How to concatenate two lists?

Question

Sorry, but elisp is not my best. I need to combine two lists of strings in this way:

("a" "b" "c") + ("d" "e" "f") -> ("a" "b" "c" "d" "e" "f")

Well, the order is not important, so I wrote this code:

(while lista
  (add-to-list 'listb (car lista)) 
  (setq lista (cdr lista)))

It works, but I'm wondering if there's a function that already does this.

Any clue? Thanks in advance.

Answer 1

You can also just use append.

(append '("a" "b" "c") '("d" "e" "f"))

Answer 2

concatenate is an alias for ‘cl-concatenate’ in ‘cl.el’.

(concatenate TYPE SEQUENCE...)

Concatenate, into a sequence of type TYPE, the argument SEQUENCEs.

So for your example

(concatenate 'list '("a" "b" "c") '("d" "e" "f"))

Since it's defined in cl you may have to (require 'cl) first, otherwise you can use cl-concatenate which seems to be loaded by default.

Also, as pointed out by @phils cl-concatenate just calls append when TYPE is 'list, here's the source from cl-extra.el:

(defun cl-concatenate (type &rest sequences)
  "Concatenate, into a sequence of type TYPE, the argument SEQUENCEs.
\n(fn TYPE SEQUENCE...)"
  (pcase type
    (`vector (apply #'vconcat sequences))
    (`string (apply #'concat sequences))
    (`list (apply #'append (append sequences '(nil))))
    (_ (error "Not a sequence type name: %S" type))))

So if you're only using lists, it's simpler to directly use append, as pointed out by @John Kitchin.

Finally, @lawlist mentionned nconc:

nconc is a built-in function in ‘C source code’.

(nconc &rest LISTS)

Concatenate any number of lists by altering them. Only the last argument is not altered, and need not be a list.

What this means:

(nconc '("a" "b" "c") '("d" "e" "f"))
=> ("a" "b" "c" "d" "e" "f")

(setq l1 '("a" "b" "c")
      l2 '("d" "e" "f"))
(nconc l1 l2)
=> ("a" "b" "c" "d" "e" "f")
l1
=> ("a" "b" "c" "d" "e" "f")
l2
=> ("d" "e" "f")