[R] Names of data frame columns in an apply
Peter Dalgaard BSA
p.dalgaard at biostat.ku.dk
Mon Jan 4 15:47:08 CET 1999
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
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