Get Value of last non-empty column for each row

Question

Take this sample data:

data.frame(a_1=c(\"Apple\",\"Grapes\",\"Melon\",\"Peach\"),a_2=c(\"Nuts\",\"Kiwi\",\"Lime\",\"Honey\"),a_3=c(\"Plum\",\"Apple\",NA,NA)         


        

Answer 1

There's no need for regex here. Just use apply + tail + na.omit:

> apply(mydf, 1, function(x) tail(na.omit(x), 1))
[1] "Cucumber" "Apple"    "Lime"     "Honey" 

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I don't know how this compares in terms of speed, but you You can also use a combination of "data.table" and "reshape2", like this:

library(data.table)
library(reshape2)
na.omit(melt(as.data.table(mydf, keep.rownames = TRUE), 
             id.vars = "rn"))[, value[.N], by = rn]
#    rn       V1
# 1:  1 Cucumber
# 2:  2    Apple
# 3:  3     Lime
# 4:  4    Honey

Or, even better:

melt(as.data.table(df, keep.rownames = TRUE), 
     id.vars = "rn", na.rm = TRUE)[, value[.N], by = rn]
#    rn       V1
# 1:  1 Cucumber
# 2:  2    Apple
# 3:  3     Lime
# 4:  4    Honey

This would be much faster. On an 800k-row dataset, apply took ~ 50 seconds while the data.table approach took about 2.5 seconds.