c++ - Creating and linkin doubly linked list -

hi have been trying debug piece of code long time cant figure out why. appreciated.

here, trying copy 2 doubly linked lists(of different lengths) new one. however, when return newly made linked list, nodes not connected. can tell me have done wrong?

struct polynode {     int coef;     int expx;     int expy;     polynode* prev;     polynode* next; }  polynode* padd(polynode* a, polynode* b) {     polynode* c = new polynode;     polynode* c_head =c;     polynode* a_head =a;     polynode* b_head =b;     c->prev = nullptr;     c->next = nullptr;      polynode* c_next = c->next;     polynode* c_prev = c->prev;      while (a != nullptr)     {         c->coef = a->coef;         c->expx = a->expx;         c->expy = a->expy;         cout << "\t\t copied c=" << c->coef << c->expx << c->expy << endl;          if(a->next != nullptr)         {             c_next = new polynode;             c_next->prev = c;             c_prev = c;             c = c_next;             = a->next;         }         else         {             c_next = new polynode;             c_next->prev = c;             c_prev = c;             c = c_next;             break;         }     }      while (b != nullptr)     {         c->coef = b->coef;         c->expx = b->expx;         c->expy = b->expy;         cout << "\t\t copied c=" << c->coef << c->expx << c->expy << endl;          if(b->next != nullptr)         {             c_next = new polynode;             c_next->prev = c;             c = c_next;             c_prev = c;             b = b->next;         }         else         {             c_next = nullptr;             break;         }     }      c_next = nullptr;      int sum = polylength(a_head) + polylength(b_head);      for(int =0; i< sum-1 ; i++)     {         if(c_prev == nullptr)         {             break;         }          c_next = c;         c = c_prev;         c_prev = c_prev->prev;     }      c_next = c;     c = c_prev;     c_prev = nullptr;      sortpoly(c); //sortpoly function sorts polynomials largest smallest exponents , sum them if same exponents.     return c;  } 

also not experienced programmer , trying develop programming habits. please suggest common best practice dealing pointers , linked list. thanks! problem here in beginning of function things go bad , returned node not have links next node.

[edit stuyckp] indented code , added problem statement last sentence.

this code full of bugs, if learning work pointers have go 1 step @ time , make drawing of pointers. did verify code , problems pop out pretty quickly.

your basic error seem think following :

polynode* c_next = c->next; polynode* c_prev = c->prev; 

will somehow create variable still belonging c node, while in fact value of c->next null , value copied in new variable. if change new variable, not changing c->next @ all.

i not going solve problems step in right direction. don't use these temporaries, instead :

while (a != nullptr) {     c->coef = a->coef;     c->expx = a->expx;     c->expy = a->expy;     cout << "\t\t copied c=" << c->coef << c->expx << c->expy << endl;      if(a->next != nullptr)     {         c->next = new polynode;//make c's next point new node         c->next->prev = c; //make new nodes prev point c         c->next->next = nullptr; //make sure indicate end         c = c_next; //now move c next position         = a->next; //now move next     }      ... 

i still don't code though, because if , b both null still allocate new node. allocate within while loop. , initialise c null @ start.

your code doing way many thinks make debugging , maintaining crazy. why there copy paste code while loop , while b loop practically same. try make fine grained functions names document doing within them. padd instance bad name large function. not adding copying 2 lists , creating new 1 both of them.

this not c++ either, plain c. in c++ use different approach behavior added datastructure in of methods. won't elaborate on this, think have enough go on.