[R] search through a matrix

[onyourmark]

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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