Sum the last n non NA values in each column of a matrix in R

Question

I have a matrix that looks like below:

x1<-c(1,2,3,4,5,6,NA)
x2<-c(1,2,NA,4,5,NA,NA)
x3<-c(1,2,3,4,NA,NA,NA)
x4<-c(1,2,3,NA,NA,NA,NA)
x5<-c(1,2,NA,NA,NA,NA,NA)
x<-cbind(x1,x2,x3,x4,x5)

If I want to calculate the last 3 non NA values of each column, and if a column has less than 3 non NA values (like column 5), then I'll sum all the non NA values in that column. I want an output that looks like

15 11 10 6 3

Thank you!

Answer 1

You can use apply with tail to sum up the last non NA like:

apply(x, 2, function(x) sum(tail(x[!is.na(x)], 3)))
#x1 x2 x3 x4 x5 
#15 11  9  6  3 

Answer 2

It also works with a customized function (@GKi answer is pretty cool):

#Build function
myfun <- function(y)
{
  #Count na
  i <- length(which(!is.na(y)))
  if(i<3)
  {
    r1 <- sum(y,na.rm=T)
  } else
  {
    y1 <- y[!is.na(y)]
    y2 <- y1[(length(y1)-2):length(y1)]
    r1 <- sum(y2)
  }
  return(r1)
}
#Apply
apply(x,2,myfun)

Output:

x1 x2 x3 x4 x5 
15 11  9  6  3

Answer 3

One dplyr option using the logic from @GKi could be:

x %>%
 data.frame() %>%
 summarise(across(everything(), ~ sum(tail(na.omit(.), 3))))

  x1 x2 x3 x4 x5
1 15 11  9  6  3

Or:

x %>%
 data.frame() %>%
 summarise(across(everything(), ~ sum(rev(na.omit(.))[1:3], na.rm = TRUE)))

Answer 4

Using sapply from base R

sapply(as.data.frame(x), function(x) sum(tail(na.omit(x), 3)))
# x1 x2 x3 x4 x5 
#15 11  9  6  3