dan - 1 year ago 47

R Question

Let's say I have a numerical

`matrix`

`set.seed(1)`

mat <- matrix(rnorm(1000),ncol=100)

And I want to generate all vectors that are the result of the element-wise product of all unique pairs of vectors in

`mat`

Is there anything faster than this:

`all.pairs <- t(combn(1:ncol(mat),2))`

res <- do.call(cbind,lapply(1:nrow(all.pairs),function(p) mat[,all.pairs[p,1]]*mat[,all.pairs[p,2]]))

Thanks

Answer Source

Of course there is:

```
n <- ncol(mat)
lst <- lapply(1:n, function (i) mat[,i] * mat[,i:n])
do.call(cbind, lst)
```

Here is a even fast way to go:

```
n <- ncol(mat)
j1 <- rep.int(1:n, n:1)
j2 <- sequence(n:1) - 1L + j1
mat[, j1] * mat[, j2]
```

Note, the above will include the multiplication of a column to itself. If you want to forbid that, use

```
n <- ncol(mat)
lst <- lapply(1:(n-1), function (i) mat[,i] * mat[,(i+1):n])
do.call(cbind, lst)
```

and

```
n <- ncol(mat)
j1 <- rep.int(1:(n-1), (n-1):1)
j2 <- sequence((n-1):1) + j1
mat[, j1] * mat[, j2]
```

Actually, `j1`

and `j2`

created above are just the 1st and 2nd row of `combn(1:ncol(mat),2)`

. So, if you still want to stay with `combn`

, use

```
all.pairs <- combn(1:ncol(mat),2)
mat[, all.pairs[1,]] * mat[, all.pairs[2,]]
```