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

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)

Reffered: https://www.geeksforgeeks.org

C++

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How to Compare Two Pairs in C++?
How to Find the Size of an Array Using Pointer to its First Element?
How to Read a File Using ifstream in C++?

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