Element is not being popped

Question

Here is the pop() function, I have written to pop an element of a stack. So far, I have succeeded in pushing elements into a stack and showing the stack. So, I guess my pop() function is doing wrong somewhere. Here is my pop function:

void pop(int newstack[], int *newtop, int bound )
{
    int item;
    if(*newtop<0)
        printf("\n CAUTION!!! UNDERFLOW");
    else
    {
        item=newstack[*newtop];
        *newtop--;
        printf("\n Element popped->%d",item);
    }
}

Taking no chance, I am also posting the show() function:

void show_stack(int newstack[], int *top)
{
    int i;
    printf("\n");
    for(i=0;i<=*top;i++)
        printf("%d",newstack[i]);
}

I guess there is no error in the show function.

I'm not sure this: `*newtop--;` does what you think it does. Actually I'll rephrase that. I'm *quite sure* it *doesn't* do what you think it does. — WhozCraig, Sep 12 '13 at 08:35
You need to post your main and pop. show is not required. And *newtop--; ==> (*newtop)--; precedence matters.... — Gangadhar, Sep 12 '13 at 08:38
@WhozCraig : You're absolutely right. Mistu4u, you should check your pointer arithmetics. — lucasg, Sep 12 '13 at 08:40
@Mistu4u in this case you might solved the issue by changing statement *newtop--; ==> (*newtop)--; but in actual sense you need to check your main function exactly how you are passing parameters to the function .. — Gangadhar, Sep 12 '13 at 08:48
My 2 cent: Put all the parameter in a struct. s_stack or something similar. — mathk, Sep 12 '13 at 09:16
Have you tried running this code step-by-step in a debugger? — PP., Sep 12 '13 at 09:36

score 5 · Accepted Answer · edited May 23 '17 at 12:20

5

The * dereference / indirection operator has lower precedence than the -- postfix decrement operator. Your statement

*newtop--;

is going to be parsed as

*(newtop--);

And since the value of newtop-- is the current value of newtop, the statement achieves precisely nothing. It dereferences newtop, and does nothing with the dereferenced value.

You actually want something like this:

*newtop = *newtop - 1;

or

(*newtop)--;

See this answer for details on expressions like *newtop--.

edited May 23 '17 at 12:20

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answered Sep 12 '13 at 08:41

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+1 or `--(*newtop);`, no reason to maintain the intermediate that post-dec creates. in this case its especially good, because `--*newtop` is equivalent. – WhozCraig Sep 12 '13 at 08:48

score 2 · Answer 2 · edited May 23 '17 at 12:20

2

*newtop--; is probably wrong

Use:

(*newtop)--;

See this

edited May 23 '17 at 12:20

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answered Sep 12 '13 at 08:37

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Element is not being popped

2 Answers2