[racket-dev] [PATCH] Add a `zip' procedure to lists

Providing `zip' is still useful IMO, both for people who don't know /
remember that trick and as a shorthand.

Vincent



At Fri, 9 Nov 2012 07:34:35 -0500 (EST),
J. Ian Johnson wrote:
> 
> [forgot reply all]
> zip is unnecessary because of n-ary map.
> (zip l0 l1) = (map list l0 l1)
> -Ian
> ----- Original Message -----
> From: "Diogo F. S. Ramos" <diogofsr at gmail.com>
> To: dev at racket-lang.org
> Sent: Thursday, November 8, 2012 11:46:15 PM GMT -05:00 US/Canada Eastern
> Subject: [racket-dev] [PATCH] Add a `zip' procedure to lists
> 
> `zip' gathers together each element of each list, in order, returning
> a list of those elements.
> 
> For example: (zip (list 4 2) (list 3 5)) => '((4 3) (2 5))
> 
> Every list has to have the same length.
> ---
> There was talk in the IRC about implementing a `zip' procedure. Here
> is my try.
> 
> Unfortunately I still don't know what is the preferred way to check
> for the validity of the parameters. I tried to copy the style of the
> rest of the file.
> 
>  collects/racket/list.rkt |    9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/collects/racket/list.rkt b/collects/racket/list.rkt
> index fe32f68..18055e5 100644
> --- a/collects/racket/list.rkt
> +++ b/collects/racket/list.rkt
> @@ -32,7 +32,8 @@
>           append-map
>           filter-not
>           shuffle
> -         range)
> +         range
> +         zip)
>  
>  (define (first x)
>    (if (and (pair? x) (list? x))
> @@ -407,3 +408,9 @@
>      [(end)            (for/list ([i (in-range end)])            i)]
>      [(start end)      (for/list ([i (in-range start end)])      i)]
>      [(start end step) (for/list ([i (in-range start end step)]) i)]))
> +
> +(define (zip list1 list2 . lists)
> +  (for ((l (list* list1 list2 lists)))
> +    (unless (list? l)
> +      (raise-argument-error 'zip "list?" l)))
> +  (apply map list (list* list1 list2 lists)))
> -- 
> 1.7.9.5
> 
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