[R] Names of data frame columns in an apply

[Peter Dalgaard BSA]

Hedderik van Rijn <rijn at swi.psy.uva.nl> writes:

> I wrote a function to recode 5 variables into one variable. These 5
> variables are stored in a data frame. For example:
> 
> subjnr time var1 var2 var3 var4 var5 outcome
> 1      1    1    0    2    1    0    20
> 2      1    0    1    2    1    0    20
> 
> When I try to recode a data set like this, using (something like):
> 
> recode <- function(dataline) <-
>   type <- 0;
>   if (dataline$var1 == 1) {
>     type <- 1;
>   }
>   if (dataline$var2 == 2) {
>     type <- 2;
>   }
>   type
> }
> 
> apply(data,1,recode)
> 
> I don't get what I want, because dataline$var1 always evaluates to NULL.
> 
> Is is at all possible to use the $xxx method of refering to columns when
> using apply? And if not, is there another way by means of which I can use
> column names instead of numbers?

You cannot use $ because the argument to recode is not a list, it is a
(non-generic) vector. Cf.

> dd<-data.frame(a=1:2,b=3:4)
> apply(dd,1,print)
a b 
1 3 
a b 
2 4 
  1 2
a 1 2
b 3 4
> apply(dd,1,mode)
        1         2 
"numeric" "numeric" 


However, dataline["var1"] should work. But are you sure this is the
way to go? How about something like:

transform(dframe, type=ifelse(var1==1,1,ifelse(var2==2,2,0)))

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907
-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !)  To: r-help-request at stat.math.ethz.ch
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._