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Check vector is empty or not | C++ STL



Here, we are going to learn how to check whether a vector is empty or not in C++ STL (Standard Template Library)? There are two ways to check whether list is empty or not 1) using vector::empty() and 2) vector::size().
Submitted by IncludeHelp, on August 24, 2018

Vector are dynamic arrays in nature, they can resize themselves according to the number of elements, if elements are added or deleted, size or memory of the vector is automatically resized.

There are two methods to check whether a given vector is an empty vector or not.

  1. Using vector::empty() function
  2. Using vector::size() function

1) vector::empty() function

It returns true, if vector is empty else it returns false.

Syntax:

 vector_name.empty();

Example:

    Input: 
    vector<int> v1{10, 20, 30, 40, 50 };
    vector<int> v2{};
    
    Function calls:
    v1.empty()
    v2.empty()
    
    Output:
    false
    true

2) vector::size() function

It returns total number of elements of a vector.

Syntax:

 vector_name.size();

Example:

    Input: 
    vector<int> v1{10, 20, 30, 40, 50 };
    vector<int> v2{};
    
    Function calls:
    v1.size()
    v2.size()
    
    Output:
    5
    0

Program 1: (Using vector::empty() function)

#include <iostream>
#include <vector>

using namespace std;

int main() {
	//declare and assign a vectors
	//non-empty
	vector<int> v1{ 10, 20, 30, 40, 50 };
	//empty 
	vector<int> v2{};

	//check whether vector are empty or not 
	//using empty () function
	if(v1.empty())
		cout << "vector: v1 is an empty vector" << endl;
	else
		cout << "vector: v1 has " << v1.size () << " elements" << endl;

	if (v2.empty ())
		cout << "vector: v2 is an empty vector" << endl;
	else
		cout << "vector: v2 has " << v2.size () << " elements" << endl ;
	
	return 0;
}

Output

    vector: v1 has 5 elements
    vector: v2 is an empty vector



Program 2: (Using vector::size() function)

#include <iostream>
#include <vector>

using namespace std;

int main() {
	//declare and assign a vectors
	//non-empty
	vector<int> v1{10, 20, 30, 40, 50};
	//empty 
	vector<int> v2{};
	
	//variable to store size 
	int n = 0;

	//check whether vector are empty or not 
	//using empty () function
	n = v1.size () ;
	if(n==0)
		cout << "Vector: v1 is an empty vector" << endl;
	else
		cout << "vector: v1 has " << n << " elements" << endl;

	n = v2.size ();
	if(n==0)
		cout << "vector: v2 is an empty vector" << endl;
	else
		cout << "vector: v2 has " << v2.size () << " elements" << endl ;

	return 0;
}

Output

    vector: v1 has 5 elements
    vector: v2 is an empty vector





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