CODEWITHSUNDEEP
cpointersreferencestack
Tony
Tony

Reputation: 23

C linked list stack and pointers

I am new to C, and am trying to implement a stack with linked list. Currently set up the Stack, all good so far. The problem has come in when I try to push a new node onto the list. I currently have

In main(), push() is called by:

push(&(s.head), 'r');

The function push is:

void push(StackNodePtr *topPtr, char value){
    printf("topPtr value %c", (*topPtr)->data); // - Is currently 'p'

    StackNodePtr sNP;
    sNP = malloc(Node_Size);
    sNP->data = value;                          // - Is currently 'r'
    sNP->nextPtr = *topPtr;

    printf("\nsNP value - %c", sNP->nextPtr->data);      // Prints p... cool
    topPtr = &sNP;      // Just assigned it???
    printf("\ntopPtr at end of push = %c", (*topPtr)->data);    // prints r... cool
    // WHY YOU NO REFERENCE sNP LATER!?!?
}

Meanwhile, back in main:

    printf("\non the stack...%c", stackTop(s.head));  // prints 'p'

It seems to work fine in push, however I call printf() on the node that topPtr pointed to and the value that topPtr used to be prints out instead (in this case 'p'). As far as I can tell from the hunting I've done, it looks and feels correct and I don't know what I have missed.

Could it be where I have done topPtr = &sNP;?

Any "push" in the right direction is a good push...

Upvotes: 1

Views: 928

Answers (5)

SeattleOrBayArea
SeattleOrBayArea

Reputation: 3118

Your function push is modifying the pointer topPtr. Since C is pass by value, the head is passed as a value and the copy passed gets modified in push. So, to modify the pointer itself, you need to pass a pointer to pointer. Also, the function push() signature needs to be corrected to pass in a pointer to pointer. Finally, the topPtr assignment in push needs to be corrected as shown in the code snippet.

Do the following changes:

push(&(s.head), 'r'); // pass pointer to pointer, assuming head is pointer, it should be fine

void push(StackNodePtr **topPtr, char value){  // make first argument pointer to pointer. 

StackNodePtr sNP;
sNP = malloc(Node_Size);
sNP->data = value;                       
sNP->nextPtr = *topPtr; 

*topPtr = &sNP;     <------*topPtr needs to be assigned. 
}

Upvotes: 0

Horonchik
Horonchik

Reputation: 477

its looks like the bug is here: topPtr = &sNP; // Just assigned it???

instead of push returning void. change it to return the new head of the stack. return sNp;

Upvotes: 0

ShinTakezou
ShinTakezou

Reputation: 9671

It should be

  *topPtr = sNP;

this way, the original head, the caller has passed the pointer to, that became the next of the new head, is "overwritten" correctly, and the caller has the correct pointer to the new head.

Upvotes: 1

Ed Swangren
Ed Swangren

Reputation: 124702

topPtr = &sNP;      // Just assigned it???

This assignment is not visible outside of the function. topPtr is passed by value, i.e., a copy of it is made and passed to the function. So, assigning a different value only modifies the copy; the original argument remains pointing to the old memory location.

If you need to modify the argument in that way you need another level of indirection, i.e., take a StackNodePtr**.

Also, I am assuming that StackNodePtr is a typedef for a StackNode*. Am I right about that? Is there a good reason to typedef this pointer type? Typically it just serves to complicate things. I would recommend typedef'ing a pointer type only when it is truly an opaque type (i.e., a HANDLE on Windows).

Upvotes: 2

aland
aland

Reputation: 5189

topPtr = &sNP;      // Just assigned it???

No, you didn't. You assigned the value to local copy of pointer. You change the value of topPtr itself, and it is does not go outside. Instead, you should write to location it points to:

*topPtr = sNP;

Upvotes: 0

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