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How to erase an element from a vector using erase() and reverse_iterator?
  • Difficulty Level : Expert
  • Last Updated : 10 Oct, 2019

Given a vector, the task is to erase an element from this vector using erase() and reverse_iterator.

Example:

Input: vector = {1, 4, 7, 10, 13, 16, 19}, element = 16
Output: 1 4 7 10 13 19

Input: vector = {99, 89, 79, 69, 59}, element = 89
Output: 99 79 69 59

Approach:

  • Get the vector and the element to be deleted
  • Initialize a reverse iterator on the vector
  • Erase the required element with the help of base() and erase()

    Reason for using base(): erase() returns a valid iterator to the new location of the element which follows the one, which was just erased, in a forward sense. So we can’t use the same process while using reverse iterators where we want to go in the reverse direction instead of forward. And also we can’t pass a reverse iterator as a parameter to erase() function or it will give a compilation error.

    A reverse_iterator is just an iterator adaptor that reverses the direction of a given iterator. All operations on the reverse_iterator really occur on that underlying iterator. We can obtain that iterator using the reverse_iterator::base() function. Infact the relationship between itr.base() and itr is:



    &*(reverse_iterator(itr))==&*(itr-1)

Below is the implementation of the above approach:

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// C++ program to delete an element of a vector
// using erase() and reverse iterator.
  
#include <iostream>
#include <vector>
  
using namespace std;
  
// Function to delete element
// 'num' from vector 'vec'
vector<int> delete_ele(vector<int> vec, int num)
{
    // initializing a reverse iterator
    vector<int>::reverse_iterator itr1;
  
    for (itr1 = vec.rbegin(); itr1 < vec.rend(); itr1++) {
  
        if (*itr1 == num) {
  
            // erasing element = 16
            vec.erase((itr1 + 1).base());
        }
    }
  
    return vec;
}
  
// Driver code
int main()
{
    vector<int> vec = { 1, 4, 7, 10, 13, 16, 19 };
  
    // we want to delete element = 16
    int num = 16;
  
    vector<int>::iterator itr1;
  
    cout << "Vector originally: \n";
    for (itr1 = vec.begin(); itr1 < vec.end(); itr1++) {
  
        // printing the original elements of vector
        cout << *itr1 << " ";
    }
  
    cout << "\n\nElement to be deleted: "
         << num << "\n\n";
  
    // reinitializing vector 'vec'
    // after deleting 'num'
    // from the vector
    // and keeping other remaining
    // elements as they are
    vec = delete_ele(vec, num);
  
    vector<int>::iterator itr2;
  
    cout << "Vector after deletion: \n";
    for (itr2 = vec.begin(); itr2 < vec.end(); itr2++) {
  
        // printing the remaining elements of vector
        cout << *itr2 << " ";
    }
  
    return 0;
}
// This code is contributed by supratik_mitra

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Output:

Vector originally: 
1 4 7 10 13 16 19 

Element to be deleted: 16

Vector after deletion: 
1 4 7 10 13 19

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