[R] Help searching a matrix for only certain records

[Matt Borkowski]

Thank you for your response Jim! I will give this one a try! But a couple followup questions...

In my search for a solution, I had seen something stating match() is much more efficient than subset() and will cut down significantly on computing time. Is there any truth to that?

Also, I found the following solution which works for matching a single condition, but I couldn't quite figure out how to  modify it it to search for both my acceptable conditions...

> testdata <- testdata[testdata$REC.TYPE == "SAO",,drop=FALSE]

-Matt




--- On Sun, 3/3/13, jim holtman <jholtman at gmail.com> wrote:

From: jim holtman <jholtman at gmail.com>
Subject: Re: [R] Help searching a matrix for only certain records
To: "Matt Borkowski" <mathias1979 at yahoo.com>
Cc: r-help at r-project.org
Date: Sunday, March 3, 2013, 8:00 AM

Try this:

dataset <- subset(dataset, grepl("(SAO |FL-15)", REC.TYPE))


On Sun, Mar 3, 2013 at 1:11 AM, Matt Borkowski <mathias1979 at yahoo.com> wrote:
> Let me start by saying I am rather new to R and generally consider myself to be a novice programmer...so don't assume I know what I'm doing :)
>
> I have a large matrix, approximately 300,000 x 14. It's essentially a 20-year dataset of 15-minute data. However, I only need the rows where the column I've named REC.TYPE contains the string "SAO  " or "FL-15".
>
> My horribly inefficient solution was to search the matrix row by row, test the REC.TYPE column and essentially delete the row if it did not match my criteria. Essentially...
>
>> j <- 1
>> for (i in 1:nrow(dataset)) {
>>    if(dataset$REC.TYPE[j] != "SAO  " && dataset$RECTYPE[j] != "FL-15") {
>>      dataset <- dataset[-j,]  }
>>    else {
>>      j <- j+1  }
>> }
>
> After watching my code get through only about 10% of the matrix in an hour and slowing with every row...I figure there must be a more efficient way of pulling out only the records I need...especially when I need to repeat this for another 8 datasets.
>
> Can anyone point me in the right direction?
>
> Thanks!
>
> Matt
>
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-- 
Jim Holtman
Data Munger Guru

What is the problem that you are trying to solve?
Tell me what you want to do, not how you want to do it.