stack::empty() and stack::size() in C++ STL

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

stack::empty()

empty() function is used to check if the stack container is empty or not.

Syntax :

stackname.empty()
Parameters :
No parameters are passed.
Returns :
True, if stack is empty
False, Otherwise

Examples:

Input :   mystack
          mystack.empty();
Output :  True
 
Input :   mystack = 1, 2, 3
Output :  False

Errors and Exceptions



1. Shows error if parameter is passed
2. Shows no exception throw guarantee.

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

int main()
{
    stack<int> mystack;
    mystack.push(1);

    // Stack becomes 1

    if (mystack.empty()) {
        cout << "True";
    }
    else {
        cout << "False";
    }
    return 0;
}

Output:

False

Application :
Given a stack of integers, find the sum of the all the integers.

Input : 1, 8, 3, 6, 2
Output: 20

Algorithm
1. Check if the stack is empty, if not add the top element to a variable initialised as 0, and pop the top element.
2. Repeat this step until the stack is empty.
3. Print the final value of the variable.

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

int main()
{
    int sum = 0;
    stack<int> mystack;
    mystack.push(1);
    mystack.push(8);
    mystack.push(3);
    mystack.push(6);
    mystack.push(2);

    // Stack becomes 1, 8, 3, 6, 2

    while (!mystack.empty()) {
        sum = sum + mystack.top();
        mystack.pop();
    }
    cout << sum;
    return 0;
}

Output:

20
stack::size()

size() function is used to return the size of the stack container or the number of elements in the stack container.

Syntax :

stackname.size()
Parameters :
No parameters are passed.
Returns :
Number of elements in the container.

Examples:

Input :   mystack = 0, 1, 2
          mystack.size();
Output :  3
 
Input :   mystack = 0, 1, 2, 3, 4, 5
          mystack.size();
Output :  6

Errors and Exceptions

1. Shows error if a parameter is passed.
2. Shows no exception throw guarantee.

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

int main()
{
    int sum = 0;
    stack<int> mystack;
    mystack.push(1);
    mystack.push(8);
    mystack.push(3);
    mystack.push(6);
    mystack.push(2);

    // Stack becomes 1, 8, 3, 6, 2

    cout << mystack.size();

    return 0;
}

Output:

5

Application :
Given a stack of integers, find the sum of the all the integers.

Input : 1, 8, 3, 6, 2
Output: 20

Algorithm
1. Check if the size of the stack is zero, if not add the top element to a variable initialised as 0, and pop the top element.
2. Repeat this step until the stack size becomes 0.
3. Print the final value of the variable.

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

int main()
{
    int sum = 0;
    stack<int> mystack;
    mystack.push(1);
    mystack.push(8);
    mystack.push(3);
    mystack.push(6);
    mystack.push(2);

    // Stack becomes 1, 8, 3, 6, 2

    while (mystack.size() > 0) {
        sum = sum + mystack.top();
        mystack.pop();
    }
    cout << sum;
    return 0;
}

Output:

20


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