Interpreting "condition has length > 1" warning from `if` function

maybe you want ifelse:

a <- c(1,1,1,1,0,0,0,0,2,2)
ifelse(a>0,a/sum(a),1)

 [1] 0.125 0.125 0.125 0.125 1.000 1.000 1.000 1.000
 [9] 0.250 0.250

if statement is not vectorized. For vectorized if statements you should use ifelse. In your case it is sufficient to write

w <- function(a){
if (any(a>0)){
  a/sum(a)
}
  else 1
}

or a short vectorised version

ifelse(a > 0, a/sum(a), 1)

It depends on which do you want to use, because first function gives output vector of length 1 (in else part) and ifelse gives output vector of length equal to length of a.