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

23 Jun, 2022
Lists are containers used in C++ to store data in a non contiguous fashion, Normally, Arrays and Vectors are contiguous in nature, therefore the insertion and deletion operations are costlier as compared to the insertion and deletion option in Lists.
empty() function is used to check if the list container is empty or not. Syntax :
listname.empty()
Parameters :
No parameters are passed.
Returns :
True, if list is empty
False, Otherwise
Examples:
Input  : list list{1, 2, 3, 4, 5};
         list.empty();
Output : False

Input  : list list{};
         list.empty();
Output : True
Errors and Exceptions
It has a no exception throw guarantee.Shows error when a parameter is passed.
It has a no exception throw guarantee.
Shows error when a parameter is passed.
CPP
// CPP program to illustrate// Implementation of empty() function#include <iostream>#include <list>using namespace std; int main(){    list<int> mylist{};    if (mylist.empty()) {        cout << "True";    }    else {        cout << "False";    }    return 0;}
Output:
True
Application : Given a list of integers, find the sum of the all the integers.
Input  : 1, 5, 6, 3, 9, 2
Output : 26
Explanation -  1+5+6+3+9+2 = 26
Algorithm
Check if the list is empty, if not add the front element to a variable initialised as 0, and pop the front element.Repeat this step until the list is empty.Print the final value of the variable.
Check if the list is empty, if not add the front element to a variable initialised as 0, and pop the front element.
Repeat this step until the list is empty.
Print the final value of the variable.
CPP
// CPP program to illustrate// Application of empty() function#include <iostream>#include <list>using namespace std; int main(){    int sum = 0;    list<int> mylist{ 1, 5, 6, 3, 9, 2 };    while (!mylist.empty()) {        sum = sum + mylist.front();        mylist.pop_front();    }    cout << sum;    return 0;}
Output:
26
size() function is used to return the size of the list container or the number of elements in the list container. Syntax :
listname.size()
Parameters :
No parameters are passed.
Returns :
Number of elements in the container.
Examples:
Input  : list list{1, 2, 3, 4, 5};
         list.size();
Output : 5

Input  : list list{};
         list.size();
Output : 0
Errors and Exceptions
It has a no exception throw guarantee.Shows error when a parameter is passed.
It has a no exception throw guarantee.
Shows error when a parameter is passed.
CPP
// CPP program to illustrate// Implementation of size() function#include <iostream>#include <list>using namespace std; int main(){    list<int> mylist{ 1, 2, 3, 4, 5 };    cout << mylist.size();    return 0;}
Output:
5
Application : Given a list of integers, find the sum of the all the integers.
Input  : 1, 5, 6, 3, 9, 2
Output : 26
Explanation -  1+5+6+3+9+2 = 26
Algorithm
Check if the size of the list is 0, if not add the front element to a variable initialised as 0, and pop the front element.Repeat this step until the list is empty.Print the final value of the variable.
Check if the size of the list is 0, if not add the front element to a variable initialised as 0, and pop the front element.
Repeat this step until the list is empty.
Print the final value of the variable.
CPP
// CPP program to illustrate// Application of size() function#include <iostream>#include <list>using namespace std; int main(){    int sum = 0;    list<int> mylist{ 1, 5, 6, 3, 9, 2 };    while (mylist.size() > 0) {        sum = sum + mylist.front();        mylist.pop_front();    }    cout << sum;    return 0;}
Output:
26
Let us see the differences in a tabular form -:
mayank007rawa
cpp-containers-library
CPP-Library
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CPP
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