[R] Help with IF command strings

[arun]

HI,
Try this:
set.seed(485)
dat1<- as.data.frame(matrix(sample(0:10,26*10,replace=TRUE),ncol=26))
mean(dat1$V21[dat1$V2==1|dat1$V2==0])
#[1] 3.8
#or
with(dat1,mean(V21[V2==1|V2==0]))
#[1] 3.8


A.K.


I have data in 26 columns, I'm trying to get a mean for column 21 only for the participants that are either 0 or 1 in column 2. 

One of the commands I tried looked something like this 

mean(data1$V21, if(V2 = 1))   

So basically I need to have the program run a mean (and later 
other forms of analysis) on participants based on their condition. 
either 0 or 1. 

Help is greatly appreciated. 

Thanks