[R] replacement functions for subsets

[Harry Mamaysky]

As I understand it rownames(aa) returns a copy of an attribute of aa. So changing the value of this vector should make the change to the copy of the row.names attribute. I would then have to set the original row.names equal to this copy to effect the change.

So my question is why "rownames(aa)[2:4] <-" changes the original attribute rather than its copy?

And the follow on question is whether it's possible to have "f(x)[2:4] <-" operate in the same way for some user defined replacement function f. 

Sent from my iPhone

On Jul 10, 2013, at 3:05 PM, David Winsemius <dwinsemius at comcast.net> wrote:


On Jul 10, 2013, at 11:47 AM, Harry Mamaysky wrote:

> I know how to define replacement functions in R (i.e. ‘foo<-‘ <- function(x,value) x<-value, etc.), but how do you define replacement functions that operate on subsets of arrays (i.e. how do you pass an index into foo)?
> For example, why does the following use of “rownames” work?

`rownames` of a dataframe is a vector, so indexing with "[" and a single vector of indices is adequate. I cannot really tell what your conceptual "why"-difficulty might be. This is just assignment within a vector. That is not really a "replacement function operating on a subset of an array" since rownames are not values of the dataframe .... and it's not an "array". (Careful use of terms is needed here.)


> 
>> aa <- data.frame( a=1:10,b=101:110 )
> 
>> aa
> 
>   a   b
> 
> 1   1 101
> 
> 2   2 102
> 
> 3   3 103
> 
> 4   4 104
> 
> 5   5 105
> 
> 6   6 106
> 
> 7   7 107
> 
> 8   8 108
> 
> 9   9 109
> 
> 10 10 110
> 
>> rownames(aa)[2:4] <- c('row2','row3','row4')
> 
>> aa
> 
>     a   b
> 
> 1     1 101
> 
> row2  2 102
> 
> row3  3 103
> 
> row4  4 104
> 
> 5     5 105
> 
> 6     6 106
> 
> 7     7 107
> 
> 8     8 108
> 
> 9     9 109
> 
> 10   10 110
> 
> 
> 
> 
> Thanks,
> 
> Harry
> 
> 
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> 
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David Winsemius
Alameda, CA, USA