[R] predict.lm removes rownames for a single row. Why?

[Prof Brian Ripley]

This is an inconsistency in drop:

> x <- matrix(1:4, 4,1, dimnames=list(letters[1:4], NULL))
> x
   [,1]
a    1
b    2
c    3
d    4
> drop(x)
a b c d
1 2 3 4
> drop(x[1,,drop=FALSE])
[1] 1

S does not do that, and I don't think R should, given its documentation.
(Note that x[1,] also drops names in R and not S.)

On Fri, 4 Jan 2008, Richard M. Heiberger wrote:

> predict.lm keeps row names when working from several rows in
> newdata, but always removes rowname from a single row.
> The rownames are removed by the line in predict.lm
>    predictor <- drop(X[, piv, drop = FALSE] %*% beta[piv])
> What is the reason for that decision?  I usually want to
> retain the row names.
>
> tmp <- data.frame(x=1:4, y=c(1,3,2,5))
> tmp.lm <- lm(y ~ x, data=tmp)
> tmp.new <- data.frame(x=5:6, y=8:7)
>
> predict(tmp.lm, newdata=tmp.new, interval="confidence")               ##
> rownames
> predict(tmp.lm, newdata=tmp.new[1,,drop=FALSE], interval="confidence")## no
> rownames
>
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-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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