Find All Occurrences of an Element in a Vector

In C++, std::vector is a dynamic array that stores elements in a similar way to C-style arrays, but it is automatically resizable and provides some built-in functions for different operations. In this article, we will learn how to find the indexes of all occurrences of an element within a vector in C++.

For example,

Input:
myVector = {2,3,2,1,5,4,2};
element = 2

Output:
The element 2 occurred at indices : 0, 2, 6

Find All Occurrences of an Element in a Vector

The C++ STL provides the std::find() function, which returns the first occurrence of a given element in a container. We can use this function with vectors and keep searching for the given element repeatedly till the end to find all the occurrences.

C++ Program To Find All Occurrences of an Element in Vector

C++

// C++ Program to Find All Occurrences of an Element in a
// Vector
#include <algorithm>
#include <iostream>
#include <vector>

using namespace std;

int main()
{
    // Initialize a vector array
    vector<int> arr = { 2, 3, 2, 1, 5, 4, 2 };

    // Element to search
    int element = 2;

    // Find the first occurrence of the element in the
    // vector
    auto it = find(arr.begin(), arr.end(), element);

    // Check if the element was found
    cout << "The element " << element
         << " occurred at indices: ";
    while (it != arr.end()) {
        // Print the index of the element
        cout << it - arr.begin() << ", ";
        // Search for the next occurrence and keeping
        // beggining of the range after the currenly found
        // occurence to avoid infinite loop
        it = find(it + 1, arr.end(), element);
    }
    cout << endl;

    return 0;
}

Output

The element 2 occurred at indices: 0, 2, 6, 

Time Complexity: O(N), because std::find() implements linear search.
Auxiliary Space:  O(1)