3 messages · Brian Ripley, Martin Morgan
colnames on a data.frame with implicit row.names
df <- data.frame(x=1:6000000)
is slow
system.time(colnames(df))
[1] 21.655 0.327 21.987 0.000 0.000
system.time(names(df))
[1] 0 0 0 0 0 because colnames calls dimnames calls row.names.data.frame calls as.character on the implicit row.names.
Martin T. Morgan Bioconductor / Computational Biology http://bioconductor.org
colnames on a data.frame with implicit row.names
df <- data.frame(x=1:6000000)
is slow
system.time(colnames(df))
[1] 21.655 0.327 21.987 0.000 0.000
system.time(names(df))
[1] 0 0 0 0 0 because colnames calls dimnames calls row.names.data.frame calls as.character on the implicit row.names.
So use names() and not colnames():
rownames and colnames for matrices
row.names and names for data frames.
All colnames assumes is that there is a dimnames method: this could be
relevant for objects inheriting from "data.frame", but there is a price
for generality.
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) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595
Yes, obviously, with hindsight. In lieu of code change, the colnames
documentation could indicate the restricted sense of 'equivalent'.
For a data frame, 'rownames' and 'colnames' are equivalent to
'row.names' and 'names' respectively.
It might help to add 'names' to See Also of ?data.frame.
Martin
Prof Brian Ripley <ripley at stats.ox.ac.uk> writes:
On Wed, 27 Sep 2006, Martin Morgan wrote:
colnames on a data.frame with implicit row.names
df <- data.frame(x=1:6000000)
is slow
system.time(colnames(df))
[1] 21.655 0.327 21.987 0.000 0.000
system.time(names(df))
[1] 0 0 0 0 0 because colnames calls dimnames calls row.names.data.frame calls as.character on the implicit row.names.
So use names() and not colnames():
rownames and colnames for matrices
row.names and names for data frames.
All colnames assumes is that there is a dimnames method: this could be
relevant for objects inheriting from "data.frame", but there is a
price for generality.
--
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)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
Martin T. Morgan Bioconductor / Computational Biology http://bioconductor.org