[jira] [Comment Edited] (ARROW-15089) [C++] Add compute kernel to get MapArray value for given key

["Dhruv Vats (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ARROW-15089?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17476066#comment-17476066 ] 

Dhruv Vats edited comment on ARROW-15089 at 1/14/22, 10:45 AM:
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I thought to ask this before I got too confused. Some (possibly very naive) questions:
 * Is {{{}MapArray{}}}, which inherits from {{{}ListArray{}}}, a collection of individual {{{}MapTypes{}}}, each of which in turn are like a {{{}List [ Struct1 \{key, value}, Struct2 \{key, value}, ... ]{}}}?
 * Arrow doesn't restrict the DataType {{key}} takes on, so what is the correct way to take it as a parameter to a {{{}kernel{}}}.
 * Some kernels use {{Call}} while others use {{{}ExecSomething{}}}, what are the prerequisites/strategies/reasonings behind these?
 * If the {{MapArray}} definition in the first point is correct, how do the {{MapArray.keys/items}} work? Do they return a _combined_ Array of keys/items? If so, then iterating and checking for matches in this {{keys}} Array is what we are looking for here?

I'm pretty sure I'm getting a lot of things mixed up. Still finding my way through the expansive Arrow type system, where each subdirectory is like a codebase in itself it seems.


was (Author: dhruv9vats):
I thought to ask this before I got too confused. Some (possibly very naive) questions:
 * Is {{{}MapArray{}}}, which inherits from {{{}ListArray{}}}, a collection of individual {{{}MapTypes{}}}, each of which in turn are a like {{{}List [ Struct1 \{key, value}, Struct2 \{key, value}, ... ]{}}}?
 * Arrow doesn't restrict the DataType {{key}} takes on, so what is the correct way to take it as a parameter to a {{{}kernel{}}}.
 * Some kernels use {{Call}} while others use {{{}ExecSomething{}}}, what are the prerequisites/strategies/reasonings behind these?
 * If the {{MapArray}} definition in the first point is correct, how do the {{MapArray.keys/items}} work? Do they return a _combined_ Array of keys/items? If so, then iterating and checking for matches in this {{keys}} Array is what we are looking for here?

I'm pretty sure I'm getting a lot of things mixed up. Still finding my way through the expansive Arrow type system, where each subdirectory is like a codebase in itself it seems.

> [C++] Add compute kernel to get MapArray value for given key
> ------------------------------------------------------------
>
>                 Key: ARROW-15089
>                 URL: https://issues.apache.org/jira/browse/ARROW-15089
>             Project: Apache Arrow
>          Issue Type: Improvement
>          Components: C++
>    Affects Versions: 6.0.1
>            Reporter: Will Jones
>            Assignee: Dhruv Vats
>            Priority: Major
>              Labels: C++, Kernels, beginner, compute, good-second-issue, kernel
>
> Given a "map", an obvious operation is to get an item corresponding to a key. The idea here is to create a kernel that does this for each map in the array.
> IIRC MapArray isn't guaranteed to have unique keys. So one version would return an array of ItemType by returning the first of last item for a given key. Yet another version could return a ListType containing all matching items. 



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