Conditions:

## add all positive numbers in the vector
x<-c(2.1, 9.3, -1.2, 2.43, -4.32, -7.3, 5.29)
a<-0
for(i in 1:length(x))
{
     if(x[i]>0) a<-a+x[i]  ## add to a if x[i]>0
}
## note: a simpler alternative, a<-sum(x[x>0])