reverse linked list

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reverse linked list

 
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  #1
Jan 4th, 2008
hello
i want to build a function which receives a pointer to a head of linked list
reverses the order of the list and returns a pointer to the new head.

my list is defined like this :

C Syntax (Toggle Plain Text) 
  1. typedef struct item {
  2.  
  3. 	int key ;
  4.  
  5. 	struct item *next ;
  6.  
  7. } item ;

my fucction is :

C Syntax (Toggle Plain Text) 
  1. void swapitem ( item *a ) {
  2.  
  3.  (a -> next) -> next = a  ;
  4.  
  5. 	a -> next = NULL ;
  6.  
  7. } // end swapitem
  8.  
  9.  
  10.   item** revList ( item *head ) {
  11.  
  12.  	item **p ;
  13.  
  14.  	if ( !head -> next ) {
  15.  
  16.  		**p  = *head ;
  17.  
  18.  		return p ;
  19.  
  20.  	}
  21.  
  22.  	else {
  23.  
  24.  	revList (head->next) ;
  25.  
  26.  	swapitem (head) ;
  27.  
  28.  	return p ;
  29.  
  30. }	
  31.  
  32.  }  // end revList

at first when i print the list it is ok
when i try to print the list after the function it shows nothing
please advice.
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Re: reverse linked list

 
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  #2
Jan 4th, 2008
When working with linked data structures, the best thing you can do is take out a piece of paper and draw the logic. Don't draw what you think is happening, draw exactly what your code does, and you'll find the bugs.
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Re: reverse linked list

 
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  #3
Jan 4th, 2008
I always take it a step farther and just tell people to forget the code. Get out the paper and crayons first, draw what must happen, then write the code to do exactly that.

Disentangling new programmers from bad code is more difficult and confusing than just going through the steps of writing the code correctly to begin with...
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Re: reverse linked list

 
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  #4
Jan 4th, 2008
>Disentangling new programmers from bad code is more
>difficult and confusing than just going through the steps
>of writing the code correctly to begin with...
Writing code correctly is all well and good, but learning to debug broken code is just as important (if not more so).
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Re: reverse linked list

 
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  #5
Jan 4th, 2008
Agreed, but when dealing with new programmers I have learned by experience (teaching and tutoring) that debugging works best once a firm concept of what should be happing is established. Anyway...
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Re: reverse linked list

 
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  #6
Jan 5th, 2008
ok i did it all i had to do is turn my pointer static

C Syntax (Toggle Plain Text) 
  1. item *revList ( item *head ) {
  2.  
  3.  	static item* p ;
  4.  
  5.  	if ( !head -> next ) {
  6.  
  7.  		p = head ;
  8.  
  9.  		return head ;
  10.  
  11.  	}
  12.  
  13.  	revList (head->next) ;
  14.  
  15.  	swapitem (head) ;
  16.  
  17.  	return p ;	
  18.  
  19.  }  // end revList

thanks everyone for the help
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