vector erase() and clear() in C++ -- vector的函数erase()和clear()

Vectors are same as dynamic arrays with the ability to resize itself automatically when an element is inserted or deleted, with their storage being handled automatically by the container. vector和动态数组一样，插入数据和删除数据可以动态改变大小，存储空间由容器处理。

vector::clear()

clear() function is used to remove all the elements of the vector container, thus making it size 0.函数clear()用来删除vector容器的所有元素。
Syntax :

vectorname.clear()
Parameters :
No parameters are passed.
Result :
All the elements of the vector are
removed ( or destroyed )

Examples:

Input  : myvector= {1, 2, 3, 4, 5};myvector.clear();
Output : myvector= {}Input  : myvector= {};myvector.clear();
Output : myvector= {}

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 clear() function
#include <iostream>
#include <vector>
using namespace std; int main()
{ vector<int> myvector; myvector.push_back(1); myvector.push_back(2); myvector.push_back(3); myvector.push_back(4); myvector.push_back(5); // Vector becomes 1, 2, 3, 4, 5 myvector.clear(); // vector becomes empty // Printing the vector for (auto it = myvector.begin(); it != myvector.end(); ++it) cout << ' ' << *it; return 0;
}

Output:

No Output

Time Complexity: O(N)
All elements are destroyed one by one.元素是一个一个删除。

vector::erase()

erase() function is used to remove elements from a container from the specified position or range.erase()函数是删除特定元素或特定范围的元素。

Syntax :

1. vectorname.erase()
2. vectorname.erase(startingposition, endingposition)
Parameters :
Position of the element to be removed in the form of iterator. position是vector中元素的下标
or the range specified using start and end iterator.
Result :
Elements are removed from the specified
position of the container.

Examples:

Input  : myvector= {1, 2, 3, 4, 5}, iterator= 2myvector.erase(iterator);
Output : 1, 2, 4, 5Input  : myvector= {1, 2, 3, 4, 5, 6, 7, 8}, iterator1= 3, iterator2= 6myvector.erase(iterator1, iterator2);
Output : 1, 2, 3, 8

Errors and Exceptions

1. It has a no exception throw guarantee, if the position is valid.
2. Shows undefined behaviour otherwise.

Removing element from particular position

// CPP program to illustrate
// working of erase() function
#include <iostream>
#include <vector>
using namespace std; int main()
{ vector<int> myvector{ 1, 2, 3, 4, 5 }; vector<int>::iterator it; it = myvector.begin(); myvector.erase(it); // Printing the Vector for (auto it = myvector.begin(); it != myvector.end(); ++it) cout << ' ' << *it; return 0;
}

Output:

2 3 4 5

Removing elements within a range

// CPP program to illustrate
// Implementation of erase() function
#include <iostream>
#include <vector>
using namespace std; int main()
{ vector<int> myvector{ 1, 2, 3, 4, 5 }; vector<int>::iterator it1, it2; it1 = myvector.begin(); it2 = myvector.end(); it2--; it2--; myvector.erase(it1, it2); // Printing the Vector for (auto it = myvector.begin(); it != myvector.end(); ++it) cout << ' ' << *it; return 0;
}

Output:

4 5

Application
Given a list of integers, remove all the even elements from the vector and print the vector. 给定一系列整数，删除偶数的元素，并打印出来。

Input  :1, 2, 3, 4, 5, 6, 7, 8, 9
Output :1 3 5 7 9
Explanation - 2, 4, 6 and 8 which are even are erased from the vector

Algorithm
1. Run a loop till the size of the vector. 运行一个循环，直到vector的尾部。
2. Check if the element at each position is divisible by 2, if yes, remove the element and decrement iterator.
3. Print the final vector.

// CPP program to illustrate
// Application of erase() function
#include <iostream>
#include <vector>
using namespace std; int main()
{ vector<int> myvector{ 1, 2, 3, 4, 5, 6, 7, 8, 9 }; for (auto i = myvector.begin(); i != myvector.end(); ++i) { if (*i % 2 == 0) { myvector.erase(i); i--; } } // Printing the vector for (auto it = myvector.begin(); it != myvector.end(); ++it) cout << ' ' << *it; return 0;
}

Output:

1 3 5 7 9

Time Complexity: O(N) in the worst case when the 1st element is deleted while O(1) in the best case when the last element is deleted. O(N)是删除第一个元素的最坏结果，O(1)是删除最后一个元素的最好的结果。

clear() vs erase(), When to use what?

clear() removes all the elements from a vector container, thus making its size 0. All the elements of the vector are removed using clear() function.删除vector容器中所有元素，大小变为0，
erase() function on the other hand, is used to remove specific elements from the container or a range of elements from the container, thus reducing its size by the number of elements removed. 删除特定位置或特定范围的元素，大小下降了删除的元素的个数。

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vector erase() and clear() in C++ -- vector的函数erase()和clear()

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