[xquery-talk] From map entry pairs to a pair of arrays

[Christian Grün]

Hi Joe, hi Emmanuel,

Here is yet another alternative that can be efficient (but it’s not
the most compact one):

  let $map := map { "a": 1, "b": 2, "c": 3 }
  return (
    array { map:keys($map) },
    array { map:for-each($map, function($key, $value) { $value }) }
  )

Last but not least, map:get($map, ...) can be replaced with $map(...):

  let $map := map { "a": 1, "b": 2, "c": 3 }
  let $keys := map:keys($map)
  return (
    array { $keys },
    array { $keys ! $map(.) }
  )

Depending on the XQuery processor, and even the specific version, one
or the other solution may be most efficient.

Cheers,
Christian



On Sat, Jul 15, 2017 at 8:16 PM, Joe Wicentowski <joewiz at gmail.com> wrote:
> Great, thank you, Emmanuel!  That is much better.
>
> I might just offer one enhancement - switching from a FLWOR to the simple
> map operator:
>
> ```
> xquery version "3.1";
>
> let $letters-numbers := map { "a": 1, "b": 2, "c": 3 }
> let $keys := map:keys($letters-numbers)
> return (
>     array { $keys },
>     array { $keys ! map:get($letters-numbers, .) }
> )
> ```
>
> Joe
>
> On Sat, Jul 15, 2017 at 12:12 PM Emmanuel Chateau <emchateau at laposte.net>
> wrote:
>>
>> Hi,
>> I would say
>>
>> ```
>> let $letters-numbers := map { "a": 1, "b": 2, "c": 3 }
>> let $keys := map:keys($letters-numbers)
>> return (
>>   array { $keys },
>>   array { for $key in $keys return map:get($letters-numbers, $key) }
>> )
>> ```
>> > Le 15 juil. 2017 à 11:41, Joe Wicentowski <joewiz at gmail.com> a écrit :
>> >
>> > Hi all,
>> >
>> > Is there an efficient way to transform entries in a map into two arrays,
>> > one containing the keys and one containing the values?  Here's my attempt:
>> >
>> > ```
>> > xquery version "3.1";
>> >
>> > let $letters-numbers := map { "a": 1, "b": 2, "c": 3 }
>> > return
>> >     (
>> >         array { map:keys($letters-numbers) },
>> >         array { $letters-numbers?* }
>> >     )
>> > ```
>> >
>> > This successfully takes `map { "a": 1, "b": 2, "c": 3 }` and returns
>> > (["a", "b", "c"], [1, 2, 3]), but the way it iterates through the map twice
>> > strikes me as somewhat inefficient.  Is there a better way to reduce this to
>> > a single pass through the map, or is this actually the best approach?
>> >
>> > Joe
>> > _______________________________________________
>> > talk at x-query.com
>> > http://x-query.com/mailman/listinfo/talk
>>
>
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