Iterate only length times over a list

Question

Let's say have the following list foo:

(setq foo '(cat dog rat bar))

When I iterate over it using say dolist, I get an extra nil:

(dolist (ele foo)
  (print ele))
cat
dog
rat
bar
nil

Why is there the extra nil ? Shouldn't dolist only iterate over the list the lenght times? (Over each element)

Of course having seen dotimes in the documentation along with dolist I tried to use that as well:

(dotimes (i (length foo))
  (print (nth i))))
cat
dog
rat
bar
nil

I also tried (- (length foo) 1)

cat
dog
rat
nil

So here are my questions

Shouldn't nil be omitted in all cases?
Is there a way I can omit that nil (and iterate over all elements)
I have a feeling that this something to do with linked lists. Can you please explain the missing concept?

Note: I am quite new to elisp and lisp in general. Please be gentle while commenting and answering. I actually am iterating over an alist and accessing the keys and values using car and cdr.

Does this answer your question? Functions print outputs twice the expected output — Drew, Nov 18 '20 at 23:40
It's due to your evaluating the dolist sexp interactively, and the command you use to do that first evaluates the sexp and then prints the result it returns. It has nothing to do with dolist itself or iteration. — Drew, Nov 18 '20 at 23:42

score 3 · Accepted Answer · answered Nov 18 '20 at 08:11

3

Welcome to SE Emacs. The nil you are seeing is not an extra iteration: it is simply the return value of the dolist or dotimes function.

answered Nov 18 '20 at 08:11

Fran Burstall

3,855
11
18

Oh ok. Thank you. I didn't notice that in the docs. – Benjamin Philip Nov 18 '20 at 08:49
@BenjaminPhilip Every expression has a value, which is one important factor that makes Emacs Lisp simple. – xuchunyang Nov 18 '20 at 10:03

score 1 · Answer 2 · answered Nov 18 '20 at 10:20

The issue come because the print output stream and the eval output stream are the same. If you create a different output stream as the default one, the trouble diseapear.

(setq foo '(cat dog rat bar ))
(dolist (ele foo)
  (print ele (get-buffer-create "kill-me")))

The print is now int the "kill-me" buffer and the nil in the default output.buffer.

Iterate only length times over a list

So here are my questions

2 Answers2