I want to work with "generalize d lists" as in Lisp, say :
((23 () . 2) () ((10))) ; only pointers and integers
So the basic element is a node, a struct with two fields car and cdr.
Each of these fields can contain either a pointer (NULL or a pointer to
another node), or an int.
I want to define the functions cons, car and cdr of Lisp.
I tried this but gcc rejects it :
#include <stdio.h>
#include <stdlib.h>
typedef struct {
object car;
object cdr;
} node;
typedef struct {
int tag; // 0==int, 1==node*
union {
int value;
node *ptr;
} val;
} object;
node* cons(object a, object b) { ...}
Can you help ? Thanks...
JG 5 3091
Jean-Guillaume Pyraksos wrote On 04/11/06 10:00,: I want to work with "generalize d lists" as in Lisp, say :
((23 () . 2) () ((10))) ; only pointers and integers
So the basic element is a node, a struct with two fields car and cdr. Each of these fields can contain either a pointer (NULL or a pointer to another node), or an int. [...]
This is Question 1.15 in the comp.lang.c Frequently
Asked Questions (FAQ) list http://www.c-faq.com/
-- Er*********@sun .com
Jean-Guillaume Pyraksos wrote: I want to work with "generalize d lists" as in Lisp, say :
[...]
See some inserted comments: typedef struct { object car; object cdr; } node;
In Lisp, the cdr of a list is a list, not an object. Thus:
object cdr => node *cdr;
that can be written in C like:
typedef struct node {
object car;
struct node *cdr;
} node;
typedef struct { int tag; // 0==int, 1==node* union { int value; node *ptr; } val; } object;
"object" must be declared before "node", because "node" uses it
node* cons(object a, object b) { ...}
"cons" in Lisp takes a list and an object, not two objects. And in C
better not to pass structures, but pointers to structures:
node *cons (object *a, node *b) { ... }
Kind regards.
On Tue, 11 Apr 2006 16:00:32 +0200, Jean-Guillaume Pyraksos wrote: I want to work with "generalize d lists" as in Lisp, say :
((23 () . 2) () ((10))) ; only pointers and integers
So the basic element is a node, a struct with two fields car and cdr. Each of these fields can contain either a pointer (NULL or a pointer to another node), or an int.
I want to define the functions cons, car and cdr of Lisp.
I tried this but gcc rejects it :
#include <stdio.h> #include <stdlib.h>
typedef struct { object car; object cdr; } node;
typedef struct { int tag; // 0==int, 1==node* union { int value; node *ptr; } val; } object;
node* cons(object a, object b) { ...}
Can you help ? Thanks...
JG
This compiles (gcc 3.4)
typedef struct
{
int tag; /* 0==int, 1==node* */
union {
int value;
struct node_tag *ptr;
} val;
} object;
typedef struct node_tag
{
object car;
object cdr;
} node;
Duncan
In article <11************ *********@i40g2 000cwc.googlegr oups.com>, "tmp123" <tm****@menta.n et> writes: Jean-Guillaume Pyraksos wrote: I want to work with "generalize d lists" as in Lisp, say :
In Lisp, the cdr of a list is a list, not an object.
[OT] The cdr of a cons cell in a list is, by definition, a list
(possibly nil), but in general the cdr of a cons cell is not
necessarily a list. See "dotted pair". To implement general
LISP-style cons cells in C, you'd have to allow for cdrs that are
not pointers to other cons cells.
Thus:
[1]> (cons 'a 'b)
(A . B)
[2]> (cdr (cons 'a 'b))
B
--
Michael Wojcik mi************@ microfocus.com
Auden often writes like Disney. Like Disney, he knows the shape of beasts --
(& incidently he, too, might have a company of artists producing his lines) --
unlike Lawrence, he does not know what shapes or motivates these beasts.
-- Dylan Thomas
Michael Wojcik wrote: In article <11************ *********@i40g2 000cwc.googlegr oups.com>, "tmp123" <tm****@menta.n et> writes: Jean-Guillaume Pyraksos wrote: I want to work with "generalize d lists" as in Lisp, say :
In Lisp, the cdr of a list is a list, not an object.
[OT] The cdr of a cons cell in a list is, by definition, a list (possibly nil), but in general the cdr of a cons cell is not necessarily a list. See "dotted pair". To implement general LISP-style cons cells in C, you'd have to allow for cdrs that are not pointers to other cons cells.
Thus:
[1]> (cons 'a 'b) (A . B) [2]> (cdr (cons 'a 'b)) B
Thanks for the clarification. I didn't known it. This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
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