Non-redundant version of expand.grid

Question

The R function expand.grid returns all possible combination between the elements of supplied parameters. e.g.

> expand.grid(c(\"         


        

Answer 1

You can use a "greater than" operation to filter redundant combinations. This works with both numeric and character vectors.

> grid <- expand.grid(c("aa", "ab", "cc"), c("aa", "ab", "cc"), stringsAsFactors = F)
> grid[grid$Var1 >= grid$Var2, ]
  Var1 Var2
1   aa   aa
2   ab   aa
3   cc   aa
5   ab   ab
6   cc   ab
9   cc   cc

This shouldn't slow down your code too much. If you're expanding vectors containing larger elements (e.g. two lists of dataframes), I recommend using numeric indices that refer to the original vectors.