How to sum a list of numbers in Emacs Lisp?

Question

This works:

(+ 1 2 3)
6

This doesn't work:

(+ '(1 2 3))

This works if 'cl-*' is loaded:

(reduce '+ '(1 2 3))
6

If reduce were always available I could write:

(defun sum (L)
  (reduce '+ L))

(sum '(1 2 3))
6

What is the best practice for defining functions such as sum?

Please do not change the question, open a new one. – kmkaplan Feb 26 '09 at 15:53 — kmkaplan, Feb 26 '09 at 15:53

score 78 · Accepted Answer · answered Feb 26 '09 at 13:45

78

(apply '+ '(1 2 3))

answered Feb 26 '09 at 13:45

kmkaplan

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Thanks. I've thought that I'm missing something trivial. – jfs Feb 26 '09 at 13:48

score 3 · Answer 2 · answered Mar 18 '14 at 11:15

3

If you manipulate lists and write functional code in Emacs, install dash.el library. Then you could use its -sum function:

(-sum '(1 2 3 4 5)) ; => 15

answered Mar 18 '14 at 11:15

Mirzhan Irkegulov

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score 2 · Answer 3 · answered Dec 21 '12 at 18:56

2

Linearly recursive function (sum L)

;;
;; sum
;;
(defun sum(list)    
    (if (null list)
        0

        (+ 
            (first list) 
            (sum (rest list))
        )   
    )   
)

answered Dec 21 '12 at 18:56

Kristo Aun

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score 0 · Answer 4 · answered Nov 02 '13 at 14:58

0

You can define your custom function to calculate the sum of a list passed to it.

(defun sum (lst) (format t "The sum is ~s~%" (write-to-string (apply '+ lst))) 
EVAL: (sum '(1 4 6 4))
-> The sum is "15"

answered Nov 02 '13 at 14:58

Sudhanshu Mishra

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score -1 · Answer 5 · answered Feb 26 '09 at 13:40

-1

This ought to do the trick:

(defun sum-list (list)
  (if list
      (+ (car list) (sum-list (cdr list)))
    0))

[source]

Edit: Here is another good link that explains car and cdr - basically they are functions that allow you to grab the first element of a list and retrieve a new list sans the first item.

answered Feb 26 '09 at 13:40

Andrew Hare

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2

Recursion is *bad* in Emacs Lisp. – kmkaplan Feb 26 '09 at 13:45
I was trying to avoid doing exactly that. – jfs Feb 26 '09 at 13:53
Most of the time, you should be able to use reduce and the varieties of map operations instead of explicitly handling cars and cdrs. – Svante Feb 26 '09 at 13:58
2

Because Emacs Lisp does not do tail call optimization. What’s more your sum-list recursion is *not* in tail position. – kmkaplan Feb 26 '09 at 14:08
Good to know! I am still learning all the ins and outs of Lisp myself. – Andrew Hare Feb 26 '09 at 14:14
Then you have an exercice: rewrite your function so that it runs in constant space. – kmkaplan Feb 26 '09 at 14:18
Yikes! I will see what I can do :) – Andrew Hare Feb 26 '09 at 14:47
@kmkaplan: Shouldn't `if` be short-circuited? In that case `sum-list` is in a tail-recursion position, isn't it? – jfs Feb 26 '09 at 15:22
The if is not the problem. The problem is the (+ ... (sum-list ...)). This creates a new frame so that computation of the + function can be done once sum-list returns. – kmkaplan Feb 26 '09 at 15:43
@kmkaplan: (defun sum-iter (L total) (if L (sum-iter (cdr L) (+ (car L) total)) total)) ? (sum-iter '(1 2 3) 0) -> 6 – jfs Feb 26 '09 at 16:12
1

Yes. But remember not to do it with Emacs Lisp, just with Lisps that give you tail call optimization. – kmkaplan Feb 26 '09 at 16:26

score -2 · Answer 6 · answered Apr 02 '17 at 17:20

-2

(insert (number-to-string (apply '+ '(1 2 3))))

answered Apr 02 '17 at 17:20

Яков Кацман

1

score -2 · Answer 7 · edited Sep 17 '11 at 11:31

-2

(eval (cons '+ '(1 2 3))) -- though not as good as 'reduce'

edited Sep 17 '11 at 11:31

Flexo

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answered Sep 16 '11 at 16:01

user946443

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3

There's no need for eval. Apply does the same thing. – jrockway Sep 17 '11 at 04:29
Yes, it was just to show that it could be done in this way as well. – user946443 Sep 17 '11 at 04:34

How to sum a list of numbers in Emacs Lisp?

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