[R] how to fetch rows with certain characteristics

[Waverley @ Palo Alto]

The reason %in% does not work is that there are might be values which
are not min in other class which are the same as the min of different
classes.  In the example I provided before, this situation did not
exist.  See the new example:

> + 1.2 1
> + 1.3 1
> + 1.5 1
> + 1.1 2
> + 1.2 2
> + 9.9 2
> + 0.1 3
> + 1.1 3
> + 1.9 3

if you are using %in%, then
1.2 2
1.1 3
will also show up in the final result.


That is why I need those row index of the min value of each class. If
I can get those, that would be best.
Thanks.

On Thu, Oct 29, 2009 at 11:58 AM, Ista Zahn <istazahn at gmail.com> wrote:
> 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
>



-- 
Waverley @ Palo Alto