Find index of an element in vector in C++

This post will discuss how to find the index of the first occurrence of a given element in vector in C++.

1. Using `std::find` with `std::distance` function

The simplest solution is to use the std::find algorithm defined in the <algorithm> header. The idea is to get the index using std::distance on the iterator returned by std::find, which points to the found value. We can also apply pointer arithmetic to the iterators. Therefore, the - operator would also work.

#include <iostream>

#include <vector>

#include <algorithm>

int main()

{

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

int key = 6;

std::vector<int>::iterator itr = std::find(v.begin(), v.end(), key);

if (itr != v.cend()) {

std::cout << "Element present at index " << std::distance(v.begin(), itr);

}

else {

std::cout << "Element not found";

}

return 0;

}

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Output:

Element present at index 2

2. Using `std::find_if` with `std::distance` function

We can also use the standard algorithm std::find_if, which accepts a predicate. This is the recommended approach if the search needs to satisfy certain conditions. For example, finding the index of the first string starting with some character in a vector of strings.

#include <iostream>

#include <vector>

#include <algorithm>

struct compare

{

int key;

compare(int const &i): key(i) {}

bool operator()(int const &i) {

return (i == key);

}

};

int main()

{

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

int key = 6;

auto itr = std::find_if(v.cbegin(), v.cend(), compare(key));

if (itr != v.cend()) {

std::cout << "Element present at index " << std::distance(v.cbegin(), itr);

}

else {

std::cout << "Element not found";

}

return 0;

}

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Output:

Element present at index 2

3. Naive Solution

Finally, we can write our own routine for this, as demonstrated below:

#include <iostream>

#include <vector>

#include <algorithm>

int main()

{

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

int key = 6;

bool found = false;

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

{

if (v[i] == key)

{

std::cout << "Element present at index " << i;

found = true;

break;

}

if (!found) {

std::cout << "Element not found";

}

return 0;

}

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Output:

Element present at index 2

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

Find index of an element in vector in C++

1. Using std::find with std::distance function

2. Using std::find_if with std::distance function

3. Naive Solution

1. Using `std::find` with `std::distance` function

2. Using `std::find_if` with `std::distance` function