stack swap() in C++ STL - GeeksforGeeks

Stacks are a type of container adaptors with LIFO(Last In First Out) type of work, where a new element is added at one end and (top) an element is removed from that end only.

stack::swap()

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

Syntax :

stackname1.swap(stackname2)

Parameters: The name of the stack with which the contents have to be swapped.

Result: All the elements of the 2 stack are swapped.

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

Note: In stack container, the elements are printed in reverse order because the top is printed first then moving on to other elements.

// 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

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