How would I print out a list with spaces in between elements

Question

So I'm trying to print out a list that looks a little bit something like this (setq lst (list '- '- '- '- '-)) and in the past I used the print command to print out the whole list, however, when printing the whole list there is parenthesis on each side which I do not want to see. I want to use something like (format t) to print every bit of my list and I have something like this set up.

(loop for item from 0 to 4
     do (progn
             (format t "~X" (nth item lst))
        )
)

This code prints out the list perfectly fine like this, ----- but as a mentioned, I want it to print spaces between each element so that it is output like this - - - - -. I used the conditional "~X" because I looked up how to output spaces with the format command and you are apparently supposed to use "~X" but it does not work so if anybody knows how I could put spaces between elements that would be greatly appreciated.

where is this 'conditional' documented? What does it mean 'it does not work'? — Rainer Joswig, Jul 03 '21 at 21:43
In `FORMAT`, `~X` is for printing in hexadecimal: http://www.lispworks.com/documentation/lw50/CLHS/Body/22_cbe.htm — Barmar, Jul 03 '21 at 21:52
Is there a reason you're using `nth` instead of `for item in lst`? — Barmar, Jul 03 '21 at 21:53
That makes sense but what's the conditional to make a space? — DartAce345, Jul 03 '21 at 21:54
that looks like a site of random garbage. Probably better to read actual documentation. http://www.lispworks.com/documentation/HyperSpec/Body/22_c.htm — Rainer Joswig, Jul 03 '21 at 21:54
Well I simplified down my code for this post but in reality I'm using nested lists so I have nested loops to run them but the code still works the same way. — DartAce345, Jul 03 '21 at 21:56
You could perhaps implement it using `~[`. See http://www.lispworks.com/documentation/lw50/CLHS/Body/22_cgb.htm — Barmar, Jul 03 '21 at 21:56
I tried "~[" but since there was no ] to go with it my computer got confused. — DartAce345, Jul 03 '21 at 21:58
Alright I'm adequately confused by this http://www.lispworks.com/documentation/lw50/CLHS/Body/22_cgb.htm could you explain your idea? — DartAce345, Jul 03 '21 at 22:02
https://stackoverflow.com/questions/8830888/whats-the-canonical-way-to-join-strings-in-a-list/41091118#41091118 — Rainer Joswig, Jul 03 '21 at 22:24

score 4 · Answer 1 · answered Jul 03 '21 at 22:01

4

Why not just use the features provided by format:

CL-USER> (defvar *my-list* '(- - - -))
*MY-LIST*
CL-USER> (format nil "~{~A~^  ~}" *my-list*)
"-  -  -  -"
CL-USER> (format t "~{~A~^  ~}" *my-list*)
-  -  -  -
NIL

Here the first call to format outputs to a string to show where the spaces are placed. ~{ is an iteration directive that takes a list as its argument. The directives between the opening ~{ and closing ~} are used repeatedly as a format string for the elements of the input list. The ~^ directive causes an early escape from the iteration context when there are no more arguments; this prevents a trailing space from being added.

The second call to format just outputs to *standard-output*.

answered Jul 03 '21 at 22:01

ad absurdum

19,498
5
37
60

The reason why I didn't do it like this is because I need to be able to index each item in a list and running functions on a string is something that I do not know how to do. – DartAce345 Jul 03 '21 at 22:07
1

Well, now I'm not exactly sure what you are trying to accomplish. Why do you need to index each list element? [Why are you using nested lists "in reality"?](https://stackoverflow.com/questions/68240118/how-would-i-print-out-a-list-with-spaces-in-between-elements/68240236#comment120605251_68240118) I answered wrt "_how would I print a list with spaces between elements_," but the new requirements are not clear; a good [MCVE] would help here. – ad absurdum Jul 04 '21 at 01:33

score 1 · Answer 2 · answered Aug 22 '21 at 09:24

Regarding your update, that you posted in the answers to your own post:

First of all, you should edit your post to show us that you found a solution, rather than having us look through all the answers to see how much progress you made on your initial problem.
As it was already mentioned in another answer, you can iterate through the elements of a list using format built-in syntax with ~{~} and ~^ (see the documentation !)
In your own solution, when you iterate over the list using loop, you can put a space at the end of the format string rather than calling format twice ...
You can use loop for <elt> in <list> rather than iterating with the indices, and calling nth at each step - which is slower, and also more verbose.
The loop ... do <stuff> already wraps the <stuff> in what we call an implicit progn, i.e. you do not need to wrap yourself all your instructions in a progn, the loop macro does that for you.
There also exists the macro dolist, which is (arguably) simpler to use in those case when you simply want to iterate over a list.

To be fair, it looks like you are a Common Lisp beginner. In this case, I suggest you read the excellent Practical Common Lisp book, which covers in details the loop macro, the format function, and a lot of basic principles. It is available for free online, and is often recommended to beginners, for good reasons !

score -3 · Answer 3 · answered Jul 03 '21 at 22:24

Ok I came up with an ingenius solution to my problem which I definitely should've seen before.

(loop for item from 0 to 4
     do (progn
             (format t "~X" (nth item lst))
             (format t " ")
        )
)

I didn't realize I could print a space like that but it works perfectly fine. Sorry for wasting you all's time but hopefully someone else can see this if they are having a brain fart like me and thanks to everyone who tried to help.

How would I print out a list with spaces in between elements

3 Answers3