Re: [Chicken-users] array-lib : prepending unit axes

[Kon Lovett]

On Nov 4, 2007, at 3:41 PM, Terrence Brannon wrote:

> i wrote a function to prepend unit axes onto an array. this is
> oftentimes necessary to get arrays to the same rank before applying
> some sort of operation to them.
>
> if there is a better way to do this, I'm all ears. Please, no comments
> about my parenthesizing style :)
>
> (define prepend-unit-axes
>   (lambda (n a)
>     (if (<= n 0)
>         a
>         (let* (
>                [ones (make-list n 1)]
>                [new-dim (flatten ones (array-dimensions a))]
>                [as-v (array->vector a)]
>                [as-l (vector->list as-v)]
>                )
>           (apply array '#() new-dim as-l)))))

Faster:

(use srfi-1)

(define (prepend-unit-axes cnt arr)
  (if (positive? cnt)
      (apply vector->array (array->vector arr)
                           arr ; Use same storage as source array
                           (append! (make-list cnt 1) (array- 
dimensions arr)))
      arr ) )

Even faster:

(define (prepend-unit-axes cnt arr)
  (if (positive? cnt)
      (apply make-shared-array arr
                               (lambda idxs (drop idxs cnt))
                               (append! (make-list cnt 1) (array- 
dimensions arr)))
      arr ) )

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Best Wishes,
Kon