how to check if a list is empty or nil?

[ssecorp]

(not (= lst nil))

(/= lst nil)

empty?, list?, nil? does not work either.

do I really have to use (= 0 (length lst))
seems inefficient if the lists can be big.

[Brian]

Post by ssecorp
(not (= lst nil))
(/= lst nil)
empty?, list?, nil? does not work either.
do I really have to use (= 0 (length lst))
seems inefficient if the lists can be big.

Use NULL. ENDP also works.

[Kenny]

Post by ssecorp
(not (= lst nil))
(/= lst nil)
empty?, list?, nil? does not work either.
do I really have to use (= 0 (length lst))
seems inefficient if the lists can be big.

lst. Or list if you realize you are not doing scheme.

kt

[ssecorp]

Post by Kenny

Post by ssecorp
(not (= lst nil))
(/= lst nil)
empty?, list?, nil? does not work either.
do I really have to use (= 0 (length lst))
seems inefficient if the lists can be big.

lst. Or list if you realize you are not doing scheme.
kt

huh?

[DeverLite]

Post by Kenny

Post by ssecorp
(not (= lst nil))
(/= lst nil)
empty?, list?, nil? does not work either.
do I really have to use (= 0 (length lst))
seems inefficient if the lists can be big.

lst. Or list if you realize you are not doing scheme.
kt

huh?

I think Kenny's point is that you can use the name "list" rather than
"lst", and this will not conflict with the function (named "list")
because, unlike scheme, CL has a separate namespace for functions.

[Pascal J. Bourguignon]

Post by DeverLite

Post by Kenny

Post by ssecorp
(not (= lst nil))
(/= lst nil)
empty?, list?, nil? does not work either.
do I really have to use (= 0 (length lst))
seems inefficient if the lists can be big.

lst. Or list if you realize you are not doing scheme.
kt

huh?

I think Kenny's point is that you can use the name "list" rather than
"lst", and this will not conflict with the function (named "list")
because, unlike scheme, CL has a separate namespace for functions.

No. Kenny's point is that to test for a not null list you can just
write list. In the same way that in C, to test for a non zero integer
you can just write integer.


And the OP should try reading some initiation book about Common Lisp,
like:

Common Lisp: A Gentle Introduction to Symbolic Computation
http://www-cgi.cs.cmu.edu/afs/cs.cmu.edu/user/dst/www/LispBook/index.html
http://www.cs.cmu.edu/~dst/LispBook/

[Kenny]

Post by Kenny

Post by ssecorp
(not (= lst nil))
(/= lst nil)
empty?, list?, nil? does not work either.
do I really have to use (= 0 (length lst))
seems inefficient if the lists can be big.

lst. Or list if you realize you are not doing scheme.
kt

huh?

1. nil is false
2. there is no need to mangle the name
3. 1 and 2 are false in Scheme

kt

[Barry Margolin]

In article

Post by ssecorp
(not (= lst nil))
(/= lst nil)

= and /= are for numbers. EQ is the predicate you should use if you
want to compare with NIL.

Post by ssecorp
empty?, list?, nil? does not work either.

Others have mentioned NULL. There's also NOT (although that should
generally only be used when you're treating it as a boolean, rather than
a list) and ENDP (which is generally used when you're iterating, hence
the name).

Post by ssecorp
do I really have to use (= 0 (length lst))
seems inefficient if the lists can be big.

[Chris Russell]

Post by ssecorp
(not (= lst nil))
(/= lst nil)
empty?, list?, nil? does not work either.
do I really have to use (= 0 (length lst))
seems inefficient if the lists can be big.

The empty list is false so you can write

(if list
(do stuff);non-empty case
(do other stuff));empty case

Otherwise do what Barry says.