Stacks in Cpp

Introduction

In the previous article, we looked into Stacks in Cpp and their implementation. In this article, we will be looking at some of the in-built functions of stacks and accessing elements in a stack.

Accessing Elements to the stack

We access the elements of a stack using the method top().

For example,

#include <iostream>
#include <stack>
using namespace std;

int main() {

  // create a stack of strings
  stack<string> colors;

  // push element into the stack
  colors.push("Red");
  colors.push("Orange");
  colors.push("Blue");
  
  // get top element
  string top = colors.top();


  cout << "Top Element: " << top;
  
  return 0;
}

Output

Top Element: Blue

In the above example, we have created a stack of strings and added some elements, and accessed them using the top() method.

string top = colors.top(); 

Getting the size of the stack

We use the size() method to get the size i.e. the number of elements in the stack.

For example,

#include <iostream>
#include <stack>
using namespace std;

int main() {

  // create a stack of int
  stack<int> prime_nums;

  // push elements into the stack
  prime_nums.push(2);
  prime_nums.push(3);
  prime_nums.push(5);
  
  // get the size of the stack
  int size = prime_nums.size();

  cout << "Size of the stack: " << size;

  return 0;
}

Output

Size of the stack: 3

In the above example, we created a stack of integers and added three elements to it. Then we used the size()  method to find the number of elements in the stack

prime_nums.size();

Since we added three elements to the stack the method returns 3.

Checking if the stack is Empty

We use the empty() method to check if a stack is empty or not.

This method returns either 0 or 1.

0 – if the stack is not empty

1 – if the stack is empty

For example,

#include <iostream>
#include <stack>
using namespace std;

int main() {

  // create a stack of double
  stack<double> nums;
  
  cout << "Is the stack empty? ";

  // check if the stack is empty  
  if (nums.empty()) {

    cout << "Yes" << endl;
  }
  else {
    cout << "No" << endl;
  }

  cout << "Pushing elements..." << endl;

  // push element into the stack
  nums.push(2.3);
  nums.push(9.7);
 
  cout << "Is the stack empty? ";

  // check if the stack is empty  
  if (nums.empty()) {

    cout << "Yes";
  }
  else {
    cout << "No";
  }

  return 0;
}

Output

Is the stack empty? Yes
Pushing elements...
Is the stack empty? No

There are still lots of functions that can be used with stack, some of them are

List of functions of Stack: 

stack::top() in C++ STL
stack::empty() and stack::size() in C++ STL
stack::push() and stack::pop() in C++ STL
stack::swap() in C++ STL
stack::emplace() in C++ STL

swap() in Stacks

This function is used to swap the contents of one stack with another stack of the same type but the size may vary. 

Syntax :

stackname1.swap(stackname2)

Examples:

contents of the stack from top to bottom are
Input  : mystack1 = {4, 3, 2, 1}
         mystack2 = {9, 7 ,5, 3}
         mystack1.swap(mystack2);
Output : mystack1 =  9, 7, 5, 3
         mystack2 =  4, 3, 2, 1

Input  : mystack1 = {7, 5, 3, 1}
         mystack2 = {8, 6, 4, 2}
         mystack1.swap(mystack2);
Output : mystack1 =  8, 6, 4, 2
         mystack2 =  7, 5, 3, 1

In the above examples, the functions helped us to swap the contents of one stack to another.

Lets look at the implementation

// CPP program to illustrate
// Implementation of swap() function
#include <stack>
#include <iostream>
using namespace std;

int main()
{
		// stack container declaration
	stack<int> mystack1;
	stack<int> mystack2;
	
	// pushing elements into first stack
	mystack1.push(1);
	mystack1.push(2);
	mystack1.push(3);
	mystack1.push(4);
	
	// pushing elements into 2nd stack
	mystack2.push(3);
	mystack2.push(5);
	mystack2.push(7);
	mystack2.push(9);

		// using swap() function to swap elements of stacks
	mystack1.swap(mystack2);

		// printing the first stack
	cout<<"mystack1 = ";
	while (!mystack1.empty()) {
		cout<<mystack1.top()<<" ";
		mystack1.pop();
	}

		// printing the second stack
	cout<<endl<<"mystack2 = ";
	while (!mystack2.empty()) {
		cout<<mystack2.top()<<" ";
		mystack2.pop();
	}
	return 0;
}

Output:

mystack1 = 9 7 5 3
mystack2 = 4 3 2 1

Conclusion

That is all for this article we learn more about the STL containers in the coming ones.

If you have any queries regarding stacks, leave a comment below.

Happy Coding!