[Rd] Re: tail(<matrix>) column numbers

Martin Maechler maechler at stat.math.ethz.ch
Fri Jul 9 10:35:42 CEST 2004

>>>>> "PatBurns" == Patrick Burns <pburns at pburns.seanet.com>
>>>>>     on Tue, 27 Jan 2004 14:20:30 +0000 writes:

[more than half a year ago]

    PatBurns> Duncan Murdoch wrote:


    DM> One other one I'll look at:
    DM> If a matrix doesn't have row names, I might add names
    DM> like '[nn,]' to it, so I get results like

      R> x <- matrix(1:100,ncol=2)
      R> tail(x)
   Rout>       [,1] [,2]
   Rout> [45,]   45   95
   Rout> [46,]   46   96
   Rout> [47,]   47   97
   Rout> [48,]   48   98
   Rout> [49,]   49   99
   Rout> [50,]   50  100
    DM> instead of the current

      R> tail(x)
   Rout>      [,1] [,2]
   Rout> [1,]   45   95
   Rout> [2,]   46   96
   Rout> [3,]   47   97
   Rout> [4,]   48   98
   Rout> [5,]   49   99
   Rout> [6,]   50  100

    DM> I just want to be careful that this doesn't mess up
    DM> something else.
    DM> Duncan Murdoch

    PatBurns> I think this could be being too "helpful".  Using
    PatBurns> tail on a matrix may often be done in a program so
    PatBurns> I think leaving things as they come is the best
    PatBurns> policy.

I tend to disagree, and would like to have us think about it

1)  Duncan's proposal was to only add row names *when* there are none.
2)  Pat is write that tail() for matrices maybe used not only interactively
    and help(tail)'s "Value:" section encourages this to some extent.

    However, how can adding column names to such a matrix-tail be harmful?

    Well, only in the case where the tail is quite large, the
    added dimnames add unneeded memory and other overhead when
    dealing with that matrix.

 But I think, programmers/users caring about efficient code
 wouldn't use tail(<matrix>) in their function code, would they?

In conclusion, I'd still argue for following Duncan's proposal,
maybe adding a \note{.} to head.Rd stating that these functions
were meant for interactive use, and for "programming", we'd
rather recommend the direct  (n-k+1):n indexing.

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