基于前一列值的起始列行值 - R

Question

I have a dataframe as follows:

xR <- data.frame("A" = c(15, 13.5, 12, 9.1, NA, NA, NA, NA), 
         "B" = c(NA, 13.6, 8.4, 6.7, 5.6, 2.0, NA, NA), 
         "C" = c(NA, NA, 8.5, 2.43, 1.23, NA, NA, NA))

Is it possible to shift values in Col B so that it starts on same row of Col A where first value of Col B is lower than corresponding row value in Col A. Shift values in Col C so that it starts on same row of Col B where first value of Col C is lower than corresponding row value in Col B and so on for each of the columns in my dataframe. My dataframe has many more columns than this so need to try automate it.

I'm hoping to end up with the following:

xR1 <- data.frame("A" = c(15, 13.5, 12, 9.1, NA, NA, NA, NA), 
             "B" = c(13.6, 8.4, 6.7, 5.6, 2.0, NA, NA, NA), 
             "C" = c(NA, 8.5, 2.43, 1.23, NA,NA, NA, NA))

Thanks

Answer 1

Here is one approach. In each column:

First, identify the first non-NA position.

Second, determine the new position to shift the first non-NA position to. This will be the minimum where the value of the first non-NA position is less than the value in the previous column.

Third, shift the vector of that column by the difference between the new position and first non-NA position.

for (i in 2:ncol(xR)) {
  first_non_NA_position <- which.min(is.na(xR[[i]]))
  new_position <- min(which(xR[[i]][first_non_NA_position] < xR[[i-1]]))
  position_diff <- first_non_NA_position - new_position
  if (position_diff > 0) {
    xR[[i]] <- c(tail(xR[[i]], -position_diff), rep(NA, position_diff))
  }
}

Output

     A    B    C
1 15.0 13.6 8.50
2 13.5  8.4 2.43
3 12.0  6.7 1.23
4  9.1  5.6   NA
5   NA  2.0   NA
6   NA   NA   NA
7   NA   NA   NA
8   NA   NA   NA