Move the head node from a first linked list to the back of a second linked list-CodePudding

When a certain if statement is passed i have to remove the head node from my first linked-list "new_queue" and add this to the back of my second linked-list "ready_queue".

When I try this the head gets removed from "new_queue" but it does not get added to the back of "ready_queue", instead it always just replaces the second node in "ready_queue".

I think this is because of the line ready_queue_prev -> next = NULL;, but if I remove this line the whole linked list will be put at the back of "ready_queue" instead of just the node.

Anyone know how to solve this?

typedef struct ST_PCB {
  int arrival_time;
  char name[9];
  int duration;
  struct ST_PCB * next;
} T_PCB;


int main(void){
    // serial list of newly arrived tasks
    T_PCB * new_queue = NULL;
    // circular list of active tasks
    T_PCB * ready_queue = NULL;
    // extra state needed to switch tasks 
    // from new_queue to ready_queue when they're started
    T_PCB * ready_queue_prev = NULL;

    //this constructs the linked-list and sorts it by arrival time
    new_queue = read_tasks();
    new_queue = sort_tasks_on_arrival(new_queue);


    if(something happends...){
        if(ready_queue != NULL){
            ready_queue_prev = new_queue;
            new_queue = new_queue->next;      
            ready_queue -> next = ready_queue_prev;
            ready_queue_prev -> next = NULL;
        }
        else{
            ready_queue = new_queue;
            new_queue = new_queue->next;
            ready_queue->next = NULL;     
        }
    }
}

CodePudding user response：

This statement is indeed making the node the second one in the list:

ready_queue -> next = ready_queue_prev;

ready_queue is the head of the queue, not the tail. You need to first find out where is the tail, and then do that assignment there:

T_PCB * tail = ready_queue;
while (tail->next != NULL) {
    tail = tail->next;
}
// We found the tail. Now make the assignment to `next`:
tail->next = ready_queue_prev;
// And now continue with what you had:
ready_queue_prev->next = NULL;

CodePudding user response：

When a certain if statement is passed i have to remove the head node from my first linked-list "new_queue" and add this to the back of my second linked-list "ready_queue".

To add a node to the tail of a linked list you have to find the tail.

First of all actually you need to check whether new_queue is not equal to NULL. If it is equal to NULL then there is nothing to append. The if statement (without any else because it is not required) can look the following way

    if ( new_queue != NULL)
    {
        T_PCB *tmp = new_queue;
        new_queue = new_queue->next;
        tmp->next = NULL;

        T_PCB **current = &ready_queue;
        while ( *current != NULL ) current = &( *current )->next;
        *current = tmp;     
    }