R help - Jul 2003 - specifying dataframe column names in loops

Dear list members,

This is probably a naive question (from a new user), but I'm having problems
getting column 
names to be recognised in loop that is supposed to apply a set of
calculations in turn
to each column of a dataframe.

The dataframe has 33 columns of output from a simulation.
The columns each have a text name and I want to apply the same set of
calculations
to each column in turn. These include some self-defined functions that refer
back to the 
column in the current iteration.

Setting up a 'for' loop and using age[s] (where 'age' is the
dataframe and
's' is the iterator) to specify
 each column works for things like 'max' and 'min'. However for
functions
like 'density' or locfit, 
error messages are generated:

lf<-locfit(~age[s])
Error in model.frame(formula, rownames, variables, varnames, extras,
extranames,  : 
        invalid variable type

Similary if I try to use the 'names' function.

lf<-locfit(~names(age[s]))
Error in model.frame(formula, rownames, variables, varnames, extras,
extranames,  : 
        invalid variable type

So the question is how do I get functions like density or locfit to 
work in the loop in the same they would for specifying the name
of the column directly e.g. d<-density(ACER), where ACER is the name of the 
first column. 

Many thanks, Simon Goodman....


-----------------------------------------------------------------------

_________________________________________________________________________
This e-mail has been sent in confidence to the named addressee(s).
If you are not the intended recipient, you must not disclose or distribute
it in any form, and you are asked to contact the sender immediately.
Views or opinions expressed in this communication may not be those
of The Zoological Society of London and, therefore, The Zoological
Society of London does not accept legal responsibility for the contents
of this message.  The recipient(s) must be aware that e-mail is not a
secure communication medium and that the contents of this mail may
have been altered by a third party in transit.
If you have any issues regarding this mail please contact:
administrator at zsl.org.

Sounds like you need to use substitute(). Something like

eval(substitute(lf <- locfit(~s, data=age), list(s=s)))

Formulae are different: I don't see why density() should be.

On Fri, 4 Jul 2003, Simon Goodman wrote:
> Dear list members,
> 
> This is probably a naive question (from a new user), but I'm having
problems
> getting column 
> names to be recognised in loop that is supposed to apply a set of
> calculations in turn
> to each column of a dataframe.
> 
> The dataframe has 33 columns of output from a simulation.
> The columns each have a text name and I want to apply the same set of
> calculations
> to each column in turn. These include some self-defined functions that
refer
> back to the 
> column in the current iteration.
> 
> Setting up a 'for' loop and using age[s] (where 'age' is
the dataframe and
> 's' is the iterator) to specify
>  each column works for things like 'max' and 'min'. However
for functions
> like 'density' or locfit, 
> error messages are generated:
> 
> lf<-locfit(~age[s])
> Error in model.frame(formula, rownames, variables, varnames, extras,
> extranames,  : 
>         invalid variable type
...

-- 
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