Thread: use with a function

can some1 help me with understanding the code

Code:

struct Edge
{
  int terminal;
  struct Edge *next;
};

struct Edge *Insert_Vertex (int , struct Edge *);

struct Edge * Insert_Vertex (int vertex_no, struct Edge *first) {
  struct Edge *new1, *current;
  new1 = (struct Edge *) malloc(sizeof(struct Edge));
  //current = (struct Edge *) malloc(sizeof(struct Edge));
  new1->terminal = vertex_no;
  new1->next = NULL;
  if (!first)
    return (new1);
  else {
    for(current = first; current->next; current = current->next)
      current->next = new1;
    return (first);
  }
}

I also put how the Linked List is made.
thanks.

Code:

struct Edge
{
  int terminal;
  struct Edge *next;
};

struct Edge *Insert_Vertex (int , struct Edge *);

struct Edge * Insert_Vertex (int vertex_no, struct Edge *first) { // this function takes verttex_no and pointer to the struct edge
  struct Edge *new1, *current;  // we define 2 new variables with struct Edge
  new1 = (struct Edge *) malloc(sizeof(struct Edge));
  //current = (struct Edge *) malloc(sizeof(struct Edge));
  new1->terminal = vertex_no; // the new1 points to terminal : its gives Vertex_no. basically
  new1->next = NULL; // if the list next is null :means not pointing to anything anymore
  if (!first)      // i dont follow this
    return (new1);
  else {
    for(current = first; current->next; current = current->next) // not even this??????
      current->next = new1;
    return (first);
  }
}

I hope I am clear with my question

thanks ....
I actually dont understand why i get error when i run my actual file (i.e 500 elements)


in Insert_Vertex (vertex_no=334, first=0x0) error at
for(current = first; current->next==NULL; current = current->next)

but dont see it when i run some small example file.(10 elements)

Originally Posted by noobiept

if (!first) is testing to see if the variable first is equal or not to 0 (NULL)
its equivalent to

Code:

if (first != NULL)
else

the cicle I don't get it what it is doing, either way, it could had been written as:

Code:

for(current = first; current->next == NULL; current = current->next)

Those are turned around. !first is equivalent to first == NULL, and current->next in a condition (like in an if statement or loop condition) is equivalent to current->next != NULL.

sorry, I got it wrong, its actually the other way around. aahh

Originally Posted by robwhit

Those are turned around. !first is equivalent to first == NULL, and current->next in a condition (like in an if statement or loop condition) is equivalent to current->next != NULL.

yep, was just thinking now and wasn't sounding right. you're right.

I still dont follow...
I understoof very much like noobiept wrote
so u say it is equivalent to :

Code:

 if (first== NULL)      // i dont follow this
    return (new1);
  else {
    for(current = first; current->next != NULL; current = current->next) 
      current->next = new1;
    return (first);
  }
}

is it ???

Either add to the head or the tail of the list, the former being the easier of the two options.
The way for loop is written currently, makes the program end up with lots of dangling pointers.