[jira] [Comment Edited] (ARROW-10640) [C++] A "where" kernel to combine two arrays based on a mask

["David Li (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ARROW-10640?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17346972#comment-17346972 ] 

David Li edited comment on ARROW-10640 at 5/18/21, 2:46 PM:
------------------------------------------------------------

I think to summarize the discussion (including from the PR), we can call this {{if_else}} which uses a Boolean mask to select from one of two input arrays/scalars.

A null mask value outputs a null.

A True mask value outputs the LHS scalar/corresponding value from the LHS array.

A False mask value outputs the RHS scalar/corresponding value from the RHS value.

Hence:
{noformat}
if_else([True, False, null], 1, 0) -> [1, 0, null]
if_else([True, False, null], null, 0) -> [null, 0, null]
if_else([True, False, null], 1, null) -> [1, null, null{noformat}
We can file separate issues for CASE WHEN/choose.


was (Author: lidavidm):
I think to summarize the discussion (including from the PR), we can call this {{if_else}} which uses a Boolean mask to select from one of two input arrays/scalars.

A null mask value outputs a null.

A True mask value outputs the LHS scalar/corresponding value from the LHS array.

A False mask value outputs the RHS scalar/corresponding value from the RHS value.

Hence:
{noformat}
if_else([True, False, null], 1, 0) -> [1, 0, null]
if_else
We can file separate issues for CASE WHEN/choose.{noformat}

> [C++] A "where" kernel to combine two arrays based on a mask
> ------------------------------------------------------------
>
>                 Key: ARROW-10640
>                 URL: https://issues.apache.org/jira/browse/ARROW-10640
>             Project: Apache Arrow
>          Issue Type: New Feature
>          Components: C++
>            Reporter: Joris Van den Bossche
>            Priority: Major
>
> (from discussion in ARROW-9489 with [~maartenbreddels])
> A general "where" kernel like {{np.where}} (https://numpy.org/doc/stable/reference/generated/numpy.where.html) seems a generally useful kernel to have, and could also help mimicking some other python (setitem-like) operations. 
> The concrete use case in ARROW-9489 is to basically do a {{fill_null(array[string], array[string])}} which could be expressed as {{where(is_null(arr), arr2, arr)}}. 



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