CODEWITHSUNDEEP
c
Nyabingi
Nyabingi

Reputation: 33

stack implementation output

Please help with the segment below. When n is removed from the top of the stack and stack is empty, output should be '-1 popped'.(I'm getting 0 atm)

void pop(void) {
    struct node *temp;
    int n;
    if (top == NULL) {
        printf("%d popped\n", top);
        return;
    }
    n = top->item;
    temp = top;
    top = top->prev;
    free(temp);
    printf("%d popped\n", n);
    return;
}

Upvotes: 0

Views: 84

Answers (3)

Useless
Useless

Reputation: 67713

I think this fits your intent better:

void pop(void) {
    int n;
    if (top == NULL) {
        // empty: magic sentinel value
        n = -1;
    } else {
        n = top->item;
        // not empty: update the stack
        struct node *temp = top;
        top = top->prev;
        free(temp);
    }
    printf("%d popped\n", n);
    return;
}

As Ed and Anon correctly point out, you can fix the original bug by explicitly printing -1. However, making your logic less fragile in the first place (and incidentally fixing this specific bug) seems like a bigger win to me.

Upvotes: 0

Ed Heal
Ed Heal

Reputation: 59987

Because NULL is the null pointer (i.e a pointer to nothing) and usually has the value of 0.

Just change the line 

printf("%d popped\n", top);

to

printf("-1 popped\n");

Upvotes: 1

perilbrain
perilbrain

Reputation: 8187

Logic fault, You are comparing against zero and want an output of -1 !!!

 if (top == NULL) {
         printf("%d popped\n", top);    
     return;  
   }

Should be

  if (top == NULL) {
         printf("%d popped\n",-1);    
     return;  
   }

Upvotes: 1

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