Top Answers

1. Match substring of two vectors and create a new vector combining them
   9 votes
2. Efficiently fill NAs by group
   7 votes
3. Going from wide to long w/ coupled-columns: Is there a more R way to do this (i.e. - without using a for loop)?
   7 votes
4. fast melt large 2d matrix to 3 column data.table
   6 votes
5. What is the most elegant way to apply a function to multiple pairs of columns in a data.table or data.frame?
   5 votes