Populate column which a range of possible values in other columns: in R -

for example dataframe:

df <- structure(list(region = structure(1:8, .label = c("a", "b", "c",  "d", "e", "f", "g", "h"), class = "factor"), y.2012 = c(5.5,  na, 4.7, 3.6, na, na, 4.6, na), y.2013 = c(5.7, na, na, 3.8,  na, 6.2, na, na), y.2014 = c(na, 6.3, na, 4.1, 5.1, na, na, na )), .names = c("region", "y.2012", "y.2013", "y.2014"), class = "data.frame", row.names = c(na,  -8l)) 

i want add column (called 'variable') populated recent year of data available. example, 2014 preferable, followed 2013, , 2012. if no data available, na can left.

i want resulting dataframe like:

  region y.2012 y.2013 y.2014 variable 1         5.5    5.7     na      5.7 2      b     na     na    6.3      6.3 3      c    4.7     na     na      4.7 4      d    3.6    3.8    4.1      4.1 5      e     na     na    5.1      5.1 6      f     na    6.2     na      6.2 7      g    4.6     na     na      4.6 8      h     na     na     na       na 

what best way tackle problem? advice appreciated.

we can use apply margin=1 loop on rows , last non-na value (as columns ordered smallest largest years)

df$variable <- apply(df[-1], 1, function(x) {                        i1 <- tail(x[!is.na(x)],1)                      if(length(i1)>0) i1 else na}) df$variable #[1] 5.7 6.3 4.7 4.1 5.1 6.2 4.6  na 

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or option max.col , rowsums

df[-1][cbind(1:nrow(df),max.col(!is.na(df[-1]), "last")*              (na^!rowsums(!is.na(df[-1]))))] #[1] 5.7 6.3 4.7 4.1 5.1 6.2 4.6  na