• kdb+

  Why do we put a 0# here: @[`.;t;@[;`sym;`g#]0#]

  Posted by mannix on March 17, 2022 at 12:00 am

  I know roughly what apply does:

  In the other apply we are applying to tables in the namespace the inner apply.

  The inner apply is grouping on the sym column and it is a projection hence the empty slot.

  But what exactly is 0# doing here?

   

  mannix replied 8 months, 1 week ago 2 Members · 1 Reply
• 1 Reply

• 

  Laura

  Administrator

  March 17, 2022 at 12:00 am

  TLDR: the expression initialises a list t of tables in the default namespace, setting the grouped attribute on the symbol column and removing all the table rows.

  How that? The function called is not Apply but Amend At @ in its ternary form.

  The first argument is the default namespace `., which, as a namespace, is also a dictionary.

  The second argument t will be a symbol vector of names of tables in the default namespace; i.e. globals.

  The third argument is the function to be applied to each table. Youve already spotted that the unary projection @[;`sym;`g#] sets the grouped attribute on the sym column. It is composed (by juxtaposition) with the unary projection 0#. Applied to a table, 0# removes all the rows.

  So the function applied to each table named in t is the composition @[;`sym;`g#] 0#. That is, remove the rows and set grouped on the sym column.

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