Combining two lists by key number

[lastknownuser]

Need a little help with some list combining. I have two lists:

(1 2 3 4)
( (1 a) (2 b) (2 c) (2 d) (3 e) (4 f) )

And I want to combine them to one list, by key number, like this:

( [1 (1 a)] [2 (2 b) (2 c) (2 d)] [3 (3 e)] [4 (4 f)] )

[lastknownuser]

I came up with something like this quickly, but I can't figure out how to group second list by one key number.

(defun c:test ( / n m len1 len2 list3 list4)
  (setq list1 (list "1" "2" "3"))
	(setq list2 (list '("1" "a") '("2" "b") '("2" "c") '("2" "d") '("3" "e") ))
        (setq n 0)
        (setq len1 (length list1))
        (setq len2 (length list2))
        (while (< n len1)
	    (setq m 0)
	    (while (< m len2)
	    (if (= (nth n list1) (nth 0 (nth m list2)) )
		(progn
		   (setq list3 (list (nth n list1) (nth m list2)))
		   (setq list4 (cons list3 list4))
);progn
);if
(setq m (+ m 1))
);while
(setq n (+ n 1))
);while
(reverse list4)
);defun

The result is:
(("1" ("1" "a")) ("2" ("2" "b")) ("2" ("2" "c")) ("2" ("2" "d")) ("3" ("3" "e"))
But I need the lists with number 2 all grouped, like this ("2" ("2" "b") ("2" "c") ("2" "d"))

Edited October 7, 2021 by lastknownuser

[Lee Mac]

Consider the following -

(defun foo ( a b / y )
    (setq a (mapcar 'list a))
    (foreach x (reverse b)
        (if (setq y (assoc (car x) a))
            (setq a (subst (vl-list* (car x) x (cdr y)) y a))
        )
    )
    a
)

 

For example:

_$ (foo '("1" "2" "3" "4") '(("1" "a") ("2" "b") ("2" "c") ("2" "d") ("3" "e") ("4" "f")))
(("1" ("1" "a")) ("2" ("2" "b") ("2" "c") ("2" "d")) ("3" ("3" "e")) ("4" ("4" "f")))

 

I'm unsure how you wish to handle these cases:

_$ (foo '("1" "2" "3" "4" "5") '(("1" "a") ("2" "b") ("2" "c") ("2" "d") ("3" "e") ("4" "f")))
(("1" ("1" "a")) ("2" ("2" "b") ("2" "c") ("2" "d")) ("3" ("3" "e")) ("4" ("4" "f")) ("5"))

_$ (foo '("1" "2" "3" "4") '(("1" "a") ("5" "f")))
(("1" ("1" "a")) ("2") ("3") ("4"))

 

Edited October 7, 2021 by Lee Mac

[ronjonp]

Another for fun:

(setq l '(("1" "a") ("2" "b") ("2" "c") ("2" "d") ("3" "e")))
(setq k '("1" "2" "3"))
(mapcar '(lambda (x) (cons x (vl-remove-if-not '(lambda (y) (= x (car y))) l))) k)

 

[lastknownuser]

@Lee Mac@ronjonp thank you both guys, works great for what I need it!