Operator Replication

If you try something like (2:3) + (1:3) in APL or Mathematica you will get some sort of length error message and no result but in R the answer will be the vector (3, 5, 5).  This occurs because under operator replication the shorter length object is recycled starting at the beginning to match the longer object.

Operator replication can be thought of as a useful generalization of the usual way the mixed scalar-vector arithmetic is handled in APL and Mathematica.  In both these cases and in R, 3+1:3 results in the vector (4, 5, 6).

Operator replication is often referred to as the recycling rule.  For more complicated cases involving dataframes and arrays, experimentation to see what is happening is a good idea.