[R] how to fetch rows with certain characteristics

[Ista Zahn]

Hi,
I guess I don't understand why you think %in% won't work.

> x <- read.table(textConnection("1.2 1
+ 1.2 1
+ 1.3 1
+ 1.5 1
+ 2.1 2
+ 2.0 2
+ 9.9 2
+ 1.4 3
+ 1.8 3
+ 1.9 3") )
> x <- as.matrix(x)
> x.min <- tapply(x[,1], x[,2], min)
> x[x[,1] %in% x.min,]
> ## all matches
      V1 V2
[1,] 1.2  1
[2,] 1.2  1
[3,] 2.0  2
[4,] 1.4  3
> ## unique matches
> unique(x[x[,1] %in% x.min,])
      V1 V2
[1,] 1.2  1
[2,] 2.0  2
[3,] 1.4  3

-Ista
On Thu, Oct 29, 2009 at 12:36 PM, Waverley @ Palo Alto
<waverley.paloalto at gmail.com> wrote:
> Thanks.  That works.
>
> However, in my own case, there are more columns of other kinds of
> data.  So to me, it is more important to get the row index of those
> that has the min values of particular column in particular class
> (which is another column).
>
> Can you help more as how to get those row index? One issue is that for
> some class they may share the same min value so that using %in% does
> not work.  My goal is to reduce the original matrix size and get the
> result back in the original matrix format.
>
>
> Thanks.
>
> On Wed, Oct 28, 2009 at 11:55 PM, Ista Zahn <istazahn at gmail.com> wrote:
>> There are various ways, including
>>
>> x <- read.table(textConnection("1.2 1
>> + 1.3 1
>> + 1.3 1
>> + 1.5 1
>> + 2.1 2
>> + 2.0 2
>> + 9.9 2
>> + 1.4 3
>> + 1.8 3
>> + 1.9 3") )
>>
>> x <- as.matrix(x)
>>
>> x.min <- cbind(tapply(x[,1], x[,2], min), unique(x[,"V2"]))
>>
>> Most of that is just formatting it in the way you requested. All you
>> need to compute the values is
>>
>> tapply(x[,1], x[,2], min)
>>
>> -Ista
>>
>> On Thu, Oct 29, 2009 at 1:47 AM, Waverley @ Palo Alto
>> <waverley.paloalto at gmail.com> wrote:
>>> Hi,
>>>
>>> I have a matrix, first column is of certain values, second column is
>>> the class labels or a factor.
>>> e.g.
>>>
>>> 1.2 1
>>> 1.3 1
>>> 1.3 1
>>> 1.5 1
>>> 2.1 2
>>> 2.0 2
>>> 9.9 2
>>> 1.4 3
>>> 1.8 3
>>> 1.9 3
>>>
>>> I want to find out what is the min values of column 1 for each
>>> corresponding class (column 2).  For the above example, I want to
>>> return a matrix of
>>> 1.2 1
>>> 2.0 2
>>> 1.3 3
>>>
>>> Can someone suggest how to code for that?  The second column can be of
>>> characters.
>>>
>>> Thanks much.
>>>
>>>
>>> --
>>> Waverley @ Palo Alto
>>>
>>> ______________________________________________
>>> R-help at r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>>> and provide commented, minimal, self-contained, reproducible code.
>>>
>>
>>
>>
>> --
>> Ista Zahn
>> Graduate student
>> University of Rochester
>> Department of Clinical and Social Psychology
>> http://yourpsyche.org
>>
>
>
>
> --
> Waverley @ Palo Alto
>



-- 
Ista Zahn
Graduate student
University of Rochester
Department of Clinical and Social Psychology
http://yourpsyche.org