When I run this code I am not getting an error message from the compiler but I can not return the new list. Am I writing down the code wrong in the MAIN part?
Input
10->20->30->40->50->60->70->80->90->100
Output must be
10->30->50->70->90
#include <stdio.h>
#include <stdlib.h>
typedef struct SinglyLinkedListItem
{
int data;
struct SinglyLinkedListItem*next;
}SLLI;
SLLI*OddNodes(SLLI*pHead)
{
int counter =1;
SLLI*pTemp=pHead;
SLLI*pList=NULL;
while(pTemp != NULL)
{
if(counter % 2 != 0)
{
if(pList==NULL)
{
pList=malloc(sizeof(SLLI));
pList->data=pTemp->data;
pList->next=NULL;
}
else
{
SLLI*pIter=pList;
SLLI*pNew=malloc(sizeof(SLLI));
pNew->data=pTemp->data;
pNew->next=NULL;
pIter->next=pNew;
pIter=pIter->next;
}
}
pTemp=pTemp->next;
counter ++;
}
return pList;
}
You are always changing the same object pList->next.
else
{
pList->next=pTemp;
}
And moreover the original list is not being changed. So the function has undefined behavior.
For starters you should pass the head of the original node by reference. Otherwise the function will deal with a copy of head and any changes of the copy will not influence on the original list.
Here is a demonstrative program that shows how the function can be implemented.
#include <stdio.h>
#include <stdlib.h>
typedef struct SinglyLinkedListItem
{
int data;
struct SinglyLinkedListItem *next;
} SLLI;
SLLI * OddNodes( SLLI **pHead )
{
int odd = 0;
SLLI *pList = NULL;
SLLI **pCurrent = &pList;
while ( *pHead != NULL )
{
if ( odd ^= 1 )
{
*pCurrent = *pHead;
*pHead = ( *pHead )->next;
( *pCurrent )->next = NULL;
pCurrent = &( *pCurrent )->next;
}
else
{
pHead = &( *pHead )->next;
}
}
return pList;
}
int insert( SLLI **pHead, int data )
{
SLLI *pCurrent = malloc( sizeof( SLLI ) );
int success = pCurrent != NULL;
if ( success )
{
pCurrent->data = data;
pCurrent->next = *pHead;
*pHead = pCurrent;
}
return success;
}
void out( SLLI *pHead )
{
for ( ; pHead != NULL; pHead = pHead->next )
{
printf( "%d -> ", pHead->data );
}
puts( "null" );
}
int main(void)
{
const int N = 10;
SLLI *pHead = NULL;
for ( int i = N; i != 0; --i )
{
insert( &pHead, 10 * i );
}
out( pHead );
SLLI *pSecondHead = OddNodes( &pHead );
out( pHead );
out( pSecondHead );
return 0;
}
The function output is
10 -> 20 -> 30 -> 40 -> 50 -> 60 -> 70 -> 80 -> 90 -> 100 -> null
20 -> 40 -> 60 -> 80 -> 100 -> null
10 -> 30 -> 50 -> 70 -> 90 -> null
If you are not going to change the original list then the function can look simpler because in this case there is no need to pass the pointer pHead to the function by reference.
Here is a demonstrative program.
#include <stdio.h>
#include <stdlib.h>
typedef struct SinglyLinkedListItem
{
int data;
struct SinglyLinkedListItem *next;
} SLLI;
SLLI * OddNodes( SLLI *pHead )
{
int odd = 0;
SLLI *pList = NULL;
SLLI **pCurrent = &pList;
for ( ; pHead != NULL; pHead = pHead->next )
{
if ( odd ^= 1 )
{
*pCurrent = malloc( sizeof( SLLI ) );
( *pCurrent )->data = pHead->data;
( *pCurrent )->next = NULL;
pCurrent = &( *pCurrent )->next;
}
}
return pList;
}
int insert( SLLI **pHead, int data )
{
SLLI *pCurrent = malloc( sizeof( SLLI ) );
int success = pCurrent != NULL;
if ( success )
{
pCurrent->data = data;
pCurrent->next = *pHead;
*pHead = pCurrent;
}
return success;
}
void out( SLLI *pHead )
{
for ( ; pHead != NULL; pHead = pHead->next )
{
printf( "%d -> ", pHead->data );
}
puts( "null" );
}
int main(void)
{
const int N = 10;
SLLI *pHead = NULL;
for ( int i = N; i != 0; --i )
{
insert( &pHead, 10 * i );
}
out( pHead );
SLLI *pSecondHead = OddNodes( pHead );
out( pHead );
out( pSecondHead );
return 0;
}
Its output is
10 -> 20 -> 30 -> 40 -> 50 -> 60 -> 70 -> 80 -> 90 -> 100 -> null
10 -> 20 -> 30 -> 40 -> 50 -> 60 -> 70 -> 80 -> 90 -> 100 -> null
10 -> 30 -> 50 -> 70 -> 90 -> null
If you do not yest understand the work with pointers by reference then the function can look the following way
SLLI * OddNodes( SLLI *pHead )
{
int odd = 0;
SLLI *pList = NULL;
for ( SLLI *pCurrent = pList; pHead != NULL; pHead = pHead->next )
{
if ( odd ^= 1 )
{
if ( pCurrent == NULL )
{
pList = malloc( sizeof( SLLI ) );
pList->data = pHead->data;
pList->next = NULL;
pCurrent = pList;
}
else
{
pCurrent->next = malloc( sizeof( SLLI ) );
pCurrent->next->data = pHead->data;
pCurrent->next->next = NULL;
pCurrent = pCurrent->next;
}
}
}
return pList;
}
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