exception related

[trish mo]

Hi !

I wrote my own little program on stack which goes like this :

class stack
{
 int stck[];
 int top = -1;

  void setSize(int sz)
  {
   stck = new int[sz];
  }  

  void push(int element)
  {
   try
   {
    top++;
    stck[top] = element;
    System.out.println(element + " pushed on stack !");
   }
   catch(ArrayIndexOutOfBoundsException ae)
   {
    System.out.println("Stack overflow !");
    System.out.println(element + " could not be pushed on stack !");
   }
  }

  int pop()
  {
   try
   {
    int value = stck[top]; 
    top--;
    return value;
   }
   catch(ArrayIndexOutOfBoundsException ae)
   {
    System.out.println("Stack underflow !");   
   }
   
   return -9999;
  }
}

class main_stack
{
 public static void main(String args[])
 {
  stack st1 = new stack();

   st1.setSize(5);

    for(int i = 0; i < 6; i++)
     st1.push(i+1); 

    for(int j = 0; j < 6; j++)
     if(st1.pop() != -9999)
      System.out.println(st1.pop() + " popped from stack !");   
 }
}

Why does the pop function work incorrectly...looks like the code never enters the try block but that's not possible !
Can anyone provide some answers ?

Thanks

[Matthew Brown]

Hi. Welcome to JavaRanch!

You've got a couple of different problems there.

1. You create a stack of size 5, and then try and add 6 items. That's fine, the error capture handles that...except the error capture does not reset the top pointer. So now any attempt to pop causes an ArrayIndexOutOfBoundsException, as the pointer is already set to an invalid value.

2. Fix that, and you'll see the second problem. You're calling pop twice on each iteration. Once to check the value is OK, but then you pop it again. So you count down in twos. I doubt that's what's intended.

[Joanne Neal]

It would help if you told us what is actually happening, but I suspect the following.

for(int i = 0; i < 6; i++)   
     st1.push(i+1); 

This pushes 6 elements on to the stack, but you have only set the stack size to 5, so I presume you are getting a stack overflow.

You then try to pop an element off the stack, but top will still be out of bounds so you will get a stack underflow.

[Wouter Oet]

Why do you use Exceptions and not a simple if-statement? e.g.:

if(top >= stck.length) ...

[trish mo]

Matthew Brown wrote:Hi. Welcome to JavaRanch!

You've got a couple of different problems there.

1. You create a stack of size 5, and then try and add 6 items. That's fine, the error capture handles that...except the error capture does not reset the top pointer. So now any attempt to pop causes an ArrayIndexOutOfBoundsException, as the pointer is already set to an invalid value.

2. Fix that, and you'll see the second problem. You're calling pop twice on each iteration. Once to check the value is OK, but then you pop it again. So you count down in twos. I doubt that's what's intended.

Yup, got it

This is the modified code :

class stack
{
 int stck[];
 int top = -1;

  void setSize(int sz)
  {
   stck = new int[sz];
  }  

  void push(int element)
  {
   try
   {
    top++;
    stck[top] = element;
    System.out.println(element + " pushed on stack !");
   }
   catch(ArrayIndexOutOfBoundsException ae)
   {
    System.out.println("Stack overflow !");
    System.out.println(element + " could not be pushed on stack !");
   }
  }

  int pop()
  {
   try
   {
    top--;
    int value = stck[top]; 
    return value;
   }
   catch(ArrayIndexOutOfBoundsException ae)
   {
    System.out.println("Stack underflow !");   
   }
   
   return -9999;
  }
}

class main_stack
{
 public static void main(String args[])
 {
  stack st1 = new stack();
  int val;

   st1.setSize(5);

    for(int i = 0; i < 6; i++)
     st1.push(i+1); 

    for(int j = 0; j < 6; j++)
     if((val = st1.pop()) != -9999)
      System.out.println(val + " popped from stack !");   
 }
}

Thanks a lot !

[Matthew Brown]

Yes, but....now try running that with a size of 6. You'll find it's not quite working right.