Thread: Singly Linked List - Insert value after given value

I have really been struggling with segmentation faults in my linked lists assignments. I think I do not totally understand the pointers. The assignment below was given to me in its entirety except that I am to create the functions. The functions are insertFront, to add a value to the front of the list (which I have gotten to work), insertBack, to insert a value to the back of the list (which works), isInList, which searches the list for a value input by the user and returns 1 if it is there and 0 if it is not (which works), findNode, which returns the node the value was found in (which mostly works, I need to tweak it), insertAfter, to insert a value after a specified node (which seg faults) and insertBefore, to insert a value before a specified node (which seg faults). I am at my wits end trying to fix these functions. If someone could help me figure out what I am doing wrong and explain it to me in the plainest English possible I would be very, very, grateful!

Here is the code: (Note insertAfter and insertBefore are done two different ways -- insertBefore with a counter to count the actual node position and insertAfter to check the actual value the next value should be added after -- because I have been trying everything to get them to work. If one way is better than the other, let me know!)

Code:

#include <stdio.h>
#include <stdlib.h>

struct lnode
{
  int data;
  struct lnode *next;
};


struct lheader
{
  struct lnode *start;
  int len;
};


struct lheader *makelist( )
{
  struct lheader *alist;
  alist = malloc( sizeof( struct lheader ) );
  alist->len = 0;
  alist->start = NULL;
  return alist;
}

struct lnode *makenode( int val )
{
  struct lnode *box;
  box = malloc( sizeof( struct lnode ) );
  box->data = val;
  box->next = NULL;
  return box;
}


void insertFront( struct lheader *L, int val )
{
  struct lnode *newnode;
  newnode = makenode(val);
  newnode->next = L->start;
  L->start = newnode;
}


void insertBack( struct lheader *L, int val, int count )
{
  struct lnode *trav;
  struct lnode *newnode;
  newnode = makenode(val);
  if ( L->start == NULL )
  {
    L->start = newnode;
  }
 else
 {
   trav = L->start;
   while( trav->next != NULL )
   {
    trav = trav->next;
   }
   trav->next = newnode;
   L->len = count;
 }
}


int isInList( struct lheader *L, int val )
{
  struct lnode *temp;
  int dupe = 0;
  temp = L->start;
  while ( temp != NULL )
  {
    if ( temp->data == val )
    {
      dupe = 1;
      break;
    }
    else
    {
      temp = temp->next;
    }
  }
  return dupe;
}

struct lnode *findNode( struct lheader *L, int val )
{
  struct lnode *ptr;
  struct lnode *check;
  int i = 0;
  ptr = L->start;
  while ( ptr != NULL )
  {
    ++i;
    check->data = i;
    if ( ptr->data == val )
    {
      break;
    }
    else
    {
      ptr = ptr->next;
    }
  }
  return check;
}


void insertAfter( struct lheader *L, struct lnode *p )
{
  int pos, value;
  struct lnode *nn;
  struct lnode *temp;
  temp = p;
  printf( "What number do you want to insert? " );
  scanf( "%d", &value );
  printf( "Insert after which value: " );
  scanf( "%d", &pos );
  nn = makenode(value);
  if ( L->start == NULL )
  {
    L->start = nn;
  }
  else
  {
    temp = L->start;
    while( temp->next != NULL && temp->data != pos )
    {
      temp = temp->next;
    }
    if ( temp->data == pos )
    {
      nn->next = temp->next;
      temp->next = nn;
      printf("Value is %d: ", nn->data);
    }
    else
    {
      printf( "Value %d not found\n", pos );
    }
  }
}


void insertBefore( struct lheader *L, int val, struct lnode *p )
{
  struct lnode *newnode;
  struct lnode *temp;
  int pos, i;
  newnode = p;
  printf("Insert before which node? (1-5) ");
  scanf( "%d", &pos );
  if ( L->start == NULL )
  {
    L->start = newnode;
  }
  else
  {
    temp = L->start;
    for( i = 0; i < pos - 1; ++i)
    {
      temp = temp->next;
    }
    newnode->next = temp->next;
    temp->next = newnode;
  }
}


void printlist( struct lnode *front )
{
  struct lnode *mov;
  mov = front;
  while (mov != NULL)
  {
    printf("%d  ", mov->data);
    mov = mov->next;
  }
  printf("\n");
}


void printer( struct lheader *alist )
{
  struct lnode *mov;
  printf("--------------------------\n");
  printf("List print, len %d\n", alist->len);
  printlist( alist->start );
  printf("--------------------------\n");
}


int main()
{
  struct lheader *L;
  struct lnode  *head, *tmp;
  struct lnode  *mark;
  int i, x;

  L = makelist();

  for (i = 1; i <= 5; ++i)
  {
    x = rand() % 25 + 1;
    printf("-- Adding -- %d\n", x);
    // insertFront( L, x );
    insertBack( L, x, i );
    printer( L );
  }


  printf(">>>>Value to search: ");
  scanf("%d", &x);
  i = isInList(L, x);
  printf("I is %d\n", i);
  tmp = findNode(L, x);
  if (tmp == NULL)
  {
    printf("NOPE\n");
  }
  else
  {
    printf("Found node %d\n", tmp->data);
  }
  printf(">>>>Value to add: ");
  scanf("%d", &x);
  mark = makenode(x);
  insertBefore( L, x, mark );
  printer( L );
  // insertAfter( L, mark );
  // printer( L );
  return 0;
}

Welcome to the forum. Good first post.

However, you should include instructions on how to trigger the error conditions you're seeing (i.e. what input you are entering, and at what point it goes wrong).

---

If you aren't getting warnings, you need to increase the warning level of your compiler.

Code:

/*
main.c||In function 'printer':|
main.c|187|warning: unused variable 'mov'|
main.c||In function 'main':|
main.c|198|warning: unused variable 'head'|
main.c||In function 'findNode':|
main.c|107|warning: 'check' is used uninitialized in this function|
||=== Build finished: 0 errors, 3 warnings ===|
*/

The third one is really worth looking into.