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list::empty() and list::size() in C++ STL
• Difficulty Level : Basic
• Last Updated : 12 Dec, 2017

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.

list::empty()

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

1. It has a no exception throw guarantee.
2. Shows error when a parameter is passed.

 `// CPP program to illustrate ` `// Implementation of empty() function ` `#include ` `#include ` `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

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

 `// CPP program to illustrate ` `// Application of empty() function ` `#include ` `#include ` `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
```
list::size()

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

1. It has a no exception throw guarantee.
2. Shows error when a parameter is passed.

 `// CPP program to illustrate ` `// Implementation of size() function ` `#include ` `#include ` `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

1. 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.
2. Repeat this step until the list is empty.
3. Print the final value of the variable.

 `// CPP program to illustrate ` `// Application of size() function ` `#include ` `#include ` `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
```

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