Use outer instead of expand.grid

Question

I'm looking for as much speed as possible and staying in base to do what `expand.grid` does. I have used `outer` for similar purposes in the past to create a vector; something like this: ``` v <- outer(letters, LETTERS, paste0) unlist(v[lower.tri(v)]) ``` Benchmarking has shown me that `outer` can be drastically faster than `expand.grid` but this time I want to create two columns just like `expand.grid` (all possible combos for 2 vectors) but my methods with `outer` do not benchmark as fast with outer this time. I'm hoping to take 2 vectors and create every possible combo as two columns as fast as possible (I think `outer` may be the route but am wide open to any base method. Here's the `expand.grid` method and `outer` method. ``` dat <- cbind(mtcars, mtcars, mtcars) expand.grid(seq_len(nrow(dat)), seq_len(ncol(dat))) FOO <- function(x, y) paste(x, y, sep=":") x <- outer(seq_len(nrow(dat)), seq_len(ncol(dat)), FOO) apply(do.call("rbind", strsplit(x, ":")), 2, as.integer) ``` The micro…

Accepted Answer

Using `rep.int`:

```
expand.grid.alt <- function(seq1,seq2) {
  cbind(rep.int(seq1, length(seq2)),
        c(t(matrix(rep.int(seq2, length(seq1)), nrow=length(seq2)))))
}

expand.grid.alt(seq_len(nrow(dat)), seq_len(ncol(dat)))
```

In my computer is like 6 times faster than `expand.grid`.

Use outer instead of expand.grid

Solution

Problem