Re: proposal: searching in array function - array_position

Hi

here is a proof concept of array_offset function

possible question:

* used comparation "=" or "IS NOT DISTINCT FROM"

In this initial proof concept I used "IS NOT DISTINCT FROM" operator - but
my opinion is not strong in this question. Both has some advantages and
disadvantages.

Regards

Pavel

2015-01-16 19:12 GMT+01:00 Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>:

>
>
> 2015-01-16 18:37 GMT+01:00 Jim Nasby <Jim(dot)Nasby(at)bluetreble(dot)com>:
>
>> On 1/16/15 11:16 AM, Pavel Stehule wrote:
>>
>>>
>>>
>>> 2015-01-16 17:57 GMT+01:00 Jim Nasby <Jim(dot)Nasby(at)bluetreble(dot)com <mailto:
>>> Jim(dot)Nasby(at)bluetreble(dot)com>>:
>>>
>>> On 1/16/15 3:39 AM, Pavel Stehule wrote:
>>>
>>> I am proposing a simple function, that returns a position of
>>> element in array.
>>>
>>>
>>> Yes please!
>>>
>>> FUNCTION array_position(anyarray, anyelement) RETURNS int
>>>
>>>
>>> That won't work on a multi-dimensional array. Ideally it needs to
>>> accept a slice or an element and return the specifier for the slice.
>>>
>>>
>>> It is question, what is a result - probably, there can be a
>>> multidimensional variant, where result will be a array
>>>
>>> array_position([1,2,3],2) --> 2
>>> array_position([[1,2],[2,3],[3,4]], [2,3]) --> 2 /* 2nd parameter
>>> should to have N-1 dimension of first parameter */
>>>
>>
>> The problem with that is you can't actually use '2' to get [2,3] back:
>>
>> select (array[[1,2,3],[4,5,6],[7,8,9]])[1] IS NULL;
>> ?column?
>> ----------
>> t
>> (1 row)
>>
>
> yes, but when you are searching a array in array you can use a full slice
> selection:
>
> postgres=# select (ARRAY[[1,2],[4,5]])[1][1:2]; -- [1:2] should be a
> constant every time in this case -- so it should not be returned
> array
> ---------
> {{1,2}}
> (1 row)
>
>
>
>
>>
>> I think the bigger problem here is we need something better than slices
>> for handling subsets of arrays. Even if the function returned [2:2] it's
>> still going to behave differently than it will in the non-array case
>> because you won't be getting the expected number of dimensions back. :(
>>
>
> you cannot to return a slice and I don't propose it, although we can
> return a range type or array of range type - but still we cannot to use
> range for a arrays.
>
>>
>> array_position_md([1,2,3],2) --> [2]
>>> array_position_md([[1,2],[2,3],[3,4]], 2) --> [2,1]
>>>
>>> another question is how to solve more than one occurrence on one value -
>>> probably two sets of functions - first returns first occurrence of value,
>>> second returns set of occurrence
>>>
>>
>> Gee, if only way had some way to return multiple elements of something...
>> ;P
>>
>> In other words, I think all of these should actually return an array of
>> positions. I think it's OK for someone that only cares about the first
>> instance to just do [1].
>
>
> there can be two functions - "position" - returns first and "positions"
> returns all as a array
>
>
>>
>> --
>> Jim Nasby, Data Architect, Blue Treble Consulting
>> Data in Trouble? Get it in Treble! http://BlueTreble.com
>>
>
>