Question

I am trying to create a linked list that will take the input from the user, order it, and print it out once the user inputs 0 or a negative number. Somewhere my code is adding a “0” to the begining of the print loop.
Example: I input 1-2-3-4-5. The program then returns 0-1-2-3-4-5.
Example2: I input 1-2-3-4-5. The program then returns 0-5-1-2-3-4. Which is also a problem for me because i eventually need to make the program order the inputed values from least to greatest. But for right now im focused on getting it to take the input 1-2-3-4-5 and print 1-2-3-4-5.

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

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

//prototypes
void insertNode(struct listNode *Head, int x);
void printList(struct listNode *Head);
int freeList(struct listNode *Head, int x);

//main
int main(){
     struct listNode Head = {0, NULL};
     int x = 1;
     printf("This program will create an odered linked list of numbers greater"
     " than 0 until the user inputs 0 or a negative number.\n");
     while (x > 0){
           printf("Please input a value to store into the list.\n");
           scanf("%d", &x);
           if (x > 0){
           insertNode(&Head, x);
           }
     }
     printList(&Head);
     system("PAUSE");
     }

void insertNode(struct listNode * Head, int x){
     struct listNode *newNode, *current;
     newNode = malloc(sizeof(struct listNode));
     newNode->data = x;
     newNode->next = NULL;
     current = Head;
     while (current->next != NULL && current->data < x) 
     {
        current = current->next;
        }

        if(current->next == NULL){
             current->next = newNode;
        }
        else{
             newNode->next = current->next;
             current->next = newNode;
        }
}
void printList(struct listNode * Head){
    struct listNode *current = Head;
    while (current != NULL){
          if(current > 0){
               printf("%d \n", *current);
          }
          current = current->next;
    }
}

Answer 1

It has a zero in the list because you put it there:

struct listNode Head = {0, NULL};

If you want a quick fix, change the line in printList() and anything else that processes the list from:

struct listNode *current = Head;

to:

struct listNode *current = Head->next;

This will start at the second element of the list, ignoring the one you put there to start with.

---

However, a better way is probably not to have that extraneous element at all:

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

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

// Prototypes (freeList removed since not defined).

void insertNode(struct listNode **pHead, int val);
void printList(struct listNode *Head);

// Main program for testing.

int main(void) {
    // List initially empty.

    struct listNode *Head = NULL;

    int x = 1;
    puts("This program will create an ordered linked list");
    puts("    of numbers greater than 0 until the user");
    puts("    enters 0, a negative number, or a non-integer.");
    for(;;) {
          puts("Please input a value to store into the list.");
          if ((scanf("%d", &x) != 1) || (x <= 0)) break;
          insertNode(&Head, x);
     }
     printList(Head);
}

void insertNode(struct listNode **pHead, int val){
    struct listNode *newNode, *current, *previous;

    // Allocate new node, should really check for failure here.

    newNode = malloc (sizeof (struct listNode));
    newNode->data = val;
    newNode->next = NULL;

    // Handle inserting into empty list.

    if (*pHead == NULL) {
        *pHead = newNode;
        return;
    }

    // Find node to insert before.

    current = *pHead;
    while (current != NULL && current->data < val)  {
        previous = current;
        current = current->next;
    }


    // Handle inserting at start of list.

    if (current == *pHead) {
        newNode->next = *pHead;
        *pHead = newNode;
        return;
    }

    // Handle inserting at end of list.

    if (current == NULL) {
        previous->next = newNode;
        return;
    }

    // Handle inserting somewhere inside the list.

    newNode->next = current;
    previous->next = newNode;
}

void printList (struct listNode *Head) {
    struct listNode *current = Head;

    if (current == NULL) {
        puts ("There are no numbers.");
        return;
    }

    puts ("Numbers are:");
    while (current != NULL) {
        printf ("   %d\n", current->data);
        current = current->next;
    }
}

There’s a couple of other things I’ve cleaned up there as well, such as changing *current to the more explicit current->data, passing a pointer to the head so you can change it, and making a slight modification to the main input loop. Here’s a sample run:

This program will create an ordered linked list
    of numbers greater than 0 until the user 
    inputs 0 or a negative number.
Please input a value to store into the list.
4
Please input a value to store into the list.
1
Please input a value to store into the list.
8
Please input a value to store into the list.
5
Please input a value to store into the list.
6
Please input a value to store into the list.
3
Please input a value to store into the list.
2
Please input a value to store into the list.
9
Please input a value to store into the list.
7
Please input a value to store into the list.
0
Numbers are:
   1 
   2 
   3 
   4 
   5 
   6 
   7 
   8 
   9