# why is this query working, and what does it do exactly ? **URL:** \ **Category:** Community Support\ **Tags:** kdb-and-q\ **Created:** [July 10, 2012, 10:35am UTC](https://forum.kx.com/t/why-is-this-query-working-and-what-does-it-do-exactly/7809 "2012-07-10T10:35:00Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![kuentang](https://avatars.discourse-cdn.com/v4/letter/k/bc8723/32.png) [@kuentang](https://forum.kx.com/u/kuentang)\ **Post date:** [July 10, 2012, 10:35am UTC](https://forum.kx.com/t/why-is-this-query-working-and-what-does-it-do-exactly/7809/1 "2012-07-10T10:35:00Z")
Hi all,can somebody explain why this query is working?a group a: til 8Regards,Kim ---
**Author:** ![Kevin\_Smyth](https://avatars.discourse-cdn.com/v4/letter/k/958977/32.png) [@Kevin\_Smyth](https://forum.kx.com/u/Kevin_Smyth)\ **Post date:** [July 10, 2012, 10:39am UTC](https://forum.kx.com/t/why-is-this-query-working-and-what-does-it-do-exactly/7809/2 "2012-07-10T10:39:00Z")
a is assigned to be an int list from 0-7, then ‘a group a’ maps each element of a to its index in the list, try ‘a group reverse a:til 8’ ---
**Author:** ![Deepak1](https://avatars.discourse-cdn.com/v4/letter/d/8dc957/32.png) [@Deepak1](https://forum.kx.com/u/Deepak1)\ **Post date:** [July 10, 2012, 10:44am UTC](https://forum.kx.com/t/why-is-this-query-working-and-what-does-it-do-exactly/7809/3 "2012-07-10T10:44:00Z")
Hi, ? This query will create a dictionary where domain contains unique elements and range have elements list appears… ? ? a: til 8 ? show a ? 0 1 2 3 4 5 6 7 ? q)a group b 0| 0 1| 1 2| 2 3| 3 4| 4 5| 5 6| 6 7| 7 ? q)c: 1 2 3 4 3 2 5 6 q)c group c 1| ,1 2| 2 2 3| 3 3 4| ,4 5| ,5 6| ,6 Regrads Hemant ? ---
**Author:** ![kuentang](https://avatars.discourse-cdn.com/v4/letter/k/bc8723/32.png) [@kuentang](https://forum.kx.com/u/kuentang)\ **Post date:** [July 10, 2012, 12:27pm UTC](https://forum.kx.com/t/why-is-this-query-working-and-what-does-it-do-exactly/7809/4 "2012-07-10T12:27:00Z")
Thx for your explanation, But why is this code executable? Normally you cannot forward a map to a list This code is not executable. `b `a`b ! 1 2 But this code is executable: `a `a`b ! 1 2 This is work as designed ? Kim ---
**Author:** ![abcfoots1](https://avatars.discourse-cdn.com/v4/letter/a/96bed5/32.png) [@abcfoots1](https://forum.kx.com/u/abcfoots1)\ **Post date:** [July 10, 2012, 1:34pm UTC](https://forum.kx.com/t/why-is-this-query-working-and-what-does-it-do-exactly/7809/5 "2012-07-10T13:34:00Z")
\>This code is not executable.  \>`b `a`b ! 1 2  \>But this code is executable:  \>`a `a`b ! 1 2  Consider the latter to behave like  q)a: 8?10       / use random vector so you can see the mapping better q){a@x}`a`b!1,2           / value is used to index list a. the return value of f[dict] is the original keys and the returned list a| 1 b| 9 This is different to the  ---
**Author:** ![kuentang](https://avatars.discourse-cdn.com/v4/letter/k/bc8723/32.png) [@kuentang](https://forum.kx.com/u/kuentang)\ **Post date:** [July 10, 2012, 2:31pm UTC](https://forum.kx.com/t/why-is-this-query-working-and-what-does-it-do-exactly/7809/6 "2012-07-10T14:31:00Z")
Cool,thx for your explaination.KimOn 10 Jul., 14:34, abcfoots \ wrote:\> \>This code is not executable.\> \>`b `a`b ! 1 2> >But this code is executable:> >`a `a`b ! 1 2\>\> Consider the latter to behave like\> q)a: 8?10 ? ? ? / use random vector so you can see the mapping better\> q){a@x}`a`b!1,2 ? ? ? ? ? / value is used to index list a. the return value\> of f[dict] is the original keys and the returned list\> a| 1\> b| 9\>\> This is different to the\>\>\>\> On Tuesday, July 10, 2012 7:27:03 AM UTC-4, kuentang wrote:\>\> \> Thx for your explanation,\>\> \> But why is this code executable?\> \> Normally you cannot forward a map to a list\>\> \> This code is not executable.\> \> `b `a`b ! 1 2>> > But this code is executable:> > `a `a`b ! 1 2\>\> \> This is work as designed ?\>\> \> Kim\>\> \> On 10 Jul., 11:39, Kevin Smyth \ wrote:\> \> \> a is assigned to be an int list from 0-7, then ‘a group a’ maps each\> \> \> element of a to its index in the list, try ‘a group reverse a:til 8’\>\> \> \> On 10 July 2012 10:35, kuentang \ wrote:\>\> \> \> \> Hi all,\>\> \> \> \> can somebody explain why this query is working?