Programming Forums > Application Development > C++

epswing · Feb 17th, 2005, 7:33 PM

Hello all,

I'm a java/python programmer plunging into C. This "pointer" business is pretty neat, although it took a while to visualize pointers TO pointers and stuff. Anyways, I thought I had it figured out, however I get wildly unexpected results with this simple singly-linked-list. All I want it to do for now is insert and print.

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#include <stdio.h>

/* node stucture */
struct node {
        int value;
        struct node * next;
};

/*
 * inserts int 'i' at the end of linkedlist 'list'
 */
void insert(struct node ** list, int i)
{
        /* create a node pointer for the node to be inserted*/
        struct node * newnode = NULL;

        /* allocate space for the node and assign to above pointer */
        newnode = (struct node *) malloc(sizeof(struct node));

        /* give our new node its info */
        newnode->value = i;
        newnode->next = NULL;

        if (*list == NULL) {
                /* the list is empty, assign the first node */
                *list = newnode;
        } else {
                /* move to the end of the list, assign the newnode */
                while ((*list)->next != NULL) {
                        *list = (*list)->next;
                }
                (*list)->next = newnode;
                free(newnode);
        }
}

/*
 * prints linklist 'list' to the screen
 */
void print(struct node * list)
{
        while (list != NULL) {
                printf("%d ", list->value);
                list = list->next;
        }
        printf("\n");
}

/*
 * gather some numbers to put in the list, then print the whole list
 */
main()
{
        struct node * first = NULL;
        int n = 0;

        printf("Enter a series of intergers, eg. 10 3 8 0. Seperate by spaces, 0 to terminate:\n");
        do {
                scanf("%d", &n);
                if (n != 0)
                        insert(&first, n);
        } while (n != 0);
        print(first);
}

Now then. Here's some output:

Terminate right away:

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Enter a series of intergers, eg. 10 3 8 0. Seperate by spaces, 0 to terminate:
0

Nothing in the list. Good!

Insert 10, then terminate:

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Enter a series of intergers, eg. 10 3 8 0. Seperate by spaces, 0 to terminate:
10 0
10

There's a 10 in the list. Good!

Insert 10, then 12, then terminate:

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Enter a series of intergers, eg. 10 3 8 0. Seperate by spaces, 0 to terminate:
10 12 0
10 0

There's my 10, then there's a 0...hm, something went wrong!

Insert 10, then 12, then 15, then terminate:

(Toggle Plain Text)

Enter a series of intergers, eg. 10 3 8 0. Seperate by spaces, 0 to terminate:
10 12 15 0
<UNENDING LOOP OF OUTPUT DEATH HERE>

...yeah...I don't know.

Anything over 2 elements to be inserted, and it goes nuts. Inserting only 2 gives unexpected output. Inserting 1 gives the correct output.

Anyone see where my problem is?

Ooble · Feb 17th, 2005, 7:40 PM

What happens if you hit enter between each number instead of using space?

epswing · Feb 17th, 2005, 7:46 PM

Wow, never though of that. Just tried, same results.

Edit: Someone suggested that I can't borrow memory for the node, stash data in it, point a link at it, then immediately free it.

I thought I was freeing the pointer to the node. Since something else is now pointing at it...isn't that ok? I guess not, I don't get why though. If I remove the line

(Toggle Plain Text)

free(newnode);

then I get the same results no matter how many input numbers there are: It only stores the last two integers before the terminating 0.

ie. 1 3 0 stores 1 and 3 in the list.
1 3 5 0 stores 3 and 5 in the list.

