Copy value from previous matching row into new matching row

Question

My example data frame is constructed as follows:

ID       <- c("1", "2", "1", "2", "1", "2")
Current  <- c(1.12, NA, 3.66, 8.95, 4.73, 7.82)
Previous <- c(NA, NA, NA, NA, NA, NA)
df       <- data.frame(ID, Current, Previous, stringsAsFactors = TRUE)

Adding an extra ID to levels(df$ID) by writing levels(df$ID) <- c(levels(df$ID), "3") results in:

> levels(df$ID)
[1] "1" "2" "3"

(I am adding an extra level because the data frame I am currently working with contains a lot of levels that do not even occur once.)

Writing df now prints:

  ID Current Previous
1  1    1.12       NA
2  2      NA       NA
3  1    3.66       NA
4  2    8.95       NA
5  1    4.73       NA
6  2    7.82       NA

Because typeof(df$Current) is "double" and typeof(df$Previous) is "logical", I would like to convert the Previous column to "double". In order to achieve this, I write:

df$Previous <- as.numeric(as.character(df$Previous))

Writing typeof(df$Previous) now results in "double".

I would now like to do the following: Take the first ID 1 entry of Current and copy it to the second ID 1 entry of Previous and so on if the Current value is not an NA. That is, the resulting table should look like this:

  ID Current Previous
1  1    1.12       NA
2  2      NA       NA
3  1    3.66     1.12
4  2    8.95       NA
5  1    4.73     3.66
6  2    7.82     8.95

I already tried doing this by writing the following for loop, but it did not work and I do not know what I did wrong:

i = 1
for(i in length(unique(df$ID))) {
  j = 1
  k = 1
  idLocationAbsolute = 1
  for(j in length(df$ID)) {
    if(unique(df$ID)[i] == df$ID[j]) {
      idLocationAbsolute[k] = j
      k = k + 1
    }
  }
  if(k > 1) {
    k = k - 1
    l = 1
    for(l in k) {
      if(!is.na(idLocationAbsolute[l])) {
        df$Previous[l + 1] = df$Current[l]
      }
    }
  }
}

Please note that I wrote idLocationAbsolute = 1 because R prints an error message otherwise. But I cannot tell whether this is a good thing to do or if it just breaks the whole code. I, however, suppose that this is not the cause of the code not doing what it should do.