R loop doesn't give an error but no output either, what could be the issue with the code?

I am new with R and got stuck with this loop code. I want to see in which rows column 3 values and column 2 values are unequal.

I have created a data frame list, by extracting columns from a dataset PFC_DATA.

list <- as.data.frame(PFC_Data[ ,c(1,6,10)])

And then created the loop with two conditions. Since the column 3 has a bunch of NAs, if it is an NA, then skip, otherwise, if column 3 has a value that is unequal to column 2 value, print the row numbers. That's the code:

`

for (i in 1:1840) {

if (list[[3]][[i]] == is.na())

  next

else {

  if (list[[3]][[i]] != list[[2]][[i]]) {

    print(i)

  }
  }
  }

`

I also tried =="NA". However, in either case nothing happens. I do not get any error, but no output either. Any ideas?