[R] search through a matrix

[Dimitri Liakhovitski]

Just try to run x[row(x) == col(x)] .
You'll see it's a vector.
It's the same as diag(x)
Dimitri

On Tue, Apr 21, 2009 at 9:02 AM, onyourmark <william108 at gmail.com> wrote:
>
> Hi again. Thanks. I get it now.
> So row(x) is a matrix with just the row number for each entry.
>
> How about x[row(x) == col(x)] ?
> Can the square bracket function take a matrix as its argument? If so, I
> guess I understand this statement.
> The argument is a boolean matrix.
>
> Thank you.
>
>
> David Winsemius wrote:
>>
>>
>> On Apr 21, 2009, at 8:33 AM, onyourmark wrote:
>>
>>>
>>> Thanks very much.
>>>
>>> I don't really understand the row() function. I looked in the
>>> reference but
>>> I don't really get it. It says:
>>> Description
>>> Returns a matrix of integers indicating their row number in a matrix-
>>> like
>>> object, or a factor indicating the row labels.
>>>
>>> Usage
>>> row(x, as.factor = FALSE)
>>>
>>> Arguments
>>> x a matrix-like object, that is one with a two-dimensional dim.
>>>
>>> I don't understand what row() does.
>>
>> It returns a matrix of the same size as its arguments populated with
>> the row numbers instead of the matrix elements.
>>>
>>>
>>> And in the example in the documentation it says:
>>> x <- matrix(1:12, 3, 4)
>>
>> Now try row(x)
>>
>>>
>>> # extract the diagonal of a matrix
>>> dx <- x[row(x) == col(x)]
>>> dx
>>> [1] 1 5 9
>>>
>>> I thought the single square bracket notation accepts a pair
>>> separated by a
>>> comma but I don't see how
>>> row(x)==col(x) produces that?
>>
>> The single square bracket notation can be used as [r,c] or  without
>> the comma which requires a logical index. In the second method the
>> matrix entries get processed serially, column-wise.
>>
>>  > x <- matrix(1:12, 3, 4)
>>  > x[TRUE]
>>   [1]  1  2  3  4  5  6  7  8  9 10 11 12
>>  > row(x)
>>       [,1] [,2] [,3] [,4]
>> [1,]    1    1    1    1
>> [2,]    2    2    2    2
>> [3,]    3    3    3    3
>>
>> --
>> David Winsemius, MD
>> Heritage Laboratories
>> West Hartford, CT
>>
>> ______________________________________________
>> 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.
>>
>>
>
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>
> ______________________________________________
> 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.
>



-- 
Dimitri Liakhovitski
MarketTools, Inc.
Dimitri.Liakhovitski at markettools.com