R Gives Error When Trying to Use "sum" Function after "if" Statement (Coursera R Programming Course Week-2 Assignment)

Question

So, I am taking the Coursera R Programming course and trying to write the function for the 2nd week's 3rd assignment. I know a lot of people have already put their answers for this assignment online, but I tried writing my own, and can't figure out why I am getting an error. It would be great if someone can offer their opinion.

This function is supposed to " take a directory of data files and a threshold for complete cases and calculates the correlation between sulfate and nitrate (two different columns) for monitor locations (each monitor is coded in a separate csv file) where the number of completely observed cases (on all variables) is greater than the threshold. The function should return a vector of correlations for the monitors that meet the threshold requirement. If no monitors meet the threshold requirement, then the function should return a numeric vector of length 0."

I wrote the function as:

corr<-function(directory,threshold=0) {
  directory<-getwd()
  fileList<-list.files(pattern=".csv", full.names = TRUE)

  for (i in 1:332) {
    file<-read.csv(fileList[i])
    sulf<-file[["sulfate"]]
    nitr<-file[["nitrate"]]

    if (sum(complete.cases(nitr&sulf)) <  threshold)  {
      return(numeric())

    }else {
      return(cor(nitr,sulf))
    }
  }
}

When I try to source this, it says:

Error in source("corr.R") : corr.R:10:8: unexpected symbol

9:

10: if sum

       ^

So, I can't figure out what is the problem with using sum function. Thanks a lot!