Fastest way to import single column?

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Brian
Brian on 13 Jan 2014
Commented: Kelly Kearney on 14 Jan 2014
I am attempting to import just a single column (a textual date field) from a large file with many columns and many rows. When I use textscan to import only a single column, it still takes a very long time to import my column (file is approx 700 MB). This leads me to believe that it's reading the entire file and bringing back the column that I am specifying. Can someone offer up a faster solution to import a single column from a large text file? The text file can have any number of rows and any number of columns.
Currently I am doing the following -
%Import Date field only to check latest date in file
frmt = '%*s %s %*[^\n]';
dates = textscan(fid,frmt,'delimiter',',','Headerlines',1);
fclose(fid);
dates = dates{1};
Thanks a lot! Brian
  1 Comment
José-Luis
José-Luis on 13 Jan 2014
There's no going around reading most of the file for these kinds of problems unless you do some kind of preprocessing or generate only the data you need.

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Answers (2)

David Sanchez
David Sanchez on 13 Jan 2014
From matlab's documentation:
Using a text editor, create a file scan1.dat that contains data in the following form:
09/12/2005 Level1 12.34 45 1.23e10 inf Nan Yes 5.1+3i
10/12/2005 Level2 23.54 60 9e19 -inf 0.001 No 2.2-.5i
11/12/2005 Level3 34.90 12 2e5 10 100 No 3.1+.1i
Read the first column of the file into a cell array, skipping the rest of the line:
fid = fopen('scan1.dat');
dates = textscan(fid, '%s %*[^\n]');
fclose(fid);
  2 Comments
David Sanchez
David Sanchez on 13 Jan 2014
Reading from a file usually is a time consuming task. Reading from such a big file ( 700 MB ) will take a long time no matter how you try to do it. The method you are using might be the best solution for the task.

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Kelly Kearney
Kelly Kearney on 13 Jan 2014
It's not a Matlab solution, but I have found that pre-processing in Perl (split columns and spit out a one-column intermediate file) and then using load to bring it into Matlab is faster than textscan.
  4 Comments
Kelly Kearney
Kelly Kearney on 14 Jan 2014
I don't think so... I've always found textscan to be the fastest Matlab-only way of reading in an ascii file. As others have said, there's really no way around reading the whole file and then extracting what you need. The perl program does the exact same thing: read in a full line, break it up based on commas, keep just one column... perl just happens to be very good and very fast when it comes to churning through large amounts of text.

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