How to Find All Occurrences of an Element in a Deque in C++?

In C++, deques are data structures similar to queues provided by the STL library in C++ but unlike queues, deques allow the insertion and deletion of elements from both ends. In this article, we will learn how to find all occurrences of a specific element in a deque in C++.

Example

Input:
deque<int>dq = {1, 2, 3, 4, 2, 5, 2, 6, 2}
Target=2

Output:
Element 2 found at index 1, 4, 6, 8

Finding All Occurrences of an Element in a Deque in C++

To find all occurrences of a specific element in a std::deque we can use the std::find() function, which returns the first occurrence of a given element in a queue and then keep searching for the given element repeatedly till the end to find all the occurrences.

Syntax of std::find()

find(first, last, target);

where,

• first: Iterator to the first element in the range.
• last: Iterator to the last element in the range.
• target: Element to search for.

C++ Program to Find All Occurrences of an Element in a Deque

The following program illustrates how we can find all occurrences of a specific element in a deque in C++.

C++

// C++ Program to illustrate how we can find all occurrences
// of a specific element in a deque
#include <algorithm>
#include <deque>
#include <iostream>

using namespace std;

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

    // Element to search
    int element = 2;

    // Find the first occurrence of the element in the
    // deque
    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
        // beginning of the range after the currently found
        // occurrence 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), where N is the size of the deque.
Auxiliary Space: O(1)