[R] Importing random subsets of a data file
Greg Snow
538280 at gmail.com
Wed Jul 23 18:56:37 CEST 2014
For speed your best choice is probably to load your data into a
database, then pull your samples from the database. A simple database
is SQLite and there are R packages that work directly with that
database.
Can the later samples contain some of the same rows as previous
samples? Or once a row is used in a sample, it can never be used
again in a later sample? If the former you could use R to choose a
sample of "row numbers" then ask the database for those rows (some
databases have the concept of rows built in, others would need a
sequential column of "row numbers" added), then repeat for each
sample. If the later then you could add a column to the database
based on randomly generated numbers and create an index (sort) by that
column, then select the 1st n observations as the 1st sample, the next
n observations as the 2nd sample, etc.
On Wed, Jul 23, 2014 at 9:33 AM, Khurram Nadeem <khurram.nadee at gmail.com> wrote:
> Hi R folks,
>
> Here is my problem.
>
> *1.* I have a large data file (say, in .csv or .txt format) containing 1
> million rows and 500 variables (columns).
>
> *2.* My statistical algorithm does not require the entire dataset but just
> a small random sample from the original 1 million rows.
>
> *3. *This algorithm needs to be applied 10000 times, each time generating a
> different random sample from the 'big' file as described in (2) above.
>
> Is there a way to 'import' only a (random) subset of rows from the .csv
> file without importing the entire dataset? A quick search on various R
> forums suggest that read.table() does not have this functionality.
> Obviously, I want to avoid importing the whole file because of memory
> issues. Looking forward to your help.
>
> Thanks,
> Khurram
> ------------------------
> Khurram Nadeem
> Postdoctoral Research Fellow
> Department of Mathematics & Statistics
> Acadia University, NS, Canada.
>
> [[alternative HTML version deleted]]
>
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--
Gregory (Greg) L. Snow Ph.D.
538280 at gmail.com
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