2}}} Tableform: And I would like to do better then pseudofunction[n_] := Transpose[{data2[[1]], data2[[n]]}]; SetAttributes[pseudofunction, Listable]; Range[2, 4] // pseudofunction Here is my benchmark data, where data3 is normal sample of real data. data3 = Drop[ExcelWorkBook[[Column1 ;; Column4]], None, 1]; data2 = {a #, b #, c #, d #} & /@ Range[1, 10^5]; data = RandomReal[{0, 1}, {10^6, 4}]; Here is my benchmark code kptnw[list_] := Transpose[{Table[First@#, {Length@# - 1}], Rest@#}, {3, 1, 2}] &@list kptnw2[list_] := Transpose[{ConstantArray[First@#, Length@# - 1], Rest@#}, {3, 1, 2}] &@list OleksandrR[list_] := Flatten[Outer[List, List@First[list], Rest[list], 1], {{2}, {1, 4}}] paradox2[list_] := Partition[Riffle[list[[1]], #], 2] & /@ Drop[list, 1] RM[list_] := FoldList[Transpose[{First@li...

dynamic - How can I make a clickable ArrayPlot that returns input?

I would like to create a dynamic ArrayPlot so that the rectangles, when clicked, provide the input. Can I use ArrayPlot for this? Or is there something else I should have to use? Answer ArrayPlot is much more than just a simple array like Grid : it represents a ranged 2D dataset, and its visualization can be finetuned by options like DataReversed and DataRange . These features make it quite complicated to reproduce the same layout and order with Grid . Here I offer AnnotatedArrayPlot which comes in handy when your dataset is more than just a flat 2D array. The dynamic interface allows highlighting individual cells and possibly interacting with them. AnnotatedArrayPlot works the same way as ArrayPlot and accepts the same options plus Enabled , HighlightCoordinates , HighlightStyle and HighlightElementFunction . data = {{Missing["HasSomeMoreData"], GrayLevel[ 1], {RGBColor[0, 1, 1], RGBColor[0, 0, 1], GrayLevel[1]}, RGBColor[0, 1, 0]}, {GrayLevel[0], GrayLevel...

Blog

Search This Blog