[R] rbind and data.frame

[Göran Broström]

On 5 Dec 2001, Peter Dalgaard BSA wrote:

> Göran Broström <gb at stat.umu.se> writes:
> 
> > Version 1.3.1  (2001-08-31) (RH 7.2):
> > 
> > > dat <- data.frame(x = 1, y = 2)
> > > x <- matrix(0, ncol = 2, nrow = 2)
> > > x
> >      [,1] [,2]
> > [1,]    0    0
> > [2,]    0    0
> > > dat
> >   x y
> > 1 1 2
> > > rbind(dat, x)
> >   x y
> > 1 1 2
> > 2 0 0
> > 
> > I expected
> > 
> > > rbind(dat, x)
> >   x y
> > 1 1 2
> > 2 0 0
> > 3 0 0
> > 
> > Is my expectation wrong?
> 
> Maybe and maybe not... The help page has
> 
>      If you want to combine other objects with data frames, it may be
>      necessary to coerce them to data frames first.
> 
> and the internal logic in rbind.data.frame is that everything that is
> not a data frame or a list is a vector. I'm slightly puzzled as to why
> we don't check the lengths but quietly truncate or recycle (whereas
> the corresponding cbind code does protest even though we don't always
> want it to...)

The help page also have:

     The functions `cbind' and `rbind' are generic, with methods for
     data frames.  The data frame method will be used if an argument is
     a data frame and the rest are vectors or matrices. 
                                           ^^^^^^^^^^^
Maybe somewhat misleading? However, this is the way to do it:

> dat <- data.frame(x = 1, y = 2)
> x <- matrix(NA, ncol = 2, nrow = 2)
> x <- data.frame(x)     ## Why no 'names =' arg?
> names(x) <- names(dat) ## Necessary 
> rbind(dat, x)
   x  y
1  1  2
2 NA NA
3 NA NA

My real problem is how to create a data frame in a sequentially growing
manner, when I know the final size (no of cases). I want to avoid to
call 'rbind' many times, and instead create an 'empty' data frame in
one call, and then fill it. Are there better ways of doing this?

Göran
-- 
 Göran Broström                      tel: +46 90 786 5223
 professor                           fax: +46 90 786 6614
 Department of Statistics            http://www.stat.umu.se/egna/gb/
 Umeå University
 SE-90187 Umeå, Sweden             e-mail: gb at stat.umu.se

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