Last Observation Carried Forward In a data frame?

Question

Hi all,

I wish to implement a "Last Observation Carried Forward" for a data set I am working on which has missing values at the end of it.

Here is a simple code to do it (question after it):

LOCF <- function(x)
{
    # Last Observation Carried Forward (for a left to right series)
    LOCF <- max(which(!is.na(x))) # the location of the Last Observation to Carry Forward
    x[LOCF:length(x)] <- x[LOCF]
    return(x)
}


# example:
LOCF(c(1,2,3,4,NA,NA))
LOCF(c(1,NA,3,4,NA,NA))

Now this works great for simple vectors. But if I where to try and use it on a data frame:

a <- data.frame(rep("a",4), 1:4,1:4, c(1,NA,NA,NA))
a
t(apply(a, 1, LOCF)) # will make a mess

It will turn my data frame into a character matrix.

Can you think of a way to do LOCF on a data.frame, without turning it into a matrix? (I could use loops and such to correct the mess, but would love for a more elegant solution)

Cheers,

Tal

Answer 1

This already exists:

library(zoo)
na.locf(data.frame(rep("a",4), 1:4,1:4, c(1,NA,NA,NA)))

Answer 2

I ended up solving this using a loop:

fillInTheBlanks <- function(S) {
  L <- !is.na(S)
  c(S[L][1], S[L])[cumsum(L)+1]
}


LOCF.DF <- function(xx)
{
    # won't work well if the first observation is NA

    orig.class <- lapply(xx, class)

    new.xx <- data.frame(t( apply(xx,1, fillInTheBlanks) ))

    for(i in seq_along(orig.class))
    {
        if(orig.class[[i]] == "factor") new.xx[,i] <- as.factor(new.xx[,i])
        if(orig.class[[i]] == "numeric") new.xx[,i] <- as.numeric(new.xx[,i])
        if(orig.class[[i]] == "integer") new.xx[,i] <- as.integer(new.xx[,i])   
    }

    #t(na.locf(t(a)))

    return(new.xx)
}

a <- data.frame(rep("a",4), 1:4,1:4, c(1,NA,NA,NA))
LOCF.DF(a)