Feb 17th, 2005, 7:33 PM	#1
epswing Newbie Join Date: Feb 2005 Posts: 14 Rep Power: 0	simple linkedlist not working Hello all, I'm a java/python programmer plunging into C. This "pointer" business is pretty neat, although it took a while to visualize pointers TO pointers and stuff. Anyways, I thought I had it figured out, however I get wildly unexpected results with this simple singly-linked-list. All I want it to do for now is insert and print. (Toggle Plain Text) #include <stdio.h> /* node stucture / struct node { int value; struct node next; }; /* * inserts int 'i' at the end of linkedlist 'list' / void insert(struct node * list, int i) { /* create a node pointer for the node to be inserted/ struct node newnode = NULL; /* allocate space for the node and assign to above pointer / newnode = (struct node ) malloc(sizeof(struct node)); /* give our new node its info / newnode->value = i; newnode->next = NULL; if (list == NULL) { /* the list is empty, assign the first node / list = newnode; } else { /* move to the end of the list, assign the newnode / while ((list)->next != NULL) { list = (list)->next; } (list)->next = newnode; free(newnode); } } / * prints linklist 'list' to the screen / void print(struct node list) { while (list != NULL) { printf("%d ", list->value); list = list->next; } printf("\n"); } /* * gather some numbers to put in the list, then print the whole list / main() { struct node first = NULL; int n = 0; printf("Enter a series of intergers, eg. 10 3 8 0. Seperate by spaces, 0 to terminate:\n"); do { scanf("%d", &n); if (n != 0) insert(&first, n); } while (n != 0); print(first); } #include <stdio.h> /* node stucture / struct node { int value; struct node next; }; /* * inserts int 'i' at the end of linkedlist 'list' / void insert(struct node * list, int i) { /* create a node pointer for the node to be inserted/ struct node newnode = NULL; /* allocate space for the node and assign to above pointer / newnode = (struct node ) malloc(sizeof(struct node)); /* give our new node its info / newnode->value = i; newnode->next = NULL; if (list == NULL) { /* the list is empty, assign the first node / list = newnode; } else { /* move to the end of the list, assign the newnode / while ((list)->next != NULL) { list = (list)->next; } (list)->next = newnode; free(newnode); } } / * prints linklist 'list' to the screen / void print(struct node list) { while (list != NULL) { printf("%d ", list->value); list = list->next; } printf("\n"); } /* * gather some numbers to put in the list, then print the whole list / main() { struct node first = NULL; int n = 0; printf("Enter a series of intergers, eg. 10 3 8 0. Seperate by spaces, 0 to terminate:\n"); do { scanf("%d", &n); if (n != 0) insert(&first, n); } while (n != 0); print(first); } Now then. Here's some output: Terminate right away: (Toggle Plain Text) Enter a series of intergers, eg. 10 3 8 0. Seperate by spaces, 0 to terminate: 0 Enter a series of intergers, eg. 10 3 8 0. Seperate by spaces, 0 to terminate: 0 Nothing in the list. Good! Insert 10, then terminate: (Toggle Plain Text) Enter a series of intergers, eg. 10 3 8 0. Seperate by spaces, 0 to terminate: 10 0 10 Enter a series of intergers, eg. 10 3 8 0. Seperate by spaces, 0 to terminate: 10 0 10 There's a 10 in the list. Good! Insert 10, then 12, then terminate: (Toggle Plain Text) Enter a series of intergers, eg. 10 3 8 0. Seperate by spaces, 0 to terminate: 10 12 0 10 0 Enter a series of intergers, eg. 10 3 8 0. Seperate by spaces, 0 to terminate: 10 12 0 10 0 There's my 10, then there's a 0...hm, something went wrong! Insert 10, then 12, then 15, then terminate: (Toggle Plain Text) Enter a series of intergers, eg. 10 3 8 0. Seperate by spaces, 0 to terminate: 10 12 15 0 <UNENDING LOOP OF OUTPUT DEATH HERE> Enter a series of intergers, eg. 10 3 8 0. Seperate by spaces, 0 to terminate: 10 12 15 0 <UNENDING LOOP OF OUTPUT DEATH HERE> ...yeah...I don't know. Anything over 2 elements to be inserted, and it goes nuts. Inserting only 2 gives unexpected output. Inserting 1 gives the correct output. Anyone see where my problem is?

Feb 17th, 2005, 7:40 PM	#2
Ooble I eat cake for breakfast. Join Date: Jul 2004 Location: In my box. Posts: 4,434 Rep Power: 9	What happens if you hit enter between each number instead of using space? __________________ Me :: You :: Them

Feb 17th, 2005, 7:46 PM	#3
epswing Newbie Join Date: Feb 2005 Posts: 14 Rep Power: 0	Wow, never though of that. Just tried, same results. Edit: Someone suggested that I can't borrow memory for the node, stash data in it, point a link at it, then immediately free it. I thought I was freeing the pointer to the node. Since something else is now pointing at it...isn't that ok? I guess not, I don't get why though. If I remove the line (Toggle Plain Text) free(newnode); free(newnode); then I get the same results no matter how many input numbers there are: It only stores the last two integers before the terminating 0. ie. 1 3 0 stores 1 and 3 in the list. 1 3 5 0 stores 3 and 5 in the list. Last edited by epswing; Feb 17th, 2005 at 10:05 PM.

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Programming Forums > Application Development > C++

simple linkedlist not working