Summation of max function with vector input in R?

I want to build this function: f(a,b)=sum_{i=1 to n}(max(a-b_i,0)) where b=(b_1,b_2,...b_n). This is what I have done:

    vec<-function(a,b){
                  z<-0
                   for(i in b){
                         ifelse(a > i,z <- z + (a-i), 0)
                       }
                return(data.frame(z))
                   }

This code gives correct output for scalar input of a. But while using vector output answers are not always correct. For example

    > vec(c(-6,5),c(3,1,3)) 
  gives -25 for a=-6 and 8 for a=5 respectively. 

 But   > vec(-6,c(3,1,3))  gives 0. And this the correct answer.

Please throw give some idea how will I correct it.