Error in initialization of a pointer to structure

Question

I have this program that I'm trying to modify it, but I don't understand why the statement: struct Link * temp = cap; doesn't print me the number that I assigned to the linked list. Thanks in advance!

struct Link
{
    int data;
    struct Link *urmatorul;
};

void Insert(Link * cap, int n)
{
    struct Link * temp = (Link*)malloc(sizeof(struct Link));
    temp->data = n;
    temp->urmatorul = NULL;
    if(cap != NULL)
        temp->urmatorul = cap;
    cap = temp;
}

void Print(Link * cap)
{
    struct Link *temp = cap;
    printf(" %d", cap->data);
    printf("The number is: ");
    while(temp != NULL)
    {
        printf(" %d", temp->data);
        temp = temp->urmatorul;
    }
    printf("\n");
}

int main()
{
    struct Link * cap;
    cap = NULL;
    printf("How many numbers? \n");
    int x, n, i;
    scanf(" %d", &x);
    for(i = 0; i < x; ++i)
    {
        printf("Enter the number: \n");
        scanf("%d", &n);
        Insert(cap, n);
        Print(cap);
    }
    return 0;
}

C passes arguments by value. Your cap is NULL, never changed after Insert(). — llllllllll
– llllllllll, Commented Feb 6, 2018 at 21:11
Think back to your text-books or class-notes. What do they say about passing arguments to functions? Do you remember anything about it be by value? Which means they are copied, and no matter how much you modify the copy inside the function, the original will never change? Do some research about emulating pass by reference in C. — Some programmer dude
– Some programmer dude, Commented Feb 6, 2018 at 21:12
Insert needs to return the new root node of your linked list. Your insert function is leaking memory. — jwdonahue
– jwdonahue, Commented Feb 6, 2018 at 21:15

cleblanc · Accepted Answer · 2018-02-06 21:26:10Z

2

You need to pass the Link * by reference to change it, that's a Link **

void Insert(Link **cap, int n)
{
    struct Link * temp = (Link*)malloc(sizeof(struct Link));
    temp->data = n;
    temp->urmatorul = NULL;
    if(*cap != NULL)
        temp->urmatorul = *cap;
    *cap = temp;
}

and in your main(...) use

Insert(&cap, n);

or you could return the new Link * from your Insert(...) like this;

Link * Insert(Link * cap, int n)
{
    struct Link * temp = (Link*)malloc(sizeof(struct Link));
    temp->data = n;
    temp->urmatorul = NULL;
    if(cap != NULL)
        temp->urmatorul = cap;
    return temp;
}

and in your main(...) use

cap = Insert(cap, n);

edited Feb 6, 2018 at 21:26

answered Feb 6, 2018 at 21:15

cleblanc

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1 Comment

jwdonahue Over a year ago

return cap = temp; is a meaningless assignment. Of course, even a weak optimizer will remove it, but it's semantically incorrect. Just return temp.

Sergey Kalinichenko · Accepted Answer · 2018-02-06 21:16:43Z

1

This line does not do anything, because cap inside Insert is a copy of cap from the main:

cap = temp;

The change gets discarded as soon as Insert exits, so main's cap remains NULL.

Change Insert's signature to return Link*, and assign it to cap in the call from main:

Link* Insert(Link * cap, int n)
{
    struct Link * temp = malloc(sizeof(struct Link)); // No need to cast
    temp->data = n;
    temp->urmatorul = cap; // No need for the conditional
    return temp;
}

The call looks like this:

cap = Insert(cap, n);

answered Feb 6, 2018 at 21:16

Sergey Kalinichenko

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