Your loop is breaking early because res iterates in
a list of only one value.

You want

for res = (length arg)

to replace the second line, probably.

On Mon, Jun 11, 2018 at 5:25 PM, Michael Brand <michael.ch.brand@gmail.com>
wrote:

> Hi all
>
> I try to replicate the following Python list comprehension with Emacs
> Lisp. It reports the list elements with its length when the length is
> even. The expectation is to calculate the length only once and to not
> use a second and nested list comprehension:
>
> #+begin_src python :results verbatim
>   return [[res, arg]
>           for arg in ["ab", "c", ""]
>           for res in [len(arg)]
>           if res % 2 == 0]
> #+end_src
> #+results:
> : [[2, 'ab'], [0, '']]
>
> The above result is as expected (Python 2.7 and 3.6).
>
> In my trial with Emacs Lisp the syntax for the equivalent expression
> seems to be accepted but the result is not the expected ((2 "ab") (0
> "")):
>
> #+begin_src emacs-lisp :results verbatim
>   (cl-loop for arg in '("ab" "c" "")
>            for res in (list (length arg))
>            if (= (% res 2) 0)
>            collect (list res arg))
> #+end_src
> #+results:
> : ((0 "ab"))
>
> Am I doing something wrong or is this a bug in
> emacs-26.1-2189-g8377ca6 and 26.1?
>
> Michael
>
> PS: A workaround would be this less elegant version:
>
> #+begin_src emacs-lisp :results verbatim
>   (let (res)
>     (cl-loop for arg in '("ab" "c" "")
>              if (progn (setq res (length arg))
>                        (= (% res 2) 0))
>              collect (list res arg)))
> #+end_src
> #+results:
> : ((2 "ab") (0 ""))
>
>


-- 
João Távora