Antwort: Re: Inconsistency with matrix indexing (PR#8214)

Dear Duncan,

you are right. In both R 2.2.0 and 2.1.1 you get the same result. What has 
actually changed is the following:
In R 2.1.1
mat[1, , drop = FALSE][1]
r1 
 1 

While in R 2.2.0
mat[1, , drop = FALSE][1]
[1] 1
I don't agree with your suggestion.  mat[1, , drop = FALSE] is a  1x3 
matrix, with both row and column names.  Indexing it by [1] says that 
you want to treat it as a vector, and give the first element.  How could 
R know whether to keep the row or column name for that [1,1] element? 
What should it do with the expression

mat[1,1 , drop = FALSE][1]

?

If you are really concerned about the names, you should use matrix 
indexing in both places with drop = FALSE:

 > mat[1, , drop = FALSE][1,1,drop=FALSE]
    c1
r1  1

Duncan Murdoch
In both cases
mat[1, , drop = FALSE]
   c1 c2 c3
r1  1  4  7

I would strongly prefer to retain the row index r1 above.
Of course, you could argue that mat[1, , drop = FALSE][1] relates to the 
column name c1. However, I think the lower dimension is a vector and this 
typically a column with row names. Most likely, a  wiser and more concise 
interpretation than the mine is necessary, but the problem of dropping and 
loosing information is quite evident in any case.

Still, I believe, my suggestion below is justified. Things like this 
happen quite often to me and it regularly takes time and effort to rectify 
the problem.
Retaining the matrix features in sames could really be an advantage and 
avoid many if's and other workarounds.

Many thanks and best regards,
Stefan

Duncan Murdoch <murdoch at stats.uwo.ca> 
2005-10-18 14:33

An
stefan.albrecht at allianz.com
Kopie
r-devel at stat.math.ethz.ch, R-bugs at biostat.ku.dk
Thema
Re: [Rd] Inconsistency with matrix indexing (PR#8214)

stefan.albrecht at allianz.com wrote:
Full_Name: Stefan Albrecht
Version: 2.2.0
OS: Windows XP
Submission from: (NULL) (194.127.2.74)

Dear all,

in the new R 2.2.0, the matrix indexing has been changed.
I just tried your code in R 2.0.1 and 2.1.1, and got the same output as 
you did.  Which version are you comparing to?

Duncan Murdoch

  In my opinion, this
leads to an inconsistency when indexing one-row matrices.
Take

(mat <- matrix(1:9, nrow = 3, dimnames = list(paste("r", 1:3, sep = ""),
paste("c", 1:3, sep = ""))))
   c1 c2 c3
r1  1  4  7
r2  2  5  8
r3  3  6  9

Now taking the first row of the first-row of mat preserves the column 
names

mat[1, , drop = FALSE][1, ]
c1 c2 c3 
 1  4  7 

This is not true for the first column of the first-row of mat

mat[1, , drop = FALSE][, 1]
[1] 1

However, for the first column of the first two rows of mat the row names 
are
retained

mat[1:2, , drop = FALSE][, 1]
r1 r2 
 1  2 

This feature can lead to problems when indexing matrixes of different 
row
numbers, since the row names are not preserved in all cases.

I suggest to preseve the row names also for 

mat[1, , drop = FALSE][, 1]

Would it be possible to implement this feature? Many thanks.

With best regards,

Stefan Albrecht

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Antwort: Re: Inconsistency with matrix indexing (PR#8214)

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