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Data Structures: Introduction to Stacks

In the previous article, we saw how data is inserted and deleted in an array. In that case we could insert data at any place throughout the array. However, there are situations when we only need to add and retrieve data form the ends of the array. Stacks are one of the examples of this.

Stacks are data structures in which data could be added and retrieved only from one end (also known as the TOP of the stack). Suppose we insert 5, 6, 9 to the stack consecutively then while retrieving the first one to be retrieved will be 9 then 6 and then 5. That is why stacks are also known as Last-In-First-Out (or LIFO) structure.

A few terms regarding stacks:

  • Stack: Stack is a user-defined data structure. It is most commonly represented by linked-lists and arrays. In this article, we will be representing stacks with arrays.

  • Push: Adding data to the stack is known as pushing.

  • Pop: Retrieving data from the stack is known as popping.

Let us have look at this process with the help of an example. Suppose we have a stack represented by an array stack [10], which is empty to start with.

push(5)

Now, stack [10] = 5

push(10)

Now, stack [10] = 5, 10

pop() [It returns    10]

Now, stack [10] = 5

pop() [now it returns 5]

Now, stack [10] is again empty

In this way, a stack can grow and shrink over time.

Example Program

The process is quite simple so we straightaway move on to the example program in c++ that illustrates the implementation of stack.

In the following program, we have defined a class that has all the function implemented to represent a stack.

  // Example Program in C++
  // to illustrate Stacks
  #include<iostream.h>

  // stack class
  class stack
  {
   int arr[100];
   // 'top' will hold the
   // index number in the
   // array from which all
   // the pushing and popping
   // will be done
   int top;
  public:
   stack();
   void push(int);
   int pop();
  };
  // stack class definition ends

  // member functions
  // of the stack class
  stack::stack()
  {
   // initialize the top
   // position
   top=-1;
  }

  void stack::push(int num)
  {
   if(top==3)
   {
    cout<<"\nStack Full!\n";
    return;
   }

   top++;
   arr[top]=num;
  }

  int stack::pop()
  {
   if(top==-1)
   {
    cout<<"\nStack Empty!\n";
    return NULL;
   }

   return arr[top--];
  }
  // member function definition ends

  void main(void)
  {
   stack s;
   int ch;
   int num;

   while(ch!=3)
   {
    cout<<"1> Push";
    cout<<"\n2> Pop";
    cout<<"\n3> Quit\n";

    cin>>ch;

    switch(ch)
    {
     case 1:
     cout<<"enter element:";
     cin>>num;

     s.push(num);
     break;

     case 2:
     cout<<"\n\nPopped: ";
     cout<<s.pop();
     cout<<"\n\n";
     break;
    }
   }
  }

Good-Bye!

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