Imlerith - 8 months ago 31

R Question

Suppose I have the following array:

`a <- sample(letters,100,replace=TRUE)`

Then suppose those letters are ordered in a sequence, I want to extract all possible 'n' sized sequences from that array. For example:

For

`n=2`

`paste0(a[1:99],"->",a[2:100])`

for

`n=3`

`paste0(a[1:98],"->",a[2:99],"->",a[3:100])`

you get the point. Now, my goal is to create a function that would take as input

`n`

`a`

I was able to do it using loops and all that but I was hoping for a high performance one liner.

I am a bit new to R so I'm not aware of all existing functions.

Answer

You can use `embed`

. For `embed(a, 3)`

, this gives a matrix with columns

`a[3:100]`

`a[2:99]`

`a[1:98]`

in that order.

To reverse the column order use matrix syntax `m[rows, cols]`

:

```
res = embed(a, 3)[, 3:1]
```

If you want arrows printed between the columns, then

```
do.call(paste, c(split(res, col(res)), sep = " -> "))
```

is one way. This is probably better than `apply(res, 1, something)`

, performance-wise, since this is vectorized while `apply`

would loop over rows.

I was hoping to say something useful about how to find obscure functions in R, but came up mostly blank on how `embed`

might be found. Maybe...

- Go to any HTML help page
- Click the "Index" hyperlink at the bottom
- Read every single page

?