CODEWITHSUNDEEP
listlisp
barankin
barankin

Reputation: 1943

Why lists created with cons() and list() are shown differently?

Why lists created with cons() and list() are shown differently despite the lists are equal? The first one shows items separated by dot, but second one do w/o.

> (cons 1 2)
(1 . 2)

> '(1 2)
(1 2)

I know that cons constructs dotted pair, but in this case the lists are same but shown differently.

Upvotes: 0

Views: 126

Answers (3)

pimgeek
pimgeek

Reputation: 259

Let me expand previous answers still further

Although we are talking about Lisp language here, I notice that a line from Page 8 and 9 of a famous Book named "The Little Schemer (4th edition)" help me understand the 2 puzzling facts altogether:

    Why is (cons 1    2) does not look like '(1 2)?
    Why is (cons 1 '(2)) does     look like '(1 2)?
    ----
    > (cons 1 2)
    (1 . 2)
    > (cons 1 '(2))
    (1 2)
    > '(1 2)
    (1 2)

Just read the "The Laws of Cons":

The primitive cons takes 2 arguments.

The 2nd argument to cons must be a list.

The result is a list.

In practice: (cons A B) works for all values A and B, And

(car (cons A B)) = A

(cdr (cons A B)) = B

Upvotes: 0

C. K. Young
C. K. Young

Reputation: 222973

To expand on Basile's answer:

(1 2)

is a proper list of length two. That is, it contains two cons cells:

#1=(1 . #2#)
#2=(2 . ())

On the other hand,

(1 . 2)

is an improper list of length one. That is, it contains one cons cell:

#1=(1 . 2)

A non-empty proper list is a list where the last cons cell's cdr that contains the empty list, (). A non-empty improper list is a list where the last cons cell's cdr contains anything else.

Upvotes: 5

Basile Starynkevitch
Basile Starynkevitch

Reputation: 1

It is not the same list; a list such as

  (1 2)

which is returned by your quoted (i.e. 2nd) expression is the same as the result of

 (cons 1 (cons 2 ())

A proper list like above is always a single-linked list of pairs whose last pair has a nil tail.

Upvotes: 8

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