# Emacs lisp list grouping **URL:** \ **Category:** Emacs\ **Tags:** emacs-lisp, lisp\ **Created:** [April 4, 2024, 12:19pm UTC](https://forum.systemcrafters.net/t/emacs-lisp-list-grouping/544 "2024-04-04T12:19:45Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![tusharhero](https://avatars.discourse-cdn.com/v4/letter/t/e99b99/32.png) [@tusharhero](https://forum.systemcrafters.net/u/tusharhero)\ **Post date:** [April 4, 2024, 12:19pm UTC](https://forum.systemcrafters.net/t/emacs-lisp-list-grouping/544/1 "2024-04-04T12:19:45Z")
Let say I have this list. ``` '((name (tag1)) (name2 (tag2)) (name3 (tag1)) (name4 (tag3)) (name5 (tag2 tag1))) ``` How do I get this? ``` '((tag1 (name name3 name5)) (tag2 (name2 name5)) (tag3 (name4))) ``` This is for my blog: [Tagged view (experimental)](https://tusharhero.codeberg.page/tags.html) ---
**Author:** ![barocio](https://yyz1.discourse-cdn.com/flex029/user_avatar/forum.systemcrafters.net/barocio/32/384_2.png) [@barocio](https://forum.systemcrafters.net/u/barocio)\ **Post date:** [April 4, 2024, 3:54pm UTC](https://forum.systemcrafters.net/t/emacs-lisp-list-grouping/544/2 "2024-04-04T15:54:16Z")
I think this could do the trick: ```lisp (progn (setq articles '((name (tag1)) (name2 (tag2)) (name3 (tag1)) (name4 (tag3)) (name5 (tag2 tag1))) the-tags '() tags-list '() tags-listp '()) (mapcar (lambda (entry) (let* ((title (car entry)) (tags (cadr entry))) (message "Processing %S" entry) (message " title %S" title) (message " tags %S" tags) (mapcar (lambda (tag) (add-to-list 'the-tags tag :append) (let ((tag-list (plist-get tags-listp tag))) (message " tag-list(%S) %S" tag tags-listp) (setq tags-listp (plist-put tags-listp tag (append `(,title) tag-list))))) tags))) articles) the-tags (mapcar (lambda (tag) (add-to-list 'tags-list `(,tag ,(plist-get tags-listp tag)))) the-tags) tags-list) ``` Most surely it is not the most efficient way to code this, but it works. ---
**Author:** ![alphapapa](https://avatars.discourse-cdn.com/v4/letter/a/5f9b8f/32.png) [@alphapapa](https://forum.systemcrafters.net/u/alphapapa)\ **Post date:** [April 5, 2024, 12:27am UTC](https://forum.systemcrafters.net/t/emacs-lisp-list-grouping/544/3 "2024-04-05T00:27:40Z")
This is what I wrote Taxy for: [GitHub - alphapapa/taxy.el: Programmable taxonomical hierarchies for arbitrary objects](https://github.com/alphapapa/taxy.el) Here’s an example of doing it with your sample code: ```lisp (let* ((articles '((name (tag1)) (name2 (tag2)) (name3 (tag1)) (name4 (tag3)) (name5 (tag2 tag1)))) (tags (delete-dups (flatten-list (mapcar #'cadr articles)))) (taxy (make-taxy :name "Blog articles" :taxys (mapcar (lambda (tag) (make-taxy :name (symbol-name tag) :predicate (lambda (article) (member tag (cadr article))) :then #'identity)) tags)))) (setf taxy (thread-last taxy taxy-emptied (taxy-fill articles)) (taxy-items taxy) nil) (taxy-plain taxy)) ``` That produces: ```lisp ("Blog articles" (("tag1" ((name5 (tag2 tag1)) (name3 (tag1)) (name (tag1)))) ("tag2" ((name5 (tag2 tag1)) (name2 (tag2)))) ("tag3" ((name4 (tag3)))))) ``` ---
**Author:** ![cryptk](https://yyz1.discourse-cdn.com/flex029/user_avatar/forum.systemcrafters.net/cryptk/32/67_2.png) [@cryptk](https://forum.systemcrafters.net/u/cryptk)\ **Post date:** [April 5, 2024, 12:59am UTC](https://forum.systemcrafters.net/t/emacs-lisp-list-grouping/544/4 "2024-04-05T00:59:32Z")
My attempt: ```auto (let* ((data '((name (tag1)) (name2 (tag2)) (name3 (tag1)) (name4 (tag3)) (name5 (tag2 tag1))))) (mapcar (lambda (tag) `(,tag ,(delete nil (mapcar (lambda (item) (when (member tag (cadr item)) (car item))) data)))) (delete-dups (flatten-tree (mapcar #'cdr data))))) ``` ---
**Author:** ![tusharhero](https://avatars.discourse-cdn.com/v4/letter/t/e99b99/32.png) [@tusharhero](https://forum.systemcrafters.net/u/tusharhero)\ **Post date:** [April 11, 2024, 11:00pm UTC](https://forum.systemcrafters.net/t/emacs-lisp-list-grouping/544/5 "2024-04-11T23:00:02Z")
This made sense to me so I adopted it. ---
**Author:** ![armindarvish](https://yyz1.discourse-cdn.com/flex029/user_avatar/forum.systemcrafters.net/armindarvish/32/464_2.png) [@armindarvish](https://forum.systemcrafters.net/u/armindarvish)\ **Post date:** [April 20, 2024, 12:10pm UTC](https://forum.systemcrafters.net/t/emacs-lisp-list-grouping/544/6 "2024-04-20T12:10:57Z")
I think you can use a hashtable or plist for the second list (the one you want to make) then you don’t have to worry about duplicate keys. and will have one less mapcar! with plist: ```auto (let ((l '((name (tag1)) (name2 (tag2)) (name3 (tag1)) (name4 (tag3)) (name5 (tag2 tag1)))) (pl (list))) (mapcar (lambda (item) (mapcar (lambda (tag) (setq pl (plist-put pl tag (append (list (car item)) (plist-get pl tag))))) (cadr item))) l) pl ) ``` but personally I’d prefer a hashtable: ```auto (let ((data '((name (tag1)) (name2 (tag2)) (name3 (tag1)) (name4 (tag3)) (name5 (tag2 tag1)))) (table (make-hash-table :test 'equal))) (mapcar (lambda (item) (mapcar (lambda (tag) (puthash tag (append (list (car item)) (gethash tag table)) table)) (cadr item))) data) table ) ``` ---
**Author:** ![tusharhero](https://avatars.discourse-cdn.com/v4/letter/t/e99b99/32.png) [@tusharhero](https://forum.systemcrafters.net/u/tusharhero)\ **Post date:** [April 20, 2024, 1:01pm UTC](https://forum.systemcrafters.net/t/emacs-lisp-list-grouping/544/7 "2024-04-20T13:01:40Z")
> I think you can use a hashtable or plist for the second list (the one you > want to make) then you don’t have to worry about duplicate keys. and will > have one less mapcar! I will make sure to take a look at it, the current solutions works fine but it has 2 mapcars as you say.