\>\> \> \> \> a group a: til 8\>\> \> \> \> Regards,\> \> \> \> Kim\>\> \> \> \> –\> \> \> \> You received this message because you are subscribed to the Google\> \> Groups “Kdb+ Personal Developers” group.\> \> \> \> To post to this group, send email to [personal-kdbplus@googlegroups.com](mailto:personal-kdbplus@googlegroups.com).\>\> \> \> \> To unsubscribe from this group, send email to\> \> [personal-kdbplus+unsubscribe@googlegroups.com](mailto:personal-kdbplus+unsubscribe@googlegroups.com).\> \> \> \> For more options, visit this group athttp://\> \> [groups.google.com/group/personal-kdbplus?hl=en.-](http://groups.google.com/group/personal-kdbplus?hl=en.-) Zitierten Text\> \> ausblenden -\>\> \> \> - Zitierten Text anzeigen – Zitierten Text ausblenden -\>\> - Zitierten Text anzeigen -\\\ ---
**Author:** ![Aaron\_Davies](https://avatars.discourse-cdn.com/v4/letter/a/8e7dd6/32.png) [@Aaron\_Davies](https://forum.kx.com/u/Aaron_Davies)\ **Post date:** [July 13, 2012, 3:52am UTC](https://forum.kx.com/t/why-is-this-query-working-and-what-does-it-do-exactly/7809/7 "2012-07-13T03:52:00Z")
To: [personal-kdbplus@googlegroups.com](mailto:personal-kdbplus@googlegroups.com) X-Mailer: Apple Mail (2.1278) \> This code is not executable. \> `b `a`b ! 1 2 \> \> But this code is executable: \> `a `a`b ! 1 2 some forms of (sym@) mean “apply with left arg passed by name” q)(a group a)~`a group a 1b q) if the object named does not exist, that’s an error q)b group a 'b q)`b group a 'b q) define it and it starts working q)b:a q)`b group a 0| 0 1| 1 2| 2 3| 3 4| 4 5| 5 6| 6 7| 7 q) the result, btw, is a form of functional composition, and imao one of = the more interesting bits of theory embedded in q: considering a dict as a (partial) function from its keys to its values, = then two dicts f and g st f’s value and g’s key are of the same type can = be composed q)f:`a`b`c!1 2 3 q)g:1 2 3!("foo";"bar";"quux") q)g f a| "foo" b| "bar" c| "quux" q)(g f)`b “bar” q) considering a vector as a dictionary from (0..n-1) to its values, then = it can be composed with a dictionary of int value q)v:42 137 23 q)h:`a`b`c!0 1 2 q)v h a| 42 b| 137 c| 23 q)(v h)`b 137 q) ---
**Author:** ![kuentang](https://avatars.discourse-cdn.com/v4/letter/k/bc8723/32.png) [@kuentang](https://forum.kx.com/u/kuentang)\ **Post date:** [July 13, 2012, 5:01am UTC](https://forum.kx.com/t/why-is-this-query-working-and-what-does-it-do-exactly/7809/8 "2012-07-13T05:01:00Z")
Cool,thx for the explanation.Btw is this feature somewhere documented ?KimAm 13.07.2012 04:52, schrieb Aaron Davies:\>\> This code is not executable.\>\> `b `a`b ! 1 2\>\>\>\> But this code is executable:\>\> `a `a`b ! 1 2\> some forms of (sym@) mean “apply with left arg passed by name”\>\> q)(a group a)~`a group a\> 1b\> q)\>\> if the object named does not exist, that's an error\>\> q)b group a\> 'b\> q)`b group a\> 'b\> q)\>\> define it and it starts working\>\> q)b:a\> q)`b group a\> 0| 0\> 1| 1\> 2| 2\> 3| 3\> 4| 4\> 5| 5\> 6| 6\> 7| 7\> q)\>\> the result, btw, is a form of functional composition, and imao one of the more interesting bits of theory embedded in q:\>\> considering a dict as a (partial) function from its keys to its values, then two dicts f and g st f's value and g's key are of the same type can be composed\>\> q)f:`a`b`c!1 2 3\> q)g:1 2 3!(“foo”;“bar”;“quux”)\> q)g f\> a| “foo”\> b| “bar”\> c| “quux”\> q)(g f)`b\> "bar"\> q)\>\> considering a vector as a dictionary from (0..n-1) to its values, then it can be composed with a dictionary of int value\>\> q)v:42 137 23\> q)h:`a`b`c!0 1 2\> q)v h\> a| 42\> b| 137\> c| 23\> q)(v h)`b\> 137\> q)\> ---
**Author:** ![felix1](https://avatars.discourse-cdn.com/v4/letter/f/3e96dc/32.png) [@felix1](https://forum.kx.com/u/felix1)\ **Post date:** [July 13, 2012, 11:27am UTC](https://forum.kx.com/t/why-is-this-query-working-and-what-does-it-do-exactly/7809/9 "2012-07-13T11:27:00Z")
yes. here: http://kx.com/q/d/a/q1.htm#Functional Forms of Amend