Find indices of all occurrences of an element in a vector in C++

This post will discuss how to find the indices of all occurrences of an element in a vector in C++.

1. Using `std::find_if`

To find the indices of all occurrences of an element in a vector, we can repeatedly call the std::find_if function within a loop. The following example efficiently calls the std::find_if function, where the search for the next element begins at the previous match.

#include <iostream>

#include <vector>

#include <algorithm>

template<typename T>

std::vector<int> findItems(std::vector<T> const &v, int target) {

std::vector<int> indices;

auto it = v.begin();

while ((it = std::find_if(it, v.end(), [&] (T const &e) { return e == target; }))

!= v.end())

{

indices.push_back(std::distance(v.begin(), it));

it++;

}

return indices;

}

int main()

{

std::vector<int> v = {2, 1, 3, 6, 6, 7, 6, 8};

int target = 6;

std::vector<int> indices = findItems(v, target);

for (auto &e: indices) {

std::cout << e << ' ';

}

return 0;

}

Download Run Code

Output:

3 4 6

2. Using Loop

The idea here is to perform a linear scan of the vector and report indices of all matching elements with the target. This is demonstrated below using iterators. Note that to get the required index, std::distance is used (or apply pointer arithmetic).

#include <iostream>

#include <vector>

#include <algorithm>

#include <iterator>

template<typename T>

std::vector<int> findItems(std::vector<T> const &v, int target) {

std::vector<int> indices;

for (auto it = v.begin(); it != v.end(); it++) {

if (*it == target) {

indices.push_back(std::distance(v.begin(), it));

}

return indices;

}

int main()

{

std::vector<int> vec = {2, 1, 3, 6, 6, 7, 6, 8};

int target = 6;

std::vector<int> indices = findItems(vec, target);

for (auto &e: indices) {

std::cout << e << ' ';

}

return 0;

}

Download Run Code

Output:

3 4 6

We can simplify the above code with a regular for-loop:

#include <iostream>

#include <vector>

#include <algorithm>

template<typename T>

std::vector<int> findItems(std::vector<T> const &v, int target) {

std::vector<int> indices;

for (int i = 0; i < v.size(); i++) {

if (v[i] == target) {

indices.push_back(i);

}

return indices;

}

int main()

{

std::vector<int> vec = {2, 1, 3, 6, 6, 7, 6, 8};

int target = 6;

std::vector<int> indices = findItems(vec, target);

for (auto &e: indices) {

std::cout << e << ' ';

}

return 0;

}

Download Run Code

Output:

3 4 6

That’s all about finding the indices of all occurrences of an element in a vector in C++.

Find indices of all occurrences of an element in a vector in C++

1. Using std::find_if

2. Using Loop

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1. Using `std::find_if`