[R] newbie question: grouping rows

[Thomas Lumley]

On Fri, 10 Mar 2006, Matthew Scholz wrote:

> Thanks for your response, Christos, but when I try this:
>
>> new = old[old$q.D1 < 0.05,]
>
> I get the following warning (and what I'm trying to do fails):
>
> Warning message:
> < not meaningful for factors in: Ops.factor(old$q.D1, 0.05)
>
> I've tried to bone up on the topic of factors and searching R help,
> but I'm still not sure why my data is being converted to that class
> nor do I know how to undo the conversion or work around it. Thanks
> again in advance of your advice. Any suggestions?


How to undo the conversion is FAQ 7.10

 	-thomas



>
> Matt
>
> On 3/9/06, Christos Hatzis <christos at silicoinsights.com> wrote:
>> You can try:
>>
>> new.dataframe <- my.dataframe[my.dataframe$p.value < 0.05, ]
>>
>> This will select all columns.  Alternatively, you can specify the columns
>> that you want after the ",".
>>
>> -Christos
>>
>> -----Original Message-----
>> From: r-help-bounces at stat.math.ethz.ch
>> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Matthew Scholz
>> Sent: Thursday, March 09, 2006 5:18 PM
>> To: r-help at stat.math.ethz.ch
>> Subject: [R] newbie question: grouping rows
>>
>> Hi all,
>>
>> I have a very simple question that I can't seem to find the answer to.
>> How do I extract rows that meet a certain criteria  from a data frame and
>> group them into a new data frame? For example, if I want to make a new data
>> frame that only includes rows of data for which the p values (given by one
>> of the columns in the data frame) are less than a certain value, how do I do
>> this? It seems that there should be a simple function that does this. I
>> looked into getGroups from the nmle package, but am not sure how to
>> construct the form argument correctly or even if it's the appropriate way to
>> tackle this.
>>
>> Thanks in advance of your answer,
>>
>> Matt
>>
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>>
>>
>
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Thomas Lumley			Assoc. Professor, Biostatistics
tlumley at u.washington.edu	University of Washington, Seattle