Select last value in a row, by row

Question

I have a data frame where each row is a vector of values of varying lengths. I would like to create a vector of the last true value in each row.

Here is an example data frame:

df <- read.table(tc <- textConnection("
   var1    var2    var3    var4
     1       2       NA      NA
     4       4       NA      6
     2       NA      3       NA                
     4       4       4       4              
     1       NA      NA      NA"), header = TRUE); close(tc)

The vector of values I want would therefore be c(2,6,3,4,1).

I just can't figure out how to get R to identify the last value.

Any help is appreciated!

Answer 1

Do this by combining three things:

• Identify NA values with is.na
• Find the last value in a vector with tail
• Use apply to apply this function to each row in the data.frame

The code:

lastValue <- function(x)   tail(x[!is.na(x)], 1)

apply(df, 1, lastValue)
[1] 2 6 3 4 1

Answer 2

Here's an answer using matrix subsetting:

df[cbind( 1:nrow(df), max.col(!is.na(df),"last") )]

This max.col call will select the position of the last non-NA value in each row (or select the first position if they are all NA).