[R] use of NA's

[tom wright]

Can someone please explain why this works: 

> > d<-c(0,2,3,2,0,3,4,0,0,0,0,0)
> > d.mat<-matrix(data=d,nrow=4,ncol=3,byrow=TRUE)
> > for(i in 1:length(d.mat[1,])){
> + d.mat[,i][d.mat[,i]==0]<-mean(d.mat[,i][d.mat[,i]>0])
> + }

Whereas: 

> d<-c(0,2,3,2,0,3,4,0,0,0,0,0)
> d.mat<-matrix(data=d,nrow=4,ncol=3,byrow=TRUE)
> d.mat[d.mat==0]<-NA
> for(i in 1:length(d.mat[1,])){
+ d.mat[,i][d.mat[,i]==NA]<-mean(d.mat[,i],na.rm=TRUE)
+ }
dosnt

Thanks
Tom