[R] proper way to process dataframe by rows

Duncan Murdoch murdoch at stats.uwo.ca
Mon Nov 29 04:38:23 CET 2004

On Sun, 28 Nov 2004 21:25:24 -0500, Jack Tanner <ihok at hotmail.com>

>This is a best practices / style question.
>The way I use RODBC is I something like this:
> > foo <- sqlQuery(db, "select * from foo")
> > apply(foo, 1, function{...})
>That is, I use apply to iterate over each result -- row -- in the 
>RODBC-produced dataframe. Is this how one generally wants to do this?
>My concern is that when apply iterates over the rows, it uses 
>as.matrix() to convert the dataframe to a character representation of 
>itself. Thus my database's carefully planned data types (that RODBC 
>carefully preserved when returning query results) get completely lost as 
>I process the data. I've taken to judiciously sprinkling as.numeric() 
>and friends here and there, but this is just begging for bugs.
>In other words, what is the smart way to process a dataframe by rows? Or 
>is there, by chance, a specific technique or practice that is available 
>for RODBC results but not for dataframes in general?

I would just use a for() loop if I didn't care about the speed too
much.  If I did, I'd avoid dealing with rows of dataframes:  access
using dataframe indexing is slow.  Depending what your function is,
you're probably better off extracting the columns of the dataframe as
vectors, and working with those.

Duncan Murdoch

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