Problem with list in Lisp
I am trying to write a simple procedure in Lisp to insert an element into binary search tree.
I represented the tree as a list:
- the first element in the tree is the root
- the second element is the left sub-tree
- the third element is the right sub-tree
This is my code:
(define Insert
(lambda (x T)
(if (null? T)
(list x '() '())
(if (> x (car T))
(list (car T)
(cadr T)
(Insert x (list (caddr T))))
(list (car T)
(Insert x (cadr T))
(list (caddr T)))))))
When I call the procedure like this: (Insert 2 '(4 '(2 '() '() ) '()))
, I get a problem with ">" because the second arg开发者_Go百科ument isn't a real number, but I don't know why.
The exception:
>: expects type <real number> as 2nd argument, given: quote; other arguments were: 2
However, when I call the procedure like this: (Insert 2 ( list 4 (list 2 '() '() ) '()))
,
it works successfully.
Why?
I know that '(1 '() '())
and (list 1 '() '())
are equals, aren't they?
No, quote
and list
are not the same at all. The meaning of 'foo
is (quote foo)
.
'(a b c)
is exactly (quote (a b c))
, that is, a list literal (the quote
operator prevents the list from being evaluated). It is comparable to "a b c"
, which is a string literal or 5
, which is a number literal. Operations that modify literals have undefined effects, as you may recognize immediately when you see some nonsense like (setf 3 4)
.
(list a b c)
, on the other hand, creates ("conses") a new list from the values of a
, b
, and c
.
I am sure that if you clear up your confusion about quote
and list
, you will be able to correct your code.
'(1 '())
is equivalent to (list 1 (list 'quote nil))
. I suspect that if you drop the "internal" quote characters, you will be fine.
So, the quoted expression that generates an expression that is equal to (list 1 '() '())
is '(1 () ())
.
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