c - Avoiding Extra Malloc in Linked List (node->next = NULL) -

in linked list, i'm trying avoid mallocing node without adding bunch of if statements , such. have following:

polynomial create() {     polynomial head = null;     polynomial temp = null;     int numterms, coefficient, exponent;     int counter = 0;      printf("enter number of terms in polynomial: ");     scanf ("%d", &numterms);     printf("\n");      if (numterms == 0) {         head = malloc(sizeof(term));         head->coef = 0; head->exp = 0; head->next = null;         return head;     }     while (numterms != counter) {         // ask input         printf("enter coefficient , exponent of term %d: ", (counter + 1));         scanf("%d %d", &coefficient, &exponent);         printf("\n");          // create term         if (temp == null) temp = malloc(sizeof(term));         temp->coef = coefficient; temp->exp = exponent; temp->next = null;         //if((numterms - 1) != counter) temp->next = malloc(sizeof(term)); -- workaround         printf("created: %d %d\n", temp->coef, temp->exp);          // if first node created, mark head         if (counter == 0) head = temp;         // increment list , counter         temp = temp->next;         counter ++;     }     return head; } 

but when go print polynomial (i have function works so), following:

polynomial 1: 3x^4 --> in case, reference head->next null

so tried following workaround - allocate memory in advance new nodes, if last iteration of user input. accomplished by:

replace temp->next = null; with

if((numterms - 1) != counter) temp->next = malloc(sizeof(term));

the numterms - 1 prevents adding 'extra node', , malloc keep reference temp->next alive. if don't use if check , always allocate memory, end following:

polynomial 1: 3x^4 - 7x^2 + 5 + 10621224x^10617028

what part of allocation missing causes reference temp->next lost? i'm really terrible pointers , memory management in general, terrible question.

you're making harder needs be. consider simple node-population linked list conforms following:

• assumes null means empty list
• allocates head pointer without having test each allocation.
• one, , only one allocation per-node required.
• nodes presented in list in entry-order. first node in list first 1 entered, second node second entered, etc.

with that, see below general algorithm how adapts code:

struct node {     // fields here     struct node *next; };  struct node* populate_list() {     struct node *head = null;     struct node **pp = &head;     int i=0, count=0;      // todo: set count: insertion limit here.       // run insertion loop     (i=0; i<count; ++i)     {         struct node *p = malloc(sizeof(*p));          // todo: initialize node members here          // save our current tail-pointer,         //  head pointer. advance new tail         //  pointer , continue loop         *pp = p;         pp = &p->next;     }      // terminate list.     *pp = null;      // , return head pointer.     return head; } 

note: p there clarity. can reduce loop body following, totally valid:

    // run insertion loop     (i=0; i<count; ++i)     {         *pp = malloc(sizeof(**pp));          // todo: initialize node members here         //  using (*pp)->member access          // save next pointer , continue.             pp = &(*pp)->next;     } 

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adapting code

now know how this, considerably reduce code this:

polynomial create() {     polynomial head = null;     polynomial *pp = &head;      int numterms, coefficient, exponent;     int counter = 0;      // prompt valid number of terms.     printf("enter number of terms in polynomial: ");     scanf ("%d", &numterms);      while (numterms != counter)     {         // ask input         printf("enter coefficient , exponent of term %d: ", (counter + 1));         if (scanf("%d %d", &coefficient, &exponent) == 2)         {             *pp = malloc(sizeof(**pp));             (*pp)->coef = coefficient;             (*pp)->exp = exponent;             pp = &(*pp)->next;             ++counter;         }         else         {   // eat line , try again.             scanf("%*[^\n]\n")         }     }     *pp = null;      return head; }