r - Remove leading NAs to align data

Question

I have a large data.frame with 'staggered' data and would like to align it. What I mean is I would like to take something like

and remove the leading (top) NAs from all columns to get

I know about the na.trim function from the zoo package, but this didn't work on either the initial data.frame presented above or its transpose. For this I used, with transposed dataframe t.df,

t.df <- na.trim(t.df, sides = 'left')

This only returned an empty data.frame, and wouldn't work the way I wanted anyway since it would create vectors of different lengths. Can anyone point me to a package or function that might be more helpful?

Here is the code for my example used above:

# example of what I have

var1 <- c(1,2,3,4,5,6,7,8,9,10)
var2 <- c(6,2,4,7,3,NA,NA,NA,NA,NA)
var3 <- c(NA,NA,8,6,3,7,NA,NA,NA,NA)
var4 <- c(NA,NA,NA,NA,5,NA,2,6,2,9)

df <- data.frame(var1, var2, var3, var4)


# transpose and (unsuccessful) attempt to remove leading NAs

t.df <- t(df)

t.df <-  na.trim(t.df, sides = 'left')

Answer 1

We can loop over the columns (lapply(..) and apply na.trim. Then, pad NAs at the end of the each of the list elements by assigning length as the maximum length from the list elements.

library(zoo)
lst <- lapply(df, na.trim)
df[] <- lapply(lst, `length<-`, max(lengths(lst)))
df
#   var1 var2 var3 var4
#1     1    6    8    5
#2     2    2    6   NA
##     3    4    3    2
#4     4    7    7    6
#5     5    3   NA    2
#6     6   NA   NA    9
#7     7   NA   NA   NA
#8     8   NA   NA   NA
#9     9   NA   NA   NA
#10   10   NA   NA   NA

Or as @G.Grothendieck mentioned in the comments

replace(df, TRUE, do.call("merge", lapply(lst, zoo)))