r - Lag a data.frame by the number of NAs

Question

In other words, I am trying to lag a data.frame that looks like this:

V1 V2 V3 V4 V5 V6 
1  1  1  1  1  1
2  2  2  2  2  NA
3  3  3  3  NA NA
4  4  4  NA NA NA
5  5  NA NA NA NA
6  NA NA NA NA NA

To something that looks like this:

V1 V2 V3 V4 V5 V6 
1  NA NA NA NA NA
2  1  NA NA NA NA
3  2  1  NA NA NA
4  3  2  1  NA NA
5  4  3  2  1  NA
6  5  4  3  2  1

So far, I have used a function that counts the number of NAs, and have tried to lag my each column in my data.frame by the corresponding number of NAs in that column.

V1 <- c(1,2,3,4,5,6)
V2 <- c(1,2,3,4,5,NA)
V3 <- c(1,2,3,4,NA,NA)
V4 <- c(1,2,3,NA,NA,NA)
V5 <- c(1,2,NA,NA,NA,NA)
V6 <- c(1,NA,NA,NA,NA,NA)
mydata <- cbind(V1,V2,V3,V4,V5,V6)
na.count <- colSums(is.na(mydata))
lag.by <- function(mydata, na.count){lag(mydata, k = na.count)}
lagged.df <- apply(mydata, 2, lag.by) 

But this code just lags the entire data.frame by one...

Answer 1

One option would be to loop through the columns with apply and append the NA elements first by subsetting the NA elements using is.na and then the non-NA element by negating the logical vector (is.na)

apply(mydata, 2, function(x) c(x[is.na(x)], x[!is.na(x)]))
#     V1 V2 V3 V4 V5 V6
#[1,]  1 NA NA NA NA NA
#[2,]  2  1 NA NA NA NA
#[3,]  3  2  1 NA NA NA
#[4,]  4  3  2  1 NA NA
#[5,]  5  4  3  2  1 NA
#[6,]  6  5  4  3  2